NTC Thermistors Application Notes

NTC Thermistors Application Notes
NTC THERMISTORS
WHAT IS A THERMISTOR
A thermistor is an electronic component that
exhibits a large change in resistance with a change
in its body temperature. The word “thermistor” is
actually a contraction of the words “thermal resistor”.
The thermistors that we shall describe herein are
ceramic semiconductors and have either large
positive temperature coefficient of resistance (PTC
devices) or large negative temperature coefficient of
resistance (NTC devices). Both types of thermistors
(PTC and NTC) have definite features and
advantages which make them ideal for certain
sensor applications.
NTC THERMISTORS
The NTC thermistors which are discussed herein
are composed of metal oxides. The most commonly
used oxides are those of manganese, nickel, cobalt,
iron, copper and titanium. The fabrication of
commercial NTC thermistors uses basic ceramics
technology and continues today much as it has for
decades. In the basic process, a mixture of two or
more metal oxide powders are combined with
suitable binders, are formed to a desired geometry,
dried, and sintered at an elevated temperature. By
varying the types of oxides used, their relative
proportions, the sintering atmosphere, and the
sintering temperature, a wide range of resistivities
and temperature coefficient characteristics can
be obtained.
Types of NTC Thermistors
Commercial NTC thermistors can be classified
into two major groups depending upon the method
by which electrodes are attached to the ceramic
body. Each group may be further subdivided into
various types of thermistors where each type is
characterized by differences in geometry, packaging
and/or processing techniques.
The first group consists of bead type thermistors.
All of the bead type thermistors have platinum alloy
leadwires which are directly sintered into the
ceramic body. Bead type thermistors include the
following:
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•
•
•
•
•
•
•
Bare Beads
Glass Coated Beads
Ruggedized Beads
Miniature Glass Probes
Glass Probes
Glass Rods
Bead-in-Glass Enclosures
The second group of thermistors have metallized
surface contacts. All of these types are available
with radial or axial leads as well as without leads for
surface mounting or mounting by means of spring
contacts.
Metallized surface contact thermistors include
the following:
•
•
•
•
•
•
Disks
Chips (Wafers)
Surface Mounts
Flakes
Rods
Washers
NTC thermistors are available in a wide variety of
configurations and protective coatings to suit almost
any application. The most stable and accurate thermistors available are those which are hermetically
sealed in glass. Hermetically sealed thermistors are
also used, almost exclusively, for applications that
require continuous exposure to temperatures above
150°C.
Fabrication of Bead Type Thermistors
Bead type thermistors are normally fabricated by
applying a small dab of a slurry of mixed metal
oxides with a suitable binder onto a pair of spaced
platinum alloy leadwires. When the proper binder is
used, surface tension draws the material into
a bead that has the shape of an ellipsoid. The
leadwires are strung in a fixture that applies a slight
amount of tension to them and carefully controls the
spacing between the wires. After the mixture has
been allowed to dry, or has been partially sintered,
the strand of beads is removed from the fixture and
sintered. During the sintering, the thermistor oxides
shrink about the platinum alloy wires to form
intimate electrical and mechanical bonds.
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The beads are then individually cut from the
strand in one of the desired lead configurations
as shown in Figure <1>. The most common configurations are those that result in the adjacent and
opposite cut leads. All commercial bead type
thermistors have platinum alloy leads which range
from .0007” to .004” (0.018 mm to 0.1 mm)
diameter.
Although it is possible to obtain strain relief for the
leads of a bare bead with organic coatings, it is more
common to hermetically seal such units in glass.
The use of an hermetic seal provides about a
ten-fold improvement in the stability of a thermistor.
Common glass structures are shown in Figure <2>.
BARE BEAD SIZES
BEAD DIAMETER: 0.003” (0.075mm) to 0.04” (1mm)
WIRE DIAMETER: 0.007” (0.018mm) to 0.004” (0.1mm)
Figure 1: NTC Bead strands
In general, bead-type thermistors offer high
stability and reliability, fast response times,
and operation at high temperatures. They are
available in small sizes and, consequently, exhibit
comparatively low dissipation constants. They are
more costly to manufacture than metallized surface
contact type thermistors and interchangeability is
normally achieved by using matched pairs of units
connected in either series or parallel circuits.
• Bare Beads: The lead wire to ceramic contacts
are not strain relieved in bare bead type thermistors
and there is no protection against the environment.
Consequently, bare beads should be used in
applications where they are provided proper
mounting and strain relief and where the environment is relatively inert.
• Glass Coated Beads: The thin glass coating on
these thermistors can easily be ruptured during
handling or assembly operations if proper care is not
exercised. Stability is very good provided that the
glass seal remains intact.
• Ruggedized Beads: The thick glass coating
on these thermistors provides greater stability than
their glass coated counterparts. They are recommended for applications where the customer will
perform further assembly or handling.
• Miniature Glass Probes: The longer body length
of the miniature glass rod makes these thermistors
easier to handle and better suited for fluid immersion applications. The longer glass seal along the
leads also provides for a more stable device.
• Glass Probes: These are bead type thermistors
which have been sealed into the tip of a large diameter solid glass rod. Larger diameter leads of various
glass sealing alloys are welded to the platinum alloy
leadwires of the thermistor bead prior to the glass
sealing operation. The glass probe type thermistors
are generally the easiest to handle, the most durable
and the most stable of the NTC devices.
• Glass Rods: These are axial lead versions of the
glass probe type thermistor. The bead thermistor is
sealed in the center of the solid glass rod. They offer
the same ease of handling as the glass probe
thermistors. The axial leads make them better
suited for mounting on printed circuit boards.
• Bead-in-Glass Enclosures: These are bead type
thermistors which are welded to larger diameter
leads of various glass sealing alloys and then sealed
into a large hollow glass enclosure (tube or bulb)
that is usually evacuated or filled with an inert gas.
Figure 2: NTC Bead types
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the familiar outline dimensions of DO-35 glass
sealed diodes.
Disk and chip thermistors are generally larger
than the bead types and so they exhibit response
times that are comparatively slower. However, they
usually have higher dissipation constants than bead
types and are thus better able to handle power in
measurement, control or compensation applications. Disks and chips are characterized by low cost.
They easily lend themselves to resistance adjustment by means of geometry modification, thus they
are more readily available with interchangeable
characteristics. Modern production processes have
improved the overall stability and reliability of disk
and chip thermistors, although most are not yet as
good as some of the bead-type thermistors.
Figure 3: NTC Metallized surface contact type
thermistor.
Fabrication of Surface Electrode Type
Thermistors
The manufacturing methods used for thermistors
that have metallized surface contacts are very
similar to those used for ceramic capacitors. The
most commonly used types are shown in
Figure <3>.
The metallized surface contact type thermistors
can be fabricated by any of several different
methods depending upon the basic geometry of the
device. Once the desired geometry has been
obtained, the devices are sintered. The metallized
surface contact is then applied by spraying, painting,
screen printing, sputtering or dipping as required
and the contact is fired onto the ceramic body.
Some further adjustments may be made to the
geometry of the device to provide for close tolerance
on resistance or interchangeability.
Metallized surface contact type thermistors can
be obtained with axial leads, radial leads or leadless.
A variety of organic coatings are available for the
metallized surface contact types. Very small chips
are available with glass encapsulation and small
disks and chips can also be obtained in a “diode
package”, so named because the glass body meets
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Although any of the metal electrodes used for
thick-film circuit components may be used for the
metallized surface contact types of thermistors,
silver is most frequently used because of its
comparatively low cost. When small chip gaps exist
(i.e. flake and microcircuit chips), for which silver
migration and solder leaching may present
problems, gold, platinum or palladium alloys are
often used. There are also some oxide systems for
which other metals are more suitable for providing
the ohmic contact.
• Disks: Disk type thermistors are made by compressing a blend of oxide powders in a round die
using presses similar to those used for making
powdered metal parts or pharmaceutical tablets.
The “green” disks are then sintered at high
temperatures.
Electrodes are applied to the flat surfaces using
spraying or screen printing techniques. The diameter of a disk thermistor may be adjusted to a
prescribed size using a centerless grinder. In this
manner, the resistance of the disk may be adjusted
to a nominal value. Diameters of disk thermistors
range from 0.030” (0.75 mm) to 1.0” (25.4 mm).
Disk type thermistors with diameters of less than
0.4” are typically used for low cost thermometry and
temperature compensation applications, while those
with diameters greater than 0.4” are used for inrush
current limiting, surge suppression and time delay
applications.
• Chips: NTC chip thermistors are usually
fabricated using a “tape-casting” or “doctor-blading”
process in which a slurry of material is spread out
into a thick film sheet similar to the process used for
making chip capacitors or ceramic substrates. The
ceramic sheet material is then subjected to a
controlled drying cycle. In the “green” state, the
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sheet of material is reasonably flexible and easy
to handle. The dried material is then cut into slabs
or squares that are stacked on ceramic setters
and sintered at high temperatures. Metallized
electrodes are applied by using standard
spraying, screening, or dipping techniques.
The typical commercial NTC chip thermistors are
available with cross sectional areas that range from
0.010” x 0.010” (0.25 mm x 0.25 mm) up to
0.120” x 0.120” (3 mm x 3 mm) and with thicknesses that range between 0.006” (0.15 mm) and
0.030” (0.75 mm). Larger size and thicker chips
are less economical to manufacture, however, they
can be available upon special order. Chips are
frequently used for precision thermometry applications because of their smaller sizes and faster
response times when compared to disks. A typical
low cost interchangeable NTC thermistor consists of
a small chip that has been attached to leads, ground
to a precision resistance tolerance at a controlled
temperature, and then provided with an epoxy
coating for protection.
Hybrid Mount type thermistors are leadless
versions of chip type thermistors. They are intended
for mounting directly to metallized pads on hybrid
microcircuits, integrated circuits or printed circuit
boards by either soldering or conductive epoxy
bonding. A leadwire is then attached to the top
electrode surface to complete the electrical
connection.
