Two Ways to Measure Temperature Using Thermocouples Feature Simplicity,

Two Ways to Measure Temperature Using Thermocouples Feature Simplicity,
Two Ways to Measure Temperature
Using Thermocouples Feature
Simplicity, Accuracy, and Flexibility
By Matthew Duff and Joseph Towey
Introduction
The thermocouple is a simple, widely used component for
measuring temperature. This article provides a basic overview
of thermocouples, describes common challenges encountered
when designing with them, and suggests two signal conditioning
solutions. The first solution combines both reference-junction
compensation and signal conditioning in a single analog IC for
convenience and ease of use; the second solution separates the
reference-junction compensation from the signal conditioning to
provide digital-output temperature sensing with greater flexibility
and accuracy.
Thermocouple Theory
A thermocouple, shown in Figure 1, consists of two wires of
dissimilar metals joined together at one end, called the measurement
(“hot”) junction. The other end, where the wires are not joined,
is connected to the signal conditioning circuitry traces, typically
made of copper. This junction between the thermocouple metals
and the copper traces is called the reference (“cold”) junction.*
THERMOCOUPLE
METAL A
WIRING TO SIGNAL
CONDITIONING
CIRCUITRY
METAL B
MEASUREMENT
JUNCTION
REFERENCE
JUNCTION
Figure 1. Thermocouple.
The voltage produced at the reference junction depends on the
temperatures at both the measurement junction and the reference
junction. Since the thermocouple is a differential device rather
than an absolute temperature measurement device, the reference
junction temperature must be known to get an accurate absolute
temperature reading. This process is known as reference junction
compensation (cold junction compensation.)
Thermocouples have become the industry-standard method for
cost-effective measurement of a wide range of temperatures with
reasonable accuracy. They are used in a variety of applications
up to approximately +2500°C in boilers, water heaters, ovens,
and aircraft engines—to name just a few. The most popular
thermocouple is the type K, consisting of Chromel® and Alumel®
(trademarked nickel alloys containing chromium, and aluminum,
manganese, and silicon, respectively), with a measurement range
of –200°C to +1250°C.
*We use the terms “measurement junction” and “reference junction”
rather than the more traditional “hot junction” and “cold junction.” The
traditional naming system can be confusing because in many applications the
measurement junction can be colder than the reference junction.
Analog Dialogue 44-10, October (2010)
Why Use a Thermocouple?
Advantages
•Temperature range: Most practical temperature ranges,
from cryogenics to jet-engine exhaust, can be served using
thermocouples. Depending on the metal wires used, a
thermocouple is capable of measuring temperature in the range
–200°C to +2500°C.
•Robust: Thermocouples are rugged devices that are immune
to shock and vibration and are suitable for use in hazardous
environments.
•Rapid response: Because they are small and have low thermal
capacity, thermocouples respond rapidly to temperature
changes, especially if the sensing junction is exposed. They
can respond to rapidly changing temperatures within a few
hundred milliseconds.
•No self heating: Because t her mocouples require no
excitation power, they are not prone to self heating and are
intrinsically safe.
Disadvantages
•Complex signal conditioning: Substantial signal conditioning
is necessary to convert the thermocouple voltage into a usable
temperature reading. Traditionally, signal conditioning has
required a large investment in design time to avoid introducing
errors that degrade accuracy.
•Accuracy: In addition to the inherent inaccuracies in
thermocouples due to their metallurgical properties, a
thermocouple measurement is only as accurate as the reference
junction temperature can be measured, traditionally within
1°C to 2°C.
•Susceptibility to corrosion: Because thermocouples consist of
two dissimilar metals, in some environments corrosion over
time may result in deteriorating accuracy. Hence, they may
need protection; and care and maintenance are essential.
•Susceptibility to noise: When measuring microvolt-level
signal changes, noise from stray electrical and magnetic
fields can be a problem. Twisting the thermocouple wire pair
can greatly reduce magnetic field pickup. Using a shielded
cable or running wires in metal conduit and guarding can
reduce electric field pickup. The measuring device should
provide signal filtering, either in hardware or by software,
with strong rejection of the line frequency (50 Hz/60 Hz)
and its harmonics.