• Surface Mounts: Surface mount type thermistors are leadless, rectangular devices which can
either be formed in a die similar to disk type
thermistors or they can be bladed and diced similar
to chip type thermistors. Electrodes are applied
such that they wrap around the edges of the device
and the body dimensions are fixed with respect to
printed circuit industry standards.
Electrical
connection to the circuit is accomplished either by
means of reflow soldering or by conductive epoxy
bonds.
• Flakes: Flake thermistors have been fabricated
using the “doctor blade” or “tape-casting” methods
as well as the standard screening techniques used
in the manufacture of thick-film capacitors and
resistors. After sintering, electrodes are applied to
the flakes using any of the standard methods for
film-type components. The low mass and high
surface-to-mass ratio provided by flakes make these
units ideally suited for passive infrared measurement.
• Rods:
Rod-type thermistors are made by extruding a mixture of oxide powders and a suitable
binder through a die. Their greater mass, longer
thermal time constants and higher dissipation
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constants makes them suitable for applications
involving temperature compensation, time delay or
surge suppression.
• Washers: Washer type thermistors are fabricated using techniques similar to those used for
disks except that a hole is formed in the center
during the pressing operation. Washers are usually
connected to circuitry by means of spring clips or
other hardware.
Comparison of NTC Thermistors to
other temperature sensors
Figure 4: Resistance-ratio vs. temperature characteristics (NTC Thermistors vs. RTD’s)
In Figure <4> the normalized resistance-temperature ratio characteristic is shown for several different NTC thermistors in comparison to that of a
commercial resistance-temperature detector (RTD).
A comparison of the curves will immediately show
inherent advantages and disadvantages for both
types of devices.
The ten-fold increase in sensitivity exhibited by
NTC thermistor makes it advantageous to use such
devices for low cost, precision temperature
measurement and control.
Another major
advantage offered by thermistors is the availability of
a wide range of relatively high resistance values. By
using high resistance thermistors, the effects of
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sensor lead resistance can be minimized. The
non-linearity of the thermistor resistance-temperature characteristics puts a practical limit on the
temperature span over which a single thermistor can
be operated in a measurement or control circuit.
RTD’s have lower sensitivity, are more linear and
can therefore be used in application where the
temperature spans are very wide.
Thermistors have other important advantages
over RTD’s in that they are available in smaller sizes,
with faster response times, at lower costs and with
greater resistance to shock and vibration effects.
NTC thermistors compare very will to thermocouples over the limited temperature ranges where both
sensors can be effectively used for temperature
measurement and control. Of course, thermocouples will operate at much higher temperatures
and over much wider spans and are available
in very fine wire diameters. However, thermocouples
have some notable disadvantages. First the
thermal EMF values produced by thermocouples
(thermoelelments) are on the order of a few
microvolts per degree. Second, the electronic
circuits used for thermocouple measurement and
control applications must provide high gain, low
noise amplification of the signal and provide
compensation for the cold junction temperature.
Third, the stability and accuracy of base metal
thermocouples can be degraded by environmental
factors and non-homogeneities. As such, NTC
thermistors provide greater sensitivity, stability and
accuracy than thermocouples and can be used with
less complex, less costly instrumentation.
NTC thermistors also have advantages over the
solid state sensors that are finding widespread use
in direct digital temperature measurement and
control applications. The solid state devices
produce an output signal that is proportional to
temperature over operationing ranges that fall within
the overall range of -55° to +150°C. The solid
state devices can be incorporated into application
specific integrated circuits for direct readout of
temperatures. They exhibit accuracy and linearity
specifications in the range of ±0.3°C (selected) up
to ±4°C over their published ranges. Packaging of
the devices can take any of the standard outline
dimensions for solid state devices.
The NTC thermistors, by comparison, offer better
sensitivity and accuracy over the operating temperature ranges, smaller sizes with faster response times
and can be obtained in a wider assortment
of device packages or sensor housings. Glass
encapsulated NTC thermistors will also perform
in much higher operating and storage temperatures
than the solid state devices.
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PROPERTIES OF NTC THERMISTORS
NTC thermistors have thermal and electrical
properties which are important considerations in
each application. These properties are a function of
the geometry of the thermistor, of the particular
“material system” of metal oxides that is being used
and of the additional materials (electrodes, inks,
solders, leadwires, etc.) that are applied to the basic
device.
These properties and other product data are
presented in the manufacturers catalogs as nominal
resistance values, resistance-vs-temperature curves
(tables), thermal time constant values, dissipation
constant values and power ratings.
Thermal Properties
When an NTC thermistor is connected in an electrical circuit, power is dissipated as heat and the
body temperature of the thermistor will rise above
the ambient temperature of its environment. The
rate at which energy is supplied must equal the rate
at which energy is lost plus the rate at which energy
is absorbed (the energy storage capacity of the
device).
dH
d HL + d HA
=
dt
dt
dt
(1)
The rate at which thermal energy is supplied to
the thermistor in an electrical circuit is equal to the
power dissipated in the thermistor.
dH
= P= I2R= EI
dt
(2)
The rate at which thermal energy is lost from the
thermistor to its surroundings is proportional to the
temperature rise of the thermistor.
dHL
= δ∆T = δ( T-TA )
dt
(3)
where: the dissipation constant ( δ ), is defined as
the ratio, at a specified ambient temperature, of a
change in the power dissipation of a thermistor to
the resultant body temperature change. The
dissipation constant depends upon the thermal
conductivity and relative motion of the medium in
which the thermistor is located, as well as the heat
transfer from the thermistor to its surroundings by
conduction through the leads, by free convection in
the medium and by radiation. The dissipation
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The rate at which thermal energy is absorbed by
the thermistor to produce a specific amount of rise
in temperature can be expressed as follows:
d HA
dt
= sm
dT
dt
=C
dT
dt
dt
= P = I2R = EI = δ(T-TA) + C
dT
dt
(5)
In order to complete our analysis of the thermal
characteristics of thermistors, we must examine the
thermistor behavior under transient and steady state
conditions. The solution of equation (5) where the
power (P) is constant is:
P
δ
[
1 – exp
{ -δC t
[{
∆T = ( T-TA ) =
(6)
Equation (6) demonstrates that when a
significant amount of power is dissipated in a
thermistor, its body temperature will rise above the
ambient temperature as a function of time. The
transient conditions at “turn on”, and all applications
that are based upon the Current-Time
Characteristics, are governed by equation (6).
A condition of equilibrium is achieved when
dT/dt=0 in equation (5) or when t>>C/δ in equation (6). In this steady state condition, the rate of
heat loss is equal to the power supplied to the
thermistor.
Therefore:
δ( T-TA ) = δ∆T = P = ET IT
(7)
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dT -δ
(T-TA)
=
dt
C
(4)
where: (s) is the specific heat and (m) is the mass
of the thermistor. The product of the specific heat
and the mass is the heat capacity (C) of the thermistor and is dependent upon thermistor materials
and construction. Thus, the heat transfer equation
for an NTC thermistor at any instant in time after
power has been applied to the circuit can be
expressed as:
dH
where: ( ET ) is the steady state or static thermistor
voltage and ( IT ) is the steady state current. The
Voltage-Current Characteristic is governed by this
equation. When the power is reduced in a thermistor to an amount where the self-heating is
considered negligible, then the heat transfer
equation can be re-written as follows:
(8)
Equation (8) is actually a mathematical
statement of Newton’s Law of Cooling and has
the following solution:
T = TA + (TI - TA) exp
{ -tτ
{
constant is not a true constant since it varies slightly
with temperature and also with temperature rise. It
is typically measured under equilibrium conditions.
(9)
where: ( TI ) is the initial body temperature, ( TA ) is
the ambient temperature and ( τ ) is the thermal
time constant of the device. Also, τ = C/δ .
The thermal time constant ( τ ) is the amount of
time required for a thermistor to reach 63.2% of the
temperature difference when subjected to a step
function change in temperature under negligible
power dissipation conditions. The thermal time
constant is dependent upon the same environmental factors as the dissipation constant, namely, the
thermal conductivity and the motion of the medium,
the conduction through the leads, the free convection in the medium and the radiation losses. The
thermal time constant and dissipation constant data
which is given in thermistor product literature must
indicate the test methods and mounting methods
employed if it is to be valuable to the designer.
Devices with leads are normally suspended by their
leads in a still medium for testing purposes.
Thus far, all of the discussions of thermal properties of NTC thermistors have been based upon a
simple device structure with a single time constant.
When any thermistor device is encapsulated into a
sensor housing, the simple exponential response
functions no longer exist. The mass of the housing
and the thermal conductivity of the materials used in
the sensor will normally increase the dissipation constant of the thermistor and will invariably increase
the thermal response time. The thermal properties
are somewhat difficult to predict by mathematical
modeling and manufacturing variances will introduce
enough uncertainty so that testing of the finished
sensor is usually required to obtain data on the
response time and dissipation constant.
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Electrical Properties
Voltage-Current Characteristic
There are three basic electrical characteristics
that account for virtually all of the applications in
which NTC thermistors may be used.
Once a self-heated thermistor has reached a
condition of equilibrium, the rate of heat loss from
the device will be equal to the power supplied. It is
mathematically expressed by equation (7).
a)
b)
c)
Current-Time Characteristic
Voltage-Current Characteristic
Resistance-Temperature Characteristic
There are also several applications where the NTC
thermistor is indirectly heated by resistive devices
or even other thermistors. These applications are
merely special cases of one of the three basic
electrical characteristics.
Current-Time Characteristic
In our analysis of the thermal properties of NTC
thermistors, we observed that a self heated thermistor exhibits a body temperature rise that is a
function of time. This is mathematically expressed
in equation (6).
A transient condition exists in a thermistor circuit
from the time at which power is first applied from a
Thevenin source, (t=0), until the time at which an
equilibrium condition is achieved, (t>>τ). Generally,
the excitation is considered to be a step function in
voltage through a Thevenin equivalent source.