Difficulties Measuring with Thermocouples
It is not easy to transform the voltage generated by a thermocouple
into an accurate temperature reading for many reasons: the
voltage signal is small, the temperature-voltage relationship is
nonlinear, reference junction compensation is required, and
thermocouples may pose grounding problems. Let’s consider
these issues one by one.
Voltage signal is small: The most common thermocouple
types are J, K, and T. At room temperature, their voltage
varies at 52 μV/°C, 41 μV/°C, and 41 μV/°C, respectively. Other
less-common types have an even smaller voltage change with
temperature. This small signal requires a high gain stage before
analog-to-digital conversion. Table 1 compares sensitivities of
various thermocouple types.
www.analog.com/analogdialogue
1
Table 1. Voltage Change vs. Temperature Rise
(Seebeck Coefficient) for Various Thermocouple Types at 25°C.
Thermocouple Seebeck Coefficient
Type
(𝛍V/°C)
E
61
J
52
K
41
N
27
R
9
S
6
T
41
Because the voltage signal is small, the signal-conditioning
circuitry typically requires gains of about 100 or so—fairly
straightforward signal conditioning. What can be more difficult
is distinguishing the actual signal from the noise picked up on the
thermocouple leads. Thermocouple leads are long and often run
through electrically noisy environments. The noise picked up on
the leads can easily overwhelm the tiny thermocouple signal.
Two approaches are commonly combined to extract the signal from
the noise. The first is to use a differential-input amplifier, such as
an instrumentation amplifier, to amplify the signal. Because much
of the noise appears on both wires (common-mode), measuring
differentially eliminates it. The second is low-pass filtering, which
removes out-of-band noise. The low-pass filter should remove
both radio-frequency interference (above 1 MHz) that may cause
rectification in the amplifier and 50 Hz/60 Hz (power-supply) hum.
It is important to place the filter for radio frequency interference
ahead of the amplifier (or use an amplifier with filtered inputs).
The location of the 50-Hz/60-Hz filter is often not critical—it can
be combined with the RFI filter, placed between the amplifier and
ADC, incorporated as part of a sigma-delta ADC, or it can be
programmed in software as an averaging filter.
Reference junction compensation: The temperature of the
thermocouple’s reference junction must be known to get an
accurate absolute-temperature reading. When thermocouples were
first used, this was done by keeping the reference junction in an
ice bath. Figure 2 depicts a thermocouple circuit with one end at
an unknown temperature and the other end in an ice bath (0°C).
This method was used to exhaustively characterize the various
thermocouple types, thus almost all thermocouple tables use 0°C
as the reference temperature.
+
There are three common ways to compensate for the nonlinearity
of the thermocouple.
Choose a portion of the curve that is relatively f lat and
approximate the slope as linear in this region—an approach
that works especially well for measurements over a limited
temperature range. No complicated computations are needed.
One of the reasons the K- and J-type thermocouples are popular
is that they both have large stretches of temperature for which the
incremental slope of the sensitivity (Seebeck coefficient) remains
fairly constant (see Figure 3).
70
V
–
IRON
REFERENCE
JUNCTIONS
ICE BATH
REFERENCE
Figure 2. Basic iron-constantan thermocouple circuit.
But keeping the reference junction of the thermocouple in an ice
bath is not practical for most measurement systems. Instead most
systems use a technique called reference-junction compensation,
(also known as cold-junction compensation). The reference
junction temperature is measured with another temperaturesensitive device—typically an IC, thermistor, diode, or RTD
2
Voltage signal is nonlinear: The slope of a thermocouple
response curve changes over temperature. For example, at 0°C a
T-type thermocouple output changes at 39 μV/°C, but at 100°C,
the slope increases to 47 μV/°C.
+
CONSTANTAN
THERMOCOUPLE
–
A variety of sensors are available for measuring the reference
temperature:
1.Thermistors: They have fast response and a small package;
but they require linearization and have limited accuracy,
especially over a wide temperature range. They also require
current for excitation, which can produce self-heating, leading
to drift. Overall system accuracy, when combined with signal
conditioning, can be poor.