During this time the current will rise from an initial
value to a final value and this current change as a
function of time is called the “Current-Time
Characteristic”.
The Current-Time Characteristic is not a simple
exponential relationship. The rate of current change
will be initially low due to the high resistance of the
thermistor and the added source resistance. As the
device begins to self-heat, the resistance will
decrease rapidly and the rate of current change will
increase. Finally, as the device approaches an
equilibrium condition, the rate of current change will
decrease as the current reaches its final value.
The factors which affect the Current-Time
Characteristic are the heat capacity of the device
(C), the dissipation constant of the device ( δ ), the
source voltage, the source resistance and the
resistance of the device at a specified ambient
temperature. The initial and final current values and
the time required to reach the final current value can
be altered as needed by proper circuit design.
The Current-Time Characteristic is used in time
delay, surge suppression, filament protection,
overload protection and sequential switching
applications.
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If the dissipation constant variations are negligible
for a specified medium and set of conditions, and
the resistance-temperature characteristic is known,
then equation (7) can be solved for the static
voltage-current characteristic. This characteristic
can be plotted on log-log coordinates where lines of
constant resistance have a slope of +1 and lines of
constant power have a slope of -1 such as shown in
Figure <5>. For some applications it is more convenient to plot the static voltage-current
characteristic on linear coordinates such as
shown in Figure <6>.
Figure 5: Typical Voltage-Current Characteristic
(log-log scale)
Figure 6: Typical Voltage-Current Characteristic
(linear scale)
When the amount of power dissipated in the
thermistor is negligible, the voltage-current characteristic will be tangential to a line of constant
resistance that is equal to the zero-power resistance
of the device at the specified ambient temperature.
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As the current continues to be increased, the
effects of self-heating become more evident and the
temperature of the thermistor rises with a resultant
decrease in its resistance. For each subsequent
incremental increase in current there is a
corresponding decrease in resistance. Hence, the
slope of the voltage-current characteristic ( ∆E/∆I )
decreases with increasing current. This continues
until a current value ( IP ) is reached for which the
slope becomes zero and the voltage reaches a
maximum value ( EP ). As the current is increased
above the value of ( I P ), the slope of the characteristic continues to decrease and the thermistor
exhibits a negative resistance characteristic.
A maximum power rating as well as power
derating curve is usually given for each thermistor
type. Care should be exercised when designing a
circuit for a self heated application so that the
thermistor is operated within the maximum power
limitations.
There are many applications which are based
upon the static voltage-current characteristic. These
applications can be grouped according to the
type of excitation which is employed to vary the
voltage-current characteristic.
The first major group involves applications where
the dissipation constant is varied. This can be
accomplished by changing the thermal conductivity
of the medium, the relative motion of the medium or
the heat transfer from the thermistor to its
surroundings. Typical applications would include
vacuum manometers, anemometers, flow meters,
liquid level, fluid velocity, thermal conductivity cells,
gas chromatography and gas analysis.
Resistance-Temperature Characteristic
There are many applications based upon the
resistance-temperature characteristic and they can
be grouped into the general categories of resistance
thermometry, temperature control or temperature
compensation. In the previous discussions of the
current-time and voltage-current characteristics, we
examined devices that were operated in a
self-heated mode (heated above the ambient
temperature by the power being dissipated in the
thermistor). For most applications based on the
R-T characteristic the self-heating effect is undesirable and one attempts to work with as close to
zero-power as possible.
The zero-power resistance of a thermistor ( RT ) at
a specified temperature ( T ) is the DC resistance
measured when the power dissipation is negligible.
By definition in MIL-PRF-23648, the power is
considered to be negligible when “any further
decrease in power will result in not more than 0.1
percent (or 1/10 of the specified measurement
tolerance, whichever is smaller) change in
resistance”.
There are two models presently used to explain
the electrical conduction mechanism for NTC
thermistors. One explanation involves the so called
“hopping” model and the other explanation is based
upon the “energy band” model. Both conduction
models have difficulty when it comes to a complete
explanation of the R-T characteristics of metal oxide
thermistors. Fortunately, there are a number of
equations that can be used to define the resistancetemperature characteristic of the devices.
The second major group involves applications
where the electrical parameters of the circuit are
varied. This would involve a change in the Thevenin
source voltage or source resistance. Typical
applications would include automatic gain or
amplitude control, voltage regulation, equalization,
volume limiters, signal compression or expansion
and switching devices.
The third and fourth major groups involve
applications where the ambient temperature is
varied. In one case the change is thermal, while in
the other case the change is due to radiation
absorbed by the thermistor. Temperature control
and alarm indication are examples of applications
where the change is thermal. Microwave power
measurement is an example of an application where
the change is due to absorbed radiation.
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Figure 7: R-T Characteristics
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The R-T characteristic of semiconductors is
very often plotted with the logarithm of specific
resistance expressed as a function of the reciprocal
of absolute temperature. In Figure <7>, the R-T
characteristics of three commonly used thermistor
materials are shown in terms of their specified resistances and inverse absolute temperatures. The
resistance-ratio versus temperature characteristics
for these materials are specified in MIL-PRF-23648.
It can be demonstrated that, over any specified
temperature range for which the slope of a given
material system curve may be considered to be
constant (straight line relationship between In RT
and 1/T), the resistance of the device at any temperature within the specified range may be
expressed as:
[
[
RT = RTO exp
β (TO-T)
TTO
(10)
where: ( RT ) is the resistance at an absolute
temperature
(T)
expressed
in
kelvins
(°C + 273.15); (β) is the “beta” or “material
constant” is the slope of the thermistor R-T characteristic (in kelvins) over the specified temperature
range; and, RT0 is the resistance at a specified
reference temperature, T0, that is also expressed
in kelvins.
Equations (10) appears most frequently in NTC
thermistor literature. Thermistor manufacturers will
provide “beta” information for each of the material
systems they offer. Temperature spans of 0 to
50°C, 25 to 85°C, 25 to 125°C and 100 to 200°F
are most common, however, any two data points
can be used for solution. The terms equation (10)
can be rearranged to solve for beta or temperature:
TTO
TO-T
1n
[
RT
RTO
[
β=
By solving equation (10) for “alpha” ( α ) we obtain:
α=–
[ [
t(°C) =
[ [
-1
-273.15
Temp. span
(°C) 10
20
1
RT
[
[
d RT
dT
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30
40
50
Uncertainty
(°C) 0.01 0.04 0.10 0.20 0.30
The more recent literature on thermistors
accounts for the non-linearity of In RT vs 1/T
characteristic by using the standard curve fitting
technique of considering In RT to be a polynomial in
1/T or vice versa.
Excellent results have been obtained with the use
of the following third order polynomials:
1
= a + b[lnRT] + c[lnRT]2 + d[lnRT]3
T
(15)
lnRT = A0 +
B +C +D
T2
T
T3
(16)
1
= b0 + b1[lnRT]2 + b3[lnRT]3
T
(17)
(11)
(12)
Also, the temperature coefficient of resistance
or “alpha” ( α ) of an NTC thermistor is defined as:
α=
(14)
Equations (10) through (14) cited above are valid
only over small temperature spans for which the
slope of the InRT vs 1/T characteristic approximates
a straight line. At temperatures above 0°C, the
uncertainties associated with the use of these
equations are approximately as follows:
lnRT = B0 +
1
RT
1
1n
+
β
RTO
TO
β
T2
(13)
B1 + B3
T
T3
(18)
The use of equation (15) was originally proposed
by Steinhart and Hart for the oceanographic range
of -2 to +30°C. They also indicated that there was
no significant loss in accuracy when the squared
term, a2 { In RT }2 , was eliminated as in equation
(17).
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Because equations (15) and (16) each have four
unknown constants, a minimum of four calibration
data points are required in order to determine the
constants. The constants may be obtained from the
solution of four simultaneous equations if only four
data points are given, or, they may be obtained by
polynomial regression analysis when more than four
points are given. Such an analysis statistically
improves the accuracy of the data.
Table 1: Summary of curve fitting errors
A)
Full third degree polynomials, such as equations (15) and (16), do not introduce interpolation errors that exceed the total measurement uncertainty (typically 0.005°C
to 0.010°C) for:
a1)
100°C spans within the overall range of -80
to +260°C.
a2)
150°C spans within the overall range of -60
to +260°C.
a3)
B)
If we define the material constant ( β ) as the
slope of the In RT vs 1/T characteristic, then from
equation (16) we can derive the following:
β=B+
b1)
.001 to .003°C error for 50 °C spans within
the overall range of 0°C to +260°C.
b2)
.010 to .020°C error for 50°C spans within
the overall range of -80 to 0°C.
b3)
.010°C error for 100°C spans within the overall range of 0 to +260°C.
b4)
.020 to .030°C error for 100°C spans within
the overall range of -80 to +25°C.
b5)
.015°C error for 150°C span (+50 to+200°C).
b6)
.045°C error for 150°C span (0 to +150°C).
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2C
3D
+
T
T3
(19)
The temperature coefficient of resistance (α)
defined by equation (14) may also be obtained from
equation (16) as:
α=-
[
B
2C
3D
+
+
2
3
T
T
T4
(20)
Equations (17) and (18) both make use of three
constants. Consequently, they require only three
calibration data points and the solution of three
simultaneous equations to determine the values of
the unknown constants. When using these
equations, the material constant (β) and the
temperature coefficient of resistance ( α ) may be
expressed as follows:
β = B1 +
150 to 200°C spans within the overall range
of 0 to +260°C except that the interpolation
error begins to approximate the measurement
uncertainties.
Third degree polynomials with the squared
term eliminated, such as equations (17) and
(18), introduce interpolation errors that do
not exceed the following conditions:
.100°C error for 150°C span (-60 to +90°C).
.080°C error for 200°C span (0 to +200°C).
α=-
[
3B3
T3
B1
3B3
+
2
T
T4
(21)
[
The results of the investigations conducted at
Thermometrics, Inc. indicate excellent curve fit using
third degree polynomials and are summarized in the
Table <1>.
b7)
b8)
[
The work of Steinhart and Hart was confirmed by
studies conducted by B.W. Mangum at NBS and R.
Koehler at Woods Hole Oceanographic Institute.
However, they found that greater accuracy
is obtained when the squared term is retained.
(22)
Although characteristic curves of In RT vs 1/T are
useful for deriving interpolation equations, it is more
common for manufacturers to provide nominal
thermistor resistance values at a standard reference
temperature (usually specified as 25°C) as well as
resistance-ratio vs temperature characteristics.
SPECIFYING NTC THERMISTORS
To the design engineer attempting to specify, or,
to the purchasing agent attempting to procure, the
task of choosing the correct NTC thermistor may
sometimes seems to be an impossible task. While
the process can be difficult at times because of
subtleties in the use of each product type, it is not
nearly impossible if one has a good understanding of
the basics.
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Product Type and Size:
Resistance Tolerance:
Usually, the designer or user will have a good idea
as to the device size, thermal response time or other
physical characteristic that they desire in the
thermistor. Even if there is only limited information
available, it is usually enough to “rule out” whole
families of NTC product types because they will be
too far from the desired characteristics. A careful
consideration of what the thermistor is intended to
do in the application will also provide clues as to
which products are inappropriate.
The standard tolerances available for each
thermistor type are given on the specific product
data sheet. Typically, the bead type thermistors will
have a distribution (3 sigma) of the zero-power
resistance at the reference temperature of about
± 20% to ± 25% depending upon sizes. The
metallized surface contact type thermistors have
typical resistance distributions of about ± 5% to
± 10%, except for flake type thermistors where
distribution may be ± 20% or greater. Specifying
the broadest possible tolerance for the application
will provide the most cost effective solution. Of
course, tighter zero-power resistance tolerances are
available for all types of thermistors, however, the
lower expected yield from the production lots will
translate into higher cost. In this regard, the
metallized surface contact type thermistors have an
advantage since their shapes can easily be adjusted
or trimmed to provide closer tolerances at
lower costs.
Resistance-Temperature Curves:
Most NTC thermistor manufacturers provide
tables of either resistance or resistance-ratio versus
temperature for each of the material systems that
they offer in their respective product lines. Often the
manufacturer will also provide the coefficients for
the various thermistor equations in order to assist
the designer or user to interpolate the R-T data.
There are a great many material systems in use and
each one has certain limitations with respect to the
type of thermistor that can be manufactured, the
size of the thermistor, temperature ranges for operation and storage, as well as the range of available
nominal resistance values.
Nominal Resistance Value:
The next common starting point when specifying
a thermistor is to choose the nominal resistance
at a specified temperature. As previously
mentioned, manufacturers will present a range of
available resistance values for each NTC product
type and its associated material systems. The usual
reference temperature is 25°C, however, many other
reference temperatures can be specified. If the
desired resistance value is not available for that
combination of product type and material system,
then the user must determined which is more important: the product type and size, or, the
material system with its defined resistance-ratio
versus temperature data. The user can not specify
all three parameters (nominal resistance, product
type/size, material system) if the combination falls
outside of the manufacturers guidelines.
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Beta Tolerance:
The beta of a thermistor is determined by the
composition and structure of the various metal
oxides being used in the device. The beta can also
be influenced by some manufacturing process
variables. The result will be a variation from unit to
unit within a production lot as well as from lot to lot.
For bead type thermistors, beta tolerances are
usually on the order of ± 1% to ± 3% (up to ±
5% is possible for some material systems). For the
metallized surface contact type thermistors, beta
tolerances will range from ±0.5% up to ± 3%.
Resistance Limits:
The maximum and minimum resistance values at
the reference temperature are fixed by the specified
tolerance. As the temperature is changed from the
reference temperature, however, the maximum and
minimum limits (as percentage of the nominal
resistance) will increase due to the effects of the
tolerance on the material constant, beta (β).
If the temperature span is small enough so that
the beta can be considered constant, then equation
(10) can be used to solve for the minimum and
maximum resistance values. The equation is solved
for all the possible combinations of high and
low resistance as well as high and low beta. A typical plot of the resulting R-T data is shown in
figure <8>.
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Curve Matching and Interchangeability:
For some applications the operating temperature
range is too wide to permit the proper use of
equation (10). In such cases, or in cases where it
is desirable to have a uniform tolerance over the
range, the resistance limits are specified for the
minimum and the maximum operating temperatures. This two point constraint (actually a limitation
on the beta) requires that devices be selected to
closely track a given resistance-temperature or
ratio-temperature characteristic. After the thermistors are manufactured they would be limit tested at
the minimum and maximum operating temperatures. The cost of such “curve matching” will
obviously depend upon the resistance tolerance
and ratio tolerance desired as well as the operating
range.
Figure 8: Effect of beta tolerance on Resistance
Limits.
The effect of beta tolerance on the resistance
limits must be considered in any thermistor
application. Therefore, in many high or low
temperature applications it is desirable to specify the
nominal resistance at the operating temperature or
at a convenient mid point in the operating
temperature range. In the example for Figure <8>
we are given:
R NOMINAL = RTO @ T0 = 10000 Ohms @ 25°C
Beta (0/50) = 3450 kelvins
x = resistance tolerance = ± 10% = ± 0.1
y = beta tolerance = ± 5% = ± 0.05
TL = 15°C
and TH = 35°C
RTLH = 16769.3 Ohms
RTHH= 7699.5 Ohms
RTLN = 14941.7 Ohms
RTHN= 6869.4 Ohms
RTLL = 13180.3 Ohms
RTHL= 6067.5 Ohms
Note that in this example the zero-power
resistance tolerance at 15°C is +12.23%, -11.79%;
while it is +12.08%, -11.67% at 35°C. This compares to the nominal tolerance of ±10% at 25°C.
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Thermistors which have very close resistance
tolerances and which can be readily substituted
without the need for circuit adjustments and
recalibration are called “Interchangeables”. Very
often these devices have curve tolerances which are
expressed as a temperature uncertainty over the
operating temperature range rather than as a
resistance uncertainty. When the curve tolerance is
so expressed, the resistance limits at the low
temperature point will be slightly greater than the
resistance limits at the high temperature point. This
is due to the greater values for the temperature
coefficient of resistance (alpha) at lower
temperatures. It follows then, that when curve
tolerances are given as a resistance uncertainty, the
temperature uncertainty will be lesser at the low
temperature point and greater at the high
temperature point.
Metallized surface contact type thermistors have
generally better control on the beta variations and
can be readily adjusted by grinding away material to
obtain a specified resistance with close tolerances.
As such, these devices are most commonly used for
applications requiring interchangeability, reasonable
stability, low to moderate cost and where operational
temperatures do not exceed 105°C.
For applications that involve operational or
storage temperatures in excess of 105°C, thermistors that have been hermetically sealed into glass
and can withstand exposure to temperatures up to
250°C, however, their interchangeable range is
generally limited to 105°C. When applications
require interchangeability as well as operational and
storage temperatures of up to 300°C, then matched
pairs of hermetically sealed bead type thermistors
are recommended.
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Calibration
The beta and resistance tolerances for bead type
thermistors are generally too broad to permit effective and economic use of a single thermistor in such
high temperature, interchangeable applications —unless the temperature span is very small. The
usual approach to a solution involves the calibration
of the bead type thermistors at two or more temperature points over the operating range. The data
is entered into computer files for sorting and
analysis. The beads are then matched such that
high and low beta values offset each other. Thus,
when the beads are connected in either a series or
parallel pair, they will behave as a single
interchangeable device and track a defined resistance-temperature characteristic.
Some applications have accuracy requirements
which are tighter than the conventional limits on
interchangeable devices. For these applications the
thermistors must be calibrated. To use one of the
interpolation equations over a specified range, the
thermistor must be calibrated at two or more
temperatures. [Reference equations (10), (15),
(16), (17) and (18)].
The accuracy of the computer R-T characteristic
over the temperature range depends upon the
proper selection of equation and reference temperatures as well as upon the calibration uncertainties.
The resistance tolerance, beta tolerance or
interchangeability of the thermistor will have no
influence upon the accuracy of the calibrations.
Thus, low cost, broad tolerance thermistors
(with suitable stability) can be purchased with precision calibrations.
The calibration schedules available for thermistors or assemblies manufactured by Thermometrics
are listed in Table IV.
Such matching of bead type thermistors, (beads,
probes or rods) is more costly than precision
grinding of metallized surface contact type thermistors. However, it may be the only acceptable
solution for applications where small sizes, continuous operation at high temperatures or high reliability
are required.
Practical limits for interchangeability are
determined by the ability to control the tolerances on
resistance and beta over the desired temperature
range. Some practical limits for interchangeable
thermistors that are both cost effective and
commercially available are listed in Table <2>.
Obviously, not all thermistors or assemblies can
be calibrated at all temperatures over the range.
There will be limitations which are imposed by the
type of thermistor and its nominal resistance as well
as by the materials used in the construction of the
assembly. The basic calibration schedules and the
types of thermistor to which they apply are
described as follows. When a current source and
digital voltmeter are used for calibration, suitable
averaging and integration techniques are used to
eliminate
noise spikes. Thermal EMF’s are
eliminated by either subtracting the zero current
readings or averaging forward and reverse polarity
readings.
Table 2: Practical limits for interchangeability.