2.Resistance temperature-detectors (RTDs): RTDs are
accurate, stable, and reasonably linear, however, package size
and cost restrict their use to process-control applications.
3.Remote thermal diodes: A diode is used to sense the
temperature near the thermocouple connector. A conditioning
chip converts the diode voltage, which is proportional to
temperature, to an analog or digital output. Its accuracy is
limited to about ±1°C.
4.Integrated temperature sensor: An integrated temperature
sensor, a standalone IC that senses the temperature locally,
should be carefully mounted close to the reference junction,
and can combine reference junction compensation and signal
conditioning. Accuracies to within small fractions of 1°C can
be achieved.
J-TYPE
60
SEEBECK COEFFICIENT (¿V/îC)
UNKNOWN
TEMPERATURE
COPPER WIRE
(resistance temperature-detector). The thermocouple voltage
reading is then compensated to reflect the reference junction
temperature. It is important that the reference junction be read
as accurately as possible—with an accurate temperature sensor
kept at the same temperature as the reference junction. Any
error in reading the reference junction temperature will show
up directly in the final thermocouple reading.
50
K-TYPE
T-TYPE
40
30
20
10
0
–200
0
200
400
600
800
1000
TEMPERATURE (îC)
Figure 3. Variation of thermocouple sensitivity with
temperature. Note that K-type’s Seebeck coefficient is
roughly constant at about 41 μV/°C from 0°C to 1000°C.
Analog Dialogue 44-10, October (2010)
Another approach is to store in memory a lookup table that matches
each of a set of thermocouple voltages to its respective temperature.
Then use linear interpolation between the two closest points in
the table to get other temperature values.
A third approach is to use higher order equations that model
the behavior of the thermocouple. While this method is the
most accurate, it is also the most computationally intensive.
There are two sets of equations for each thermocouple. One
set converts temperature to thermocouple voltage (useful for
reference junction compensation). The other set converts
t her mocouple volt age to temper at u re. T her mocouple
tables and the higher order thermocouple equations can be
found at http://srdata.nist.gov/its90/main/. The tables and
equations are all based on a reference junction temperature
of 0°C. Reference-junction compensation must be used if the
reference-junction is at any other temperature.
Grounding requirements: Thermocouple manufacturers make
thermocouples with both insulated and grounded tips for the
measurement junction (Figure 4).
INSULATED
GROUNDED
EXPOSED
all three tip cases if the amplifier’s common-mode range has some
ability to measure below ground in the single-supply configuration.
To deal with the common-mode limitation in some single-supply
systems, biasing the thermocouple to a midscale voltage is useful.
This works well for insulated thermocouple tips, or if the overall
measurement system is isolated. However, it is not recommended
for nonisolated systems that are designed to measure grounded
or exposed thermocouples.
Practical thermocouple solutions: Thermocouple signal
conditioning is more complex than that of other temperature
measurement systems. The time required for the design and
debugging of the signal conditioning can increase a product’s
time to market. Errors in the signal conditioning, especially in
the reference junction compensation section, can lead to lower
accuracy. The following two solutions address these concerns.
The first details a simple analog integrated hardware solution
combining direct thermocouple measurement with reference
junction compensation using a single IC. The second solution
details a software-based reference-junction compensation scheme
providing improved accuracy for the thermocouple measurement
and the flexibility to use many types of thermocouples.
Measurement Solution 1: Optimized for Simplicity
Fi g u r e 6 s how s a sc hem at ic for me a su r i n g a K- t y pe
thermocouple. It is based on using the AD8495 thermocouple
amplifier, which is designed specifically to measure K-type
thermocouples. This analog solution is optimized for minimum
design time: It has a straightforward signal chain and requires
no software coding.
Figure 4. Thermocouple measurement junction types.
The thermocouple signal conditioning should be designed so as to
avoid ground loops when measuring a grounded thermocouple, yet
also have a path for the amplifier input bias currents when measuring
an insulated thermocouple. In addition, if the thermocouple tip is
grounded, the amplifier input range should be designed to handle
any differences in ground potential between the thermocouple tip
and the measurement system ground (Figure 5).