All NTC thermistor types with an overall operational
range of -80°C to 105°C:
• ±0.05°C for spans of up to 50°C
• ±0.10°C for spans of up to 75°C
• ±0.20°C for spans of up to 100°C
ALL glass sealed NTC thermistor types with
an overall operations range of -80°C to +300°C
• ±0.50°C over spans of up to 200°C
Temperature Accuracy (±ºC) for ranges shown
TABLE IV
Calibration
Schedule
Resistance
Accuracy
1
1A
1B
2
3
3M
4
5
0.005%
0.005%
0.005%
0.01%
0.01%
0.05%
0.05%
0.1%
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-140ºC
-80ºC
0.005ºC
0.005ºC
0.010ºC
0.010ºC
0.050ºC
0.050ºC
-80ºC
0ºC
0.003ºC
0.005ºC
0.005ºC
0.010ºC
0.010ºC
0.050ºC
0.050ºC
0ºC
+60ºC
+125ºC
+60ºC
+125ºC
+260ºC
0.001ºC
0.003ºC
0.005ºC
0.005ºC
0.010ºC
0.010ºC
0.050ºC
0.050ºC
0.003ºC
0.005ºC
0.005ºC
0.010ºC
0.010ºC
0.050ºC
0.050ºC
0.005ºC
0.005ºC
0.010ºC
0.010ºC
0.050ºC
0.050ºC
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SCHEDULE 1:
SCHEDULE 3M or 4:
Available only for thermistor standards, ultrastable thermistor probes or assemblies which
incorporate these devices. Calibrations are made
in an integrating block submerged in a precision
constant temperature bath. The bath and block
temperature is established using an SPRT, resistor
standards and a four wire comparison bridge.
Resistance measurements of the thermistors are
made using a precision Wheatstone bridge or a
stable, precision current source and digital voltmeter in conjunction with a data acquisition system verified against standard resistors and an
ohmic standard precision resistance decade.
Available for all thermistors and sensor assemblies. A constant temperature bath is set using two
or more thermistor standards. Resistance measurements are performed using a calibrated
Wheatstone bridge, digital meter or data acquisition
system.
SCHEDULE 1A and 1B:
Available only for thermistor standards, ultrastable thermistor probes or assemblies which incorporate these devices. The bath and block temperature is established using two or more thermistor
temperature standards which have been calibrated
against an SPRT. Resistance measurements are
performed the same as for Schedule 1.
SCHEDULE 2:
Available for all glass probe thermistors or assemblies which incorporate these devices. Stability
requirements with respect to temperature range and
time span must be verified prior to calibration. The
bath and block temperature is established using two
or more thermistor temperature standards which
have been calibrated against an SPRT. Resistance
measurements are performed using a precision
Wheatstone bridge or a stable, precision current
source and digital voltmeter in conjunction with a
data acquisition system verified against standard
resistors and an ohmic standard precision resistance decade.
SCHEDULE 3:
Available for all glass enclosed beads and probes
as well as epoxy encapsulated discs or chips and
sensor assemblies using these devices. It is advised
that stability required be verified prior to calibration.
A precision constant temperature bath is set using
two or more thermistor temperature standards.
Resistance measurements are performed using a
precision Wheatstone bridge or a stable precision
current source and digital voltmeter in conjunction
with a data acquisition system verified against standard resistors and an ohmic standard precision
resistance decade.
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SCHEDULE 5:
Available for all thermistors and sensor assemblies. A constant temperature bath is set using two
or more thermistor standards. Resistance measurements are performed using a digital meter.
In addition to the calibration services described
above, Thermometrics, Inc. can provide constants
and/or computer generated tables of resistance versus temperature, for any of the equations given in
the previous discussions.
Traceability
Temperature measurements at Thermometrics,
Inc. are traceable to the International Temperature
Scale of 1990 (ITS-90) as maintained by the
National Institute of Standards and Technology
(NIST). ITS-90 became the official international
temperature scale as of January 1, 1990. It supersedes the International Practical Temperature Scale
of 1968, amended edition of 1975 [IPTS-68(75)].
The new international temperature scale was
designed such that temperatures on this scale are
in much better agreement with thermodynamic
temperatures than the previous international temperature scales.
Traceability is achieved by means of triple point of
water cells for the defining fixed point of 0.01ºC
(273.16K) and through the use of standard platinum resistance thermometers (SPRT’s) which are
calibrated at other fixed points at NIST. Resistance
Traceability is achieved by means of standard resistors calibrated by NIST.
In the text of ITS-90, the standard platinum resistance thermometer is specified as the standard
interpolation instrument for realizing the scale
between the defining fixed points. The calibration
services offered by Thermometrics cover the range
of -140ºC to +260ºC.
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The defining fixed points of ITS-90 with respect to
this range are:
Defined Fixed Point
Ar (Argon) triple point
Hg (Mercury) triple point
H2O (Water) triple point
Ga (Gallium) melting point
In (Indium) freezing point
Sn (Tin) freezing point
Zn (Zinc) freezing point
T90 (K)
t90 (ºC)
83.8058 -189.3442
234.3156
-38.8344
273.16
0.01
302.9146
29.7646
429.7485 156.5985
505.078
231.928
692.677
419.527
Upon request, for a nominal fee, documentation for traceability to the National Institute of
Standards and Technology can be furnished for all
calibrations performed at Thermometrics.
Testing and Calibration of Thermistors
The very factors which make thermistors more
useful than other temperature detectors, namely
small size, fast response, and high sensitivity, also
present problems in their testing and calibration.
In general, thermistors and assemblies should be
calibrated in a well-circulated temperature-controlled liquid bath. The bath liquid should be chosen
to provide low electrical conductivity, low viscosity
and high thermal conductivity. The liquid volume of
the bath should be at least 1000 times the volume
of the test fixture and assemblies that are placed in
the bath. The heat capacity of the bath should be
high enough so that the bath temperature is not
changed significantly by the immersion of the thermistor fixture and assembly.
Thermistors are specified in terms of nominal
resistance values at one or more discrete temperatures with stated tolerances on both the resistance
and temperature. To properly evaluate whether or
not a thermistor meets its specifications, it is necessary that the testing facility either pull their limits
in (for manufacturers) or open their limits (for users)
by the total measurement uncertainty. The total
uncertainty includes both the temperature measurement uncertainty and the resistance measurement uncertainty.
Temperature Measurement Uncertainties
The factors which affect the temperature measurement uncertainty are:
Temperature Control of the Testing Medium
Accuracy and Precision of the
Temperature Monitor
The temperature monitor can consist of any of
the following units. The accuracy and stability of
each unit is an important consideration. The reference works cited contain more detailed information
regarding each type of temperature monitor.
1) Standard platinum resistance thermometer
(SPRT). These standards have an uncertainty
of about 0.001ºC within the temperature
ranges over which most thermistors are operated. [ Reference: J.L. Riddle, G.T.
Furukawa, and H.H. Plumb, “Platinum
Resistance Thermometry”, National Bureau of
Standards Monograph 126 (April 1972),
Available from U.S. Government Printing
Office, Washington, DC, Stock No. 030301052, B. W. Mangum, “Platinum Resistance
Thermometer Calibrations”, NBS Special
Publication 250-22, 1987, B.W. Mangum and
G.T. Furukawa, “Guidelines for Realizing the
Internation Temperature Scale of 1990
(ITS-90)”, NIST Technical Note 1265, 1990.]
2) Thermistor temperature standards. Thermistor
standards are available from Thermometrics,
Inc. with accuracies that vary between
0.001ºC and 0.01ºC. [ Reference: Product
Data Section of this Catalog; Type “S”, “AS”,
“ES” and “CSP”. ]
3) Precision mercury-in-glass thermometers for
0ºC and the range of 24ºC to 38ºC. The
maximum uncertainty associated with these
thermometers is 0.03ºC. [ Reference:
Mangum, B.W., and J.A. Wise, Standard
Reference Materials: Description and Use of
Precision Thermometers for the Clinical
Laboratory, SRM 933 and SRM 934, NBS
Special Publication 260-48 (May 1974),
Available from US Government Printing Office,
Washington, DC, SD Catalog No.
C13.10:260-48. ]
4) Resistance Temperature Detectors (RTD’s).
Platinum RTD’s for commercial and industrial
use (not SPRT’s) are available with accuracies
of ±0.1ºC or ±0.25ºC. The stability and
repeatability of some of the better RTD’s on
the market makes them suitable for calibration
of individual units to within ±0.01ºC. Such
units should be checked for stability and
hysteresis prior to calibration.
This can vary between ±0.001ºC for a sophisticated precision laboratory bath to as much as
±3ºC for bench top testing.
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5) Liquid-in-glass thermometers. Accuracies
attainable with liquid-in-glass thermometers for
various graduation intervals have been published by ASTM. Although accuracies in the
range of ±0.01ºC to ±0.03ºC are shown for
totally immersed thermometers under some
conditions, the practical realization of accuracies better than ±0.03ºC is very difficult to
achieve. Typically, the accuracy of liquid-inglass thermometers ranges between ±0.1ºC
and ±0.5ºC.
6) Thermocouples. A joint ANSI / ASTM specification lists the limits of error for thermocouples. The errors for various thermocouple
types and temperature ranges are expressed
in terms of a temperature accuracy or a per
cent of reading, whichever is greater. As a
result, the best accuracies one can expect
from a thermocouple (special limits of error)
are on the order of ±0.5ºC to ±1.1ºC.
Typically, the best accuracies obtained are on
the order of 1.0ºC to 2.2ºC. [Reference:
Standard Temperature-Electromotive Force
(EMF) Tables for Thermocouples, ANSI/ASTM
E220-77, ASTM Standards on Thermocouples,
#06-5200077-40, p. 24, American Society for
Testing and Materials (January 1978).]
7) Quartz thermometers. A quartz thermometer
is also available which has a reported accuracy
of ±0.04ºC from -50ºC to +150ºC and
±0.075ºC from -80ºC to +250ºC.
[Reference: Quartz Thermometer, Model
2804A, Test and Measurement Catalog,
p. 427, Hewlett Packard Company (1991).]
Temperature Gradients Within the Medium
Of particular importance are temperature gradients between the thermistor under test and the
temperature monitor (sensor). Such gradients can
be minimized by using a well-stirred liquid bath. The
thermal conductivity, and dielectric constant of the
liquid should be high and its viscosity should be low.
Immersion Errors or Stem Effects for the
Temperature Monitor
Generally, there is a heat-transfer path between
the actual sensing element and the surrounding
ambient environment which normally is at a different temperature than the calibration temperature.
This can result in a monitor temperature that differs
from the calibration medium by some factor. Unless
the monitor has been calibrated for partial immersion, total immersion is recommended.
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Self-heating Effects
For sensors, such as RTD’s and thermistors,
which require some power to be dissipated in the
sensor during measurement, self-heating effects
in the monitor must be considered.
Equipment Uncertainties
The uncertainties associated with any auxiliary
equipment required for reading the monitor adds to
the temperature uncertainty. For example, platinum
SPRT’s and RTD’s have a temperature coefficient
of resistance of about 0.4%/ºC. To realize a
temperature uncertainty of 0.001ºC with an SPRT, a
precision 4-wire bridge is required which is capable
of accurately resolving better than 1 PPM (preferably
1 part in 107 ). For such measurements, a ratio
bridge is frequently used which compares the reading to a 4-terminal standard resistor. To realize a
0.01ºC measurement with a platinum RTD, the
resistance measuring equipment must have an
accuracy which is better than 0.004% and preferably 0.001%. With a thermistor standard, a 0.01ºC
measurement requires an instrument having an
accuracy of better than 0.04% and preferably at least 0.01%.
Thermal Response of Monitor
The difference between the response time of the
monitor and the thermistor under test is a major
consideration. This is particularly true when an
attempt is made to calibrate a fast response
thermistor such as a small bead. The thermal mass
of the monitor has the property of integrating the
temperature fluctuations of the test medium. It is
possible for the monitor to indicate that there is only
a slight fluctuation in the temperature of the
medium while, in fact, the thermistor under test
could be experiencing very large temperature fluctuations. To minimize this problem, the thermistor can
be mass-loaded, or, a heat sink can be affixed to
both the monitor and the unit under test. A thermal
integrating block is often used for this purpose.
Heat Capacity of the Medium
The heat capacity of the test medium must be
sufficiently large compared with that of the thermistor or thermistor assembly and its associated fixture
so that the temperature of the test medium is not
changed when they are immersed. When this is not
the case, enough time must be permitted to elapse
for the total system to reach an equilibrium
condition, after immersion.
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Resistance Measurement
Uncertainties
The factors which affect the resistance measurement uncertainty are:
Resistance Measuring Equipment
The accuracy, precision, stability, and temperature coefficient of the resistance measuring
equipment must be considered.
Self-heating Error
The ratio of the power dissipated in the thermistor (by the test equipment) to the dissipation
constant of the thermistor in the test medium
determines the self-heat error.
Stability of the Thermistor Under Test
If a thermistor which has not had sufficient
stability conditioning is calibrated at a low temperature and is subse-quently calibrated at an elevated
temperature, the high temperature exposure may
create a shift in resistance which gives the appearance of a hysteresis effect. This may also occur if
the thermistor is calibrated at a temperature above
its maximum rated temperature.
Thermistor and Circuit Lead Resistance
In general, when the desired accuracy of the
measurement requires a resistance resolution that
is less than 1 Ohm, or when the absolute resistance
of the thermistor at a specified temperature will be
less than 1000 Ohms, the user must consider the
following:
1. The contact resistance between the
instrument and its measuring leads.
2. The contact resistance between the
instrument leads and the fixture
used to hold the thermistor.
3. The temperature coefficient of the
instrument and measuring leads.
4. The resistance of the thermistor leads
and the ability to make contact on the
thermistor leads at exactly the same
point each time a measurement is
made. This is important when attempt
ing to make measurements on thermistors with fine gauge platinum alloy lead
wires such as small beads.
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Thermal EMF Effects
Thermal EMF’s can result at the instrument terminals, the connections between the instrument
leads and the thermistor fixture, the connection
between the fixture and the thermistor leads, and
electrical connections between dissimilar metals
within the fixture itself.
QUALITY ASSURANCE
Thermometrics maintains a system which complies with MIL-Q-9858. All of our bead and chip
thermistors are designed to comply with
MIL-PRF-23648. Every Thermometrics thermistor
receives 100% electrical inspection as well as
100% visual and mechanical inspection. This is verified by additional QA sampling which varies with the
application requirements (typically 0.65 AQL or 1.0
AQL). Our Applications Engineering Department will
gladly provide assistance in selecting the best qualification and acceptance testing programs for any
specific application. We can provide a low cost
screening test or sophisticated qualification testing
depending upon your requirements.
APPLICATIONS
The thermistor is a versatile component that can
be used in a wide variety of applications where the
measurand is temperature dependent.
Table I gives a partial listing of thermistor
applications which are grouped according to one of
the three fundamental electrical characteristics:
the resistance-temperature characteristic, the
voltage-current characteristic, and the currenttime characteristic. The current-time and voltagecurrent characteristics are associated with
self-heated thermistors. The resistance-temperature characteristic is applicable to thermistors
operated with negligible self-heat.
Applications which depend on the resistancetemperature characteristic include temperature
measurement, control, and compensation. Also
included are those applications for which the
temperature of the thermistor is related to some
other physical phenomena. An example would be
the use of thermistor type cardiac catheters for
thermodilution studies. With this type of device, a
saline or dextrose solution, having a known
volume and temperature, is injected into the
blood stream through one of the catheter lumens.
The solution mixes with the blood and is diluted as
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it is carried downstream past a thermistor located
at the surface of another catheter lumen. At the
thermistor location, the temperature of the bloodinjectate mixture is measured over a period of
time. The cardiac output (efficiency) is computed
from the temperature-time response data.
Another example is an hypsometer, an instrument
in which the temperature of a boiling fluid is used
to determine the pressure to which the fluid is
exposed.
Applications based on the voltage-current characteristic of a thermistor generally involve
changes in the environmental conditions or circuit
variations which, in turn, result in changes in the
operating point on any given curve or family of
curves.
The current-time characteristic of a thermistor
depends on its heat capacity and dissipation
constant as well as the circuit in which it is used.
Applications which make use of the currenttime characteristic include time delay and surge
suppression.
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NTC THERMISTOR APPLICATIONS
The NTC thermistor is a versatile component that
can be used in a wide variety of applications where
the measured is temperature dependent. Table
<3> gives a partial listing of thermistor applications
that are grouped according to one of the three
fundamental electrical characteristics; the
current-time characteristics, the voltage-current
characteristic, and the resistance-temperature
characteristic.
Applications based on the voltage-current characteristic generally involve changes in the environmental conditions or the electrical circuit parameters of a self-heated thermistor. In turn, these
changes will result in a shift of the operating point
on any given voltage-current curve or family of such
curves. These applications are further subdivided
into four major categories depending on the type of
excitation that causes the operating point to
change.
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Table 3: NTC Thermistor Applications.
APPLICATIONS BASED ON R-vs-T CHARACTERISTIC:
Automotive and Transportation Applications
Emission controls
Differential temperature controls
Fire protection and safety equipment
Engine temperatures
Aircraft temperatures
Rotor/bearing temperatures
General Industrial Applications
Industrial process controls
Plastic laminating equipment
Hot glue dispensing equipment
Auto & truck tire curing
Fiber processing & manufacturing
Pyrometers (non-contact)
Photographic processing
Copy machines
Soldering irons (controlled)
Hot mold equipment (thermoplastics)
Solar energy equipment
Laboratory and Scientific Applications
Temperature standards
Chemical analysis
Oceanographic research
Meteorology
Bathythermography
Calorimetry
Titration studies
Geological temperature studies
Spectrophotometry
Bolometry
Osmometers
Consumer / Household Appliances
Thermostats
Small appliance controls
Burglar alarm detectors
Oven temperature control
Refrigeration and air conditioning
Fire detection
Medical Applications
Fever thermometers
Dialysis equipment
Rectal temperature monitoring
Myocardial probes
Esophageal tubes
Skin and muscle temperature
Thermodilution catheters
Respiration rate measurement
Blood analysis equipment
Respirators
Hypodermic needle probes
Fluid temperature
Food Handling Applications
Fast food processing
Perishable shipping
Oven temperature control
Food storage
Coffee makers
Freezing point studies
Instrumentation Applications
Motor winding compensation
Infrared sensing compensation
Instrument winding compensation
Communications Applications
Transistor temperature compensation
Gain stabilization
Piezoelectric temperature compensation
High Reliability Applications
Missiles & spacecraft temperatures
Aircraft temperature
Submarines & underwater monitoring
Fire control equipment
APPLICATIONS BASED ON E-vs-I CHARACTERISTIC
Group 1 = change in Dissipation Constant
Vacuum manometers
Anemometers
Flow meter
Liquid level control / alarm
Fluid velocity
Thermal conductivity analysis
Gas detection / analysis
Group 2 = change in Electrical Parameters
Automatic gain / level control
Amplitude control
Voltage regulation
Volume limiting
Signal expansion / compression
Switching devices
Group 3 = change in Ambient Temperature
Temperature control
Alarm indication
Group 4 = change in Radiation Absorbed
RF / Microwave power measurement
APPLICATIONS BASED ON CURRENT-TIME CHARACTERISTIC
Time delay devices
Sequential switching
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Surge suppression
Inrush current limiting
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Applications Based on
Current-Time Characteristic
Time delay, surge suppression, inrush current
limiting and sequential switching represent some of
the earliest, high volume uses of thermistors. These
thermistor applications are all based upon the
current-time characteristic which was described
in the section on Electrical Properties. The currenttime characteristic of a thermistor depends on its
heat capacity and dissipation constant as well as the
circuit in which it is used.
-tº
thermistor is connected in the circuit, however, the
initial current will be low due to the high resistance
of the thermistor. As the thermistor self-heats, its
resistance will decrease and the current gradually
increases in the circuit thus protecting the filaments
or sensitive circuit elements.
Thermistors can be designed so that their initial
resistance and heat capacity are sufficiently large
enough to discriminate against high-voltage surges
of short duration. In this matter, the time delay
characteristic of thermistors can be used to prevent
false triggering voltages. This suppression may be
accomplished without interfering with the normal
operation of the devices at lower voltages.