WHEN USING ISOLATED
THERMOCOUPLE TIPS
MEASUREMENT
JUNCTION
PCB
TRACES
THERMOCOUPLE
RFI
FILTER
10k±
10k±
1M±
REFERENCE
JUNCTION
THERMOCOUPLE
AMPLIFIER
5V
1nF
10nF
1nF
AD8495
REF
FILTER FOR
50Hz/60Hz
5mV/îC
100k±
1¿F
fC = 1.6Hz
INCLUDES
GROUND COMMON MODE REFERENCE
CONNECTION
fC = 16kHz
JUNCTION
COMPENSATION
DIFFERENTIAL
fC = 1.3kHz
Figure 6. Measurement solution 1: optimized for simplicity.
WHEN USING EXPOSED OR
GROUNDED THERMOCOUPLE TIPS
ELECTRICAL CONNECTION
OCCURS AT TIP. VOLTAGE MUST
STAY IN COMMON-MODE INPUT
RANGE OF AMPLIFIER
WHEN THERMOCOUPLE TIP TYPE IS
UNKNOWN
1M±
Figure 5. Grounding options when using different tip types.
For nonisolated systems, a dual-supply signal-conditioning system
will typically be more robust for grounded tip and exposed tip
types. Because of its wide common-mode input range, a dualsupply amplifier can handle a large voltage differential between
the PCB (printed-circuit board) ground and the ground at the
thermocouple tip. Single-supply systems can work satisfactorily in
Analog Dialogue 44-10, October (2010)
How does this simple signal chain address the signal conditioning
requirements for K-type thermocouples?
Gain and output scale factor: The small thermocouple signal
is amplified by the AD8495’s gain of 122, resulting in a 5-mV/°C
output signal sensitivity (200°C/V).
Noise reduction: High-frequency common-mode and differential
noise are removed by the external RFI filter. Low frequency commonmode noise is rejected by the AD8495’s instrumentation amplifier.
Any remaining noise is addressed by the external post filter.
Reference junction compensation: The AD8495, which
includes a temperature sensor to compensate for changes in
ambient temperature, must be placed near the reference junction
to maintain both at the same temperature for accurate referencejunction compensation.
Nonlinearity correction: The AD8495 is calibrated to
give a 5 mV/°C output on the linear portion of the K-type
thermocouple curve, with less than 2°C of linearity error in the
–25°C to +400°C temperature range. If temperatures beyond
this range are needed, Analog Devices Application Note
AN-1087 describes how a lookup table or equation could be
used in a microprocessor to extend the temperature range.
3
Table 2. Solution 1 (Figure 6) Performance Summary
Thermocouple Measurement Junction
Type
Temperature Range
–25°C to +400°C
K
Reference Junction
Temperature Range
±3°C (A grade)
±1°C (C grade)
0°C to 50°C
Handling insulated, grounded, and exposed thermocouples:
Figure 5 shows a 1-MΩ resistor connected to ground, which allows
for all thermocouple tip types. The AD8495 was specifically
designed to be able to measure a few hundred millivolts below
ground when used with a single supply as shown. If a larger ground
differential is expected, the AD8495 can also be operated with
dual supplies.
Power
Consumption
Accuracy at 25°C
1.25 mW
Table 2 summarizes the performance of the integrated hardware
solution using the AD8495:
Measurement Solution 2: Optimized for Accuracy and Flexibility
Figure 8 shows a schematic for measuring a J-, K-, or T-type
thermocouple with a high degree of accuracy. This circuit includes
a high-precision ADC to measure the small-signal thermocouple
voltage and a high-accuracy temperature sensor to measure the
reference junction temperature. Both devices are controlled using
an SPI interface from an external microcontroller.
More about the AD8495: Figure 7 shows a block diagram of
the AD8495 thermocouple amplifier. Amplifiers A1, A2, and
A3—and the resistors shown—form an instrumentation amplifier
that amplifies the K-type thermocouple’s output with a gain
appropriate to produce an output voltage of 5 mV/°C. Inside
the box labeled “Ref junction compensation” is an ambient
temperature sensor. With the measurement junction temperature
held constant, the differential voltage from the thermocouple
will decrease if the reference junction temperature rises for any
reason. If the tiny (3.2 mm × 3.2 mm × 1.2 mm) AD8495 is in
close thermal proximity to the reference junction, the referencejunction compensation circuitry injects additional voltage into
the amplifier, so that the output voltage stays constant, thus
compensating for the reference temperature change.