Applications involving sequential switching involve
the use of two or more time delay circuits. The
circuits can involve series or parallel combinations of
thermistors and loads and are designed such that
the individual thermistor-load combinations operate
in a prescribed sequence. The choice of thermistor
and circuit parameters determine the time delays
and thus the sequence of events.
In all applications based upon the current-time
characteristics, proper consideration must be given
to the peak instantaneous power applied to the
thermistor. The thermistor may be damaged if
voltage, current or power ratings of the device are
exceeded.
Applications Based on Voltage-Current
Characteristic
Figure 9: Example of current-time delay circuit.
The thermistor current-time characteristic can be
used to discriminate against high voltage spikes of
short duration and against initial current surges.
Typical surge protection applications involve the
protection of filaments in vacuum tubes or light
bulbs, or the protection of circuit elements that are
connected in series with switching power supplies.
When filaments are cold they have a low resistance and so the initial current at “turn-on” can be
high. This current surge can shorten the useful life
of the filament. When a switching power supply is
first turned on, the large filter capacitors will attempt
to quickly charge. This large inrush current can
cause circuit breakers to trip, fuses to blow or
contacts in relays and switches to weld. When a
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All applications based on the voltage-current
characteristic use one or more thermistors which
are operated in a self-heated, steady state
condition. In the section on Electrical Properties, we
described the static voltage-current characteristic.
This characteristic can be expressed mathematically, by equation (7) and can be graphically plotted
on log-log coordinates, Figure <5>, or on linear
coordinates Figure, <6>.
The applications based on the voltage-current
characteristic shown in Table <3> were subdivided
into four major categories depending on the type of
excitation that causes the operating point to charge.
The operating point is defined as the intersection
of the voltage-current characteristic and the load
line. The load line is obtained by considering the
Thevenin equivalent circuit with respect to the
thermistor terminals. The excitations, either thermal
or electrical, will cause either the voltage-current
characteristic or the load line to shift. The result is
that a new operating point is established for a
steady state condition.
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Figure 11: E-I Characteristics with changes in
Electrical Parameters.
Figure 10: E-I Characteristics with changes in
Dissipation Constant.
Log-log coordinates are generally used to display the
voltage-current characteristic in manufacturers’ data
sheets and they are useful for demonstrating
the effects of a given type of excitation. Linear
coordinates are generally more useful when trying
to solve an actual circuit problem where a load line
intersects a voltage-current characteristic curve or
family of such curves.
In the Group 1 category of Applications based on
the E-I Characteristic shown in Table <3>, a change
in the heat-transfer properties of the thermistor
environment results in a change in the thermistor
dissipation constant and a resultant shift in the
voltage-current characteristic curve. The operating
point therefore changes to the intersection of the
load line with the shifted curve. This change in the
dissipation constant can be produced by a change
in the heat transfer from the thermistor to its
surroundings, by a change in the thermal conductivity of the medium or by a change in the relative
motion of the medium in which the thermistor is
suspended. The effect of the change in dissipation
constant is to translate the voltage-current
characteristic along a line of constant resistance
since the ambient temperature and the zero-power
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resistance of the thermistor have not changed. For
a constant amount of power applied, however, the
temperature rise above ambient will be changed by
a factor equal to the ratio of the change in dissipation constants. Figure <10> illustrates this type of
excitation.
In the Group 2 category of Applications based on
the E-I Characteristic shown in Table <3>, the
excitation is electrical and can be represented as a
change in the Thevenin voltage or resistance
(or both) at the thermistor terminals. This results in
a rotation and/or translation (or both) of the load line
with respect to the fixed voltage-current characteristic of the thermistor. For reasons of clarity, this type
of excitation is almost always plotted on linear
coordinates. Figure <11> illustrates this type of
excitation.
In the Group 3 category of Applications based on
the E-I Characteristic shown in Table <3>, the excitation is thermal which results in a change in the
voltage-current characteristic curve. If the source
resistance is a non-self-heated thermistor, the load
line may also shift. Figure <12> illustrates this type
of excitation.
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Figure 12: E-I Characteristics with changes in
Ambient Temperature.
Figure 13: E-I Characteristics with changes in
Absorbed Radiation.
It appears as if the voltage-current characteristic
has been translated along a line of constant power,
however, this is not correct. The R-T characteristic is
non-linear and so the voltage-current characteristic
curves must be generated for the new ambient temperature conditions.
thermistor beads are suspended by their leadwires
across a gap on a header or fixture for gaseous
environments and long probes are used for
immersion in liquid environments.
In the Group 4 category of Applications based on
the E-I Characteristic shown in Table <3>, the
excitation is a change in radiation absorbed by the
thermistor. The operating point has caused shift in
the same manner as if the temperature is changed.
Figure <13> illustrates this type of excitation.
In all of the applications involving self-heated
thermistors, consideration must be given to the
maximum power ratings of the thermistor utilized.
Excessive power levels can cause degradation of the
thermistor or possibly even catastrophic failure.
Particular consideration must be given to the type
mounting. The heat transfer properties of the
thermistor will be affected by the way the thermistor
is mounted and will therefore show up as a
change in the dissipation constant. For many of the
applications, it is desirable to have the thermistor
suspended in such a fashion that it is isolated from
the mount as much as possible. As an example,
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Applications Based on Resistance-Temperature
Characteristic
Applications that are based upon the resistancetemperature characteristics include temperature
measurement, control, and compensation. Also
included are those applications for which the
temperature of the thermistor is related to some
other physical phenomena. Unlike the application
based upon the current-time or voltage-current
characteristics, these applications require that the
thermistor be operated in a zero-power condition.
In the previous treatment of the ResistanceTemperature Characteristic, data was presented on
the derivation of interpolation equations that can be
used for NTC thermistors. The various equations
discussed, when used under the proper set of
conditions, can adequately and accurately define
the zero-power resistance-temperature characteristic of the NTC thermistors.
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-tº
Figure 14: Simple Voltage Divider.
For the case of the voltage divider circuit under
consideration, we obtain the normalized output from
equation (23) as follows:
eo ( T )
R
=
=
es
(R+RT)
Linear Voltage Divider
The simplest thermistor network used in many
applications is the voltage divider circuit shown in
Figure <14>. In this circuit, the output voltage is
taken across the fixed resistor. This has the advantages of providing an increasing output voltage for
increasing temperatures and allows the loading
effect of any external measurement circuitry to be
included into the computations for the resistor, R ,
and thus the loading will not affect the output
voltage as temperature varies.
The output voltage as a function of temperature
can be expressed as follows:
[
R
(R + RT)
[
eo ( T ) = es
[
(24)
RT has been defined as zero-power resistance of
a thermistor at a temperature, T , and, RTO as the
zero-power resistance of a thermistor at a standard
reference temperature, T0 . In most thermistor
literature, the reference temperature, TO , is 25°C
(298.15K) and the thermistors are cataloged by
their nominal resistance value at 25°C. The usual
practice is to furnish resistance ratio versus temperature information for each type of thermistor and
material system (composition of metal oxides) listed
in the catalog. Thus, the thermistor resistance is
normalized with respect to its resistance at the
specified temperature.
(23)
rT = RT I RTO
From the plot of the output voltage, we can
observe that a range of temperatures exists where
the circuit is reasonably linear with good sensitivity.
Therefore, the objective will be to solve for a fixed
resistor value, R , that provides optimum linearity for
a given resistance-temperature characteristic and a
given temperature range.
A very useful approach to the solution of a linear
voltage divider circuit is to normalize the output
voltage with respect to the input voltage. The result
will be a standard output function (per unit volt) that
can be used in many design problems.
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1
R
1+ T
R
[
There are a variety of instrumentation / telemetry
circuits in which a thermistor may be used for
temperature measurements. In most cases, a
major criterion is that the circuit provides an output
that is linear with temperature. When the use of a
constant-current source is desired, the circuit used
should be a two-terminal network that exhibits a
linear resistance-temperature characteristic. The
output of this network is a linear voltage-temperature function. Under these conditions, a digital
voltmeter connected across the network can display
temperature directly when the proper combination of
current and resistance level are selected. If the use
of a constant voltage source is more desirable, the
circuit used should be a two-terminal network that
exhibits a linear conductance-temperature characteristic. Conversely, the output of this network is a
linear current-temperature function.
Consequently, the design of thermistor networks
for most instrumentation / telemetry applications is
focused on creating linear resistance-temperature or
linear conductance-temperature circuits.
.
..
rT RTO = RT
(25)
Note: In the actual solution of many applications
problems, it is desirable for TO and RTO to be
specified at the midpoint of the intended operating
temperature range.
The ratio of the zero-power resistance of the
thermistor at the desired reference temperature to
the fixed value resistor in the voltage divider circuit is
a constant which we shall call “s”. Thus we have:
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S=
RTO
R
(26)
By substitution, we obtain the standard function
as follows:
F( T )=
eo ( T )
1
=
eS
1+ srT
(27)
The standard function, F(T), is dependent upon
the circuit constant, “s”, and the resistance-ratio
versus temperature characteristic, rT. If we allow the
circuit constant to assume a series of constant
values and solve for the standard function, we shall
generate a family of “s” curves. Figure <15>
illustrates a family of such curves. These curves were
generated using the resistance-ratio temperature
characteristic given for the “A” material system as
defined in MIL-PRF-23648D.
optimum linearity conditions of the divider network.
They are the “Inflection Point Method” and the
“Equal Slope Method”.
Inflection Point Method
In this method, it is desired to have the inflection
point of the standard function occur at the midpoint
of the operating temperature range. The inflection
point is the point where the slope of the curve is a
maximum. The sensitivity of the divider network
would therefore be at a maximum at this point.
This method is recommended for the solution of
temperature control applications.
This method, however, does not provide good
linearity over wide temperature ranges. Its use
should be restricted to temperature spans that are
narrow enough for beta to be considered constant
and thus the intrinsic equations can be used.
At the inflection point, the slope of the standard
curve (first derivative with respect to temperature) is
at a maximum and the curvature (second derivative
with respect to temperature) is zero. The reference
temperature will be selected as the midpoint
temperature of the desired operating range.