How does this configuration address the signal conditioning
requirements mentioned earlier?
Remove noise and amplify voltage: The AD7793, shown
in detail in Figure 9—a high-precision, low-power analog
front end—is used to measure the thermocouple voltage. The
thermocouple output is filtered externally and connects to a set
of differential inputs, AIN1(+) and AIN1(–). The signal is then
routed through a multiplexer, a buffer, and an instrumentation
amplifier—which amplifies the small thermocouple signal—and
to an ADC, which converts the signal to digital.
GND
AVDD
REFIN(+)/AIN3(+) REFIN(–)/AIN3(–)
–IN
1M±
ESD AND
OVP
AD8494/AD8495/
AD8496/AD8497
REF JUNCTION
COMPENSATION
ESD AND
OVP
BAND GAP
REFERENCE
A3
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
OUT
MUX
BUF
AVDD
A1
IOUT1
IN-AMP
INTERNAL
CLOCK
REF
FILTERING
THERMOCOUPLE
MEASUREMENT
JUNCTION
AD7793
DOUT/RDY
DIN
SCLK
CS
DVDD
CLK
Figure 7. AD8495 functional block diagram.
REFERENCE
JUNCTION
«-œ
ADC
GND
IOUT2
Figure 9. AD7793 functional block diagram.
«-œADC
AIN1+
VDD
DECOUPLING
VDD
AD7793
AIN1–
TEMPERATURE
SENSOR
ADT7320
SPI_CLK
SPI_MISO
SPI_MOSI
SPI_SEL_A
GND
AVDD
THERMOCOUPLE
+IN
VBIAS
A2
SERIAL INTERFACE
AND CONTROL LOGIC
SENSE
SCLK
DOUT
DIN
CS
VDD
GND
EP
INT
CT
VDD
GND
MICROCONTROLLER
DOUT
SPI_MISO
DIN
SPI_MOSI
SCLK
CS
SPI_SEL_A
VDD
DECOUPLING
VDD
SPI_CLK
SPI_SEL_B
SPI_SEL_A GND
Figure 8. Measurement solution 2: Optimized for accuracy and flexibility.
4
Analog Dialogue 44-10, October (2010)
Table 3. Solution 2 (Figure 8) Performance Summary
Thermocouple Measurement Junction Reference Junction
Type
Temperature Range
Temperature Range Accuracy
J, K, T
Full Range
–10°C to +85°C
–20°C to +105°C
C ompensate for reference ju nct ion te mper at u re:
The ADT7320 (detailed in Figure 10), if placed close enough
to the reference junction, can measure the reference-junction
temperature accurately, to ±0.2°C, from –10°C to +85°C. An
on-chip temperature sensor generates a voltage proportional to
absolute temperature, which is compared to an internal voltage
reference and applied to a precision digital modulator. The
digitized result from the modulator updates a 16-bit temperature
value register. The temperature value register can then be
read back from a microcontroller, using an SPI interface, and
combined with the temperature reading from the ADC to effect
the compensation.
Correct nonlinearity: The ADT7320 provides excellent linearity
over its entire rated temperature range (–40°C to +125°C),
requiring no correction or calibration by the user. Its digital
output can thus be considered an accurate representation of the
reference-junction state.
To determine the actual thermocouple temperature, this
reference temperature measurement must be converted into
an equivalent thermoelectric voltage using equations provided
by the National Institute of Standards and Technolog y
(NIST). This voltage then gets added to the thermocouple
voltage measured by the AD7793; and the summation is then
translated back into a thermocouple temperature, again using
NIST equations.
Handle insulated and grounded thermocouples: Figure 8
shows a thermocouple with an exposed tip. This provides the best
response time, but the same configuration could also be used with
an insulated-tip thermocouple.