Equal Slope Method
Figure 15: Design curves for “A” Material System
(MIL-PRF-23648D).
It is obvious from the design curves that a value
for the circuit constant, “s”, exists such that
optimum linearity can be achieved for the divider
network over a specified temperature range. The
design curves can be used to provide a graphical
solution or a first approximation for many applications. For the best solution to a design problem,
however, an analytical approach is required. There
are two analytical methods employed to solve for the
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In this method it is desired to set the slopes of the
standard function equal to each other at the
endpoints of the temperature range (TL and TH).
This method can provide good linearity over wider
temperature ranges. When using this method for
solution, the polynomial equations for the resistance-temperature characteristic are used.
Specifically, equations (15) through (18) provide
suitable interpolation accuracies for most linearized
network applications.
Both of the analytical methods discussed above
have been based on a single thermistor voltage
divider. When the thermistor is connected to more
complex circuits which contain only resistances and
voltage sources, the problem can be reduced back
to the simple voltage divider by considering the
Thevenin equivalent circuit as seen at the thermistor
terminals. As a short refresher, the Thevenin voltage
is the open circuit voltage across the thermistor
terminals and the Thevenin resistance is the
resistance as seen by the thermistor with the voltage
source short circuited.
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Modified Voltage Divider Circuit
Figure <16> shows two simple modifications to
the basic voltage divider which can be converted to
or from a Thevenin equivalent circuit as required for
any given application.
The voltage divider of Figure <16a> is used
where it is desired to reduce the output signal while
Figure <16b> is used where it is desired to reduce
the source voltage and translate the output signal by
adding a bias voltage.
Of the two circuits, Figure <16b> is more
commonly used, especially in bridge circuits. It
permits the use of conventional source voltages and
reduces the voltage placed across the thermistor to
an acceptable level of self-heating. The bias voltage
can be compensated in the bridge design.
When the Wheatstone Bridge circuit is more complex and the load resistance cannot be considered
infinite, the Thevenin theorem is used to reduce the
circuit to its equivalent form. Figure <18a> shows
the basic Wheatstone Bridge circuit for a finite load
resistance, while Figure <18b> shows the Thevenin
equivalent circuit.
A
A
B
Figure 17:
Load).
Wheatstone Bridge Circuits (Infinite
B
Figure 16: Modified Voltage Divider Circuits.
Bridge Circuit
Bridge circuits are actually two voltage divider
circuits. In most applications, the bridge consists
of a linear thermistor voltage divider and a fixed
resistor voltage divider. For differential temperature
applications, the bridge consists of matching
thermistor linear voltage dividers. Figure <17a>
illustrates a basic Wheatstone Bridge circuit with one
linearized thermistor voltage divider and Figure
<17b> illustrates the Wheatstone Bridge circuit
used for differential temperature applications. Both
of the circuits in Figure <17> represent cases
where the load resistance is infinite and thus does
not affect the output voltage of the voltage divider
or dividers.
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A
B
Figure 18: Wheatstone Bridge Circuits (Finite Load).
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Ohmmeter Circuit
Another circuit which is commonly employed in
temperature measurement applications is the basic
Ohmmeter circuit which is shown in Figure <19>.
This circuit is also a basic voltage divider of sorts. It
is generally used for low cost temperature measurement applications; thus, the trimming potentiometer
may not always be in the circuit.
In the Ohmmeter circuit, the objective is to
produce a linear current. This current can be
expressed as a constant times the standards
function, F(T). The value of the constant is the
source voltage divided by the circuit resistance as
seen by the thermistor. Note that the circuit
consisting of a thermistor in series with a fixed
resistance is a linear conductance versus temperature network.
The voltage divider circuits, the Wheatstone
Bridge circuits and the Ohmmeter circuit discussed
so far have all been examples of linear conductance
versus temperature networks. They may all be
solved by the use of the standard function “s”
curves, the inflection point method or the equal
slope method as preferred.
The linear conductance-temperature networks
are driven by a constant voltage source, whereas,
the linear resistance-temperature networks will be
driven by a constant current source. Note that one
is the dual of the other.
Figure <20> illustrates the basic linear
resistance-temperature networks used in most
compensation applications. The simplest network is
obviously that shown in Figure <20a>. If we
normalize the network resistance with respect to the
shunt resistor, we observe that the standard
function, F(T), can be used for the design of linear
resistance networks.
In order to increase the overall network resistance
to a higher value, a series resistor can be inserted as
illustrated by Figure <20b>. This can also be done
to increase the voltage drop across the network
when a constant current is applied to the terminals.
Obviously, the linear resistance-temperature
characteristic is translated by the series resistor and
the slope remains unchanged.
Figure <20c> shows the circuit of Figure
<20b> with the addition of a resistor in series with
the thermistor. This circuit is used to permit the use
of a standard value for the thermistor. The standard
value thermistor must be slightly lower than the
desired value for optimum linearity and both
thermistors must have the same resistance
ratio-temperature characteristic.
Figure <20d> shows the basic circuit of Figure
<20a> with the addition of a resistor in series with
the thermistor, again for the purpose of utilizing
a standard value of thermistor.
Figure 19: Ohmmeter Circuit.
Linear Resistance Networks
Many applications based upon the resistancetemperature characteristic require the use of a
linearized resistance network. Among the most
common applications are networks to compensate
for the positive temperature coefficient resistance
changes of devices and circuits as well as coil
windings in relays, motors, instrument movements
or generators.
Another important area of
applications involve the need for a voltage which has
a negative slope with respect to temperature in
order to compensate for temperature drifts of
amplifiers, oscillators or other circuits containing
active components.
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A
B
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T1
T3
T2
C
Figure 21: Multiple Thermistor Networks.
Digital Instrumentation
D
Figure 20: Linear Resistance Networks.
Multiple Thermistor Networks
There is a practical limit to obtaining linearized
voltage divider or linearized resistance networks
using a single thermistor. In order to obtain better
linearization over wider temperature spans, two
or more thermistors are sometimes used in combination with resistor networks. Figure <21>
illustrates two and three thermistor networks,
respectively.
The analytical solutions for these multiple
thermistor networks are too complex for presentation. The usual procedure is to obtain the optimum
solution by means of a computer model of the
desired circuit and an iterative program. Based
upon the temperature range and desired circuit
characteristics, a reasonably good first approximation can be made for circuit element values. The
computer will then adjust the element values
incrementally and examine the results until an
optimum network is achieved.
T
T
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To date, a major limitation to the use of thermistors for resistance thermometry has been the
non-linearity of the resistance-temperature characteristic. Even a carefully designed linear voltage
divider circuit or a linear resistance network can only
operate over a relatively narrow range of temperatures before the non-linearity errors become
significant. Generally speaking, the wider the
temperature span, the more non-linearity one must
be willing to accept. In order to handle the wide
temperature ranges of some applications, it is often
necessary to provide range switches on the analog
instrument. In this manner, the overall temperature
range can be broken down into smaller segments
and the voltage divider or bridge circuits can be
optimized for each of the smaller temperature
ranges. This approach is not always convenient
when a continuous readout is desired for the
time-temperature characteristic of a process or
experiment.
In the past few years, digital thermometers for
thermistor sensors have been introduced that
display the measured temperature over relatively
wide ranges and have accuracies that were not
possible with the analog instruments. The digital
thermometers can also be equipped with output
options (BCD, RS-232C, GPIB, etc.) so that the
temperature data can be networked to computers,
data-loggers, or continuous process controllers.
The basic voltage divider circuit or Wheatstone
Bridge circuit is still utilized to provide an analog
input stage for these instruments. For example, a
constant current source may be used in conjunction
with a linear-resistance network, or, a constant
voltage source may be used in conjunction with a
linear-conductance network. However, the linearity
of the analog signal is no longer a major consideration since the data is now processed with the aid
of microprocessor circuits.
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The input signal from the thermistor transducer
circuit first is amplified and/or linearized as required
and fed to an A/D (analog to digital) converter. The
output of the A/D converter is now a digital signal
that can be operated upon by the instrument microprocessor chip. The overall precision of the A/D
converter will be a major determining factor in the
overall system accuracy. That is to say, a 16 bit A/D
is better than a 14 bit A/D converter. There are
many possible combinations of microprocessor
chips and software programming which can be used
for the digital thermometer. There will be high cost
or low cost solutions that are dictated by the
accuracy requirements of the particular thermistor
application.
An example of a low cost instrument is the
hand-held type thermometer that is designed to
work with an interchangeable thermistor. In this type
of instrument, the resistance-temperature curve of
the interchangeable device is know over the desired
temperature range and is stored in a ROM (read only
memory) chip. Each of the thermistor sensors
designed to be used with the instrument are
interchangeable to the same resistance-temperature curve. Thus, the accuracy of the instrument is
basically determined by the interchangeability of the
individual sensors. This is usually specified in the
form of a temperature accuracy over a given
temperature range. The typical accuracy for this
type of hand held thermometer is on the order of
±0.1°C to ±0.2°C over the specified temperature
range, which is usually 0°C to 100°C.
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High accuracy digital thermometers are currently
available as bench-type instruments. These instruments are not dedicated to a particular R-T
characteristic such as the hand-held thermometers.
These instruments are usually programmable so that
they can work with any thermistor whose resistancetemperature characteristic has been defined over a
specified temperature range. Major features of this
class of digital thermometer include: memory
portions of the instrument that are both RAM
(random access memory) as well as ROM, and,
I/O options (input/output ports) to interface with
displays, keypads, CRT’s, printers, etc.
These resistance-temperature characteristic of
each thermistor must first be determined by calibration at selected reference temperatures over the
desired operating range. The test data is then used
to compute the constants for one of the standard
thermistor curve fitting equations. The accuracy of
these digital thermometers will thus be dependent
upon the exactness of fit of the thermistor R-T data
using the equation. The equation interpolation error
is, on turn, a function of the accuracy of the initial
calibration data points as well as the temperature
range.
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