±0.2°C
±0.25°C
Power
Consumption
3 mW
3 mW
Table 3 summarizes the performance of the software-based
reference-junction measurement solution, using NIST data:
Conclusion
Thermocouples offer robust temperature measurement over a
quite wide temperature range, but they are often not a first choice
for temperature measurement because of the required trade-offs
between design time and accuracy. This article proposes costeffective ways of resolving these concerns.
The first solution concentrates on reducing the complexity of the
measurement by means of a hardware-based analog reference
junction compensation technique. It results in a straightforward
signal chain with no software programming required, relying on
the integration provided by the AD8495 thermocouple amplifier,
which produces a 5-mV/°C output signal that can be fed into the
analog input of a wide variety of microcontrollers.
The second solution provides the highest accuracy measurement
and also enables the use of various thermocouple types. A
software-based reference junction compensation technique, it
relies on the high-accuracy ADT7320 digital temperature sensor
to provide a much more accurate reference junction compensation
measurement than had been achievable until now. The ADT7320
comes fully calibrated and specified over the –40°C to +125°C
temperature range. Completely transparent, unlike a traditional
thermistor or RTD sensor measurement, it neither requires a
costly calibration step after board assembly, nor does it consume
processor or memory resources with calibration coefficients or
linearization routines. Consuming only microwatts of power,
it avoids self-heating issues that undermine the accuracy of
traditional resistive sensor solutions.
SCLK 1
DOUT 2
SPI
INTERFACE
DIN 3
INTERNAL
REFERENCE
CS 4
TEMPERATURE
VALUE
REGISTER
CONFIGURATION
AND STATUS
REGISTER
THYST
REGISTER
TCRT
REGISTER
THIGH
REGISTER
TLOW
REGISTER
INTERNAL
OSCILLATOR
ADT7320
TCRIT
TEMPERATURE
SENSOR
«-œ
MODULATOR
THIGH
FILTER
LOGIC
TLOW
6 CT
5 INT
7
GND
8
VDD
Figure 10. ADT7320 functional block diagram.
Analog Dialogue 44-10, October (2010)
5
Appendix
Use of NIST Equation to Convert ADT7320 Temperature to Voltage
The thermocouple reference junction compensation is based on
the relationship:
(1)
where:
ΔV = thermocouple output voltage
V @ J1 = voltage generated at the thermocouple junction
V @ J 2 = voltage generated at the reference junction
Authors
For this compensation relationship to be valid, both terminals
of the reference junction must be maintained at the same
temperature. Temperature equalization is accomplished with an
isothermal terminal block that permits the temperature of both
terminals to equalize while maintaining electrical isolation.
After the reference junction temperature is measured, it must be
converted into the equivalent thermoelectric voltage that would
be generated with the junction at the measured temperature. One
technique uses a power series polynomial. The thermoelectric
voltage is calculated:
(2)
where:
E = thermoelectric voltage (microvolts)
an = thermocouple-type-dependent polynomial coefficients
T = temperature (°C)
n = order of polynomial
6
NIST publishes tables of polynomial coefficients for each type
of thermocouple. In these tables are lists of coefficients, order
(the number of terms in the polynomial), valid temperature
ranges for each list of coefficients, and error range. Some types
of thermocouples require more than one table of coefficients to
cover the entire temperature operating range. Tables for the power
series polynomial are listed in the main text.
Matthew Duff [[email protected]]
joined Analog Devices in 2005 as an applications
engineer in the Integrated Amplifier Products
Group. Prior to joining Analog Devices, Matt
worked for National Instr uments in both
design and project management positions on
instrumentation and automotive products. He
received his BS from Texas A&M and MS from Georgia Tech,
both in electrical engineering.
Jose ph Towey [ joe.towe y @ a n a log.com]
joined Analog Devices in 2002 as a senior
test development engineer with the Thermal
Sensing Group. Joe is currently applications
manager for the Thermal Sensing and Switch/
Multiplexer Group. Prior to joining Analog
Devices, Joe worked for Tellabs and Motorola
in both test development and project management positions.
He is qualified with a BSc (Hons) degree in computer science
and a diploma in electronic engineering.
Analog Dialogue 44-10, October (2010)
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