Thermal System Glossary
Abolute Zero–The lowest theoretical temperature. At absolute zero,
a body would have no molecular motion of heat energy. Absolute
zero is the zero point on the Rankine and Kelvin scale. (-273.15°C or
state transforms to the gaseous state. Commonly refers to the
boiling point of water (100°C or 212°F at sea level).
AC–Alternating Current; an electric current that reverses direction at
regularly occurring intervals.
Bumpless Transfer–The smooth, automatic transition from
automatic control (closed loop) to manual control (open Loop). The
control output is maintained during the transfer.
Btu–British Thermal Unit; the amount of thermal energy required to
raise one pound of water, 1°F .
Calibration Accuracy–the potential error of a device compared to
a physical constant or agency standard.
Burst Firing–a fast cycling control output type (3-32VDC for Ogden
products) used in conjunction with a solid state relay.
Control Accuracy–maintaining a process at the desired setting.
The errors or combination of errors in the entire system including
the sensor, control, power, load and design inefficiencies affect
control accuracy.
Calibration–the process of adjusting an instrument so that the
indication is accurate compared to the actual value.
Display Accuracy–the amount of potential error between a
measured value and the control’s displayed value.
Calorie–the amount of thermal energy required to raise one gram of
water 1°C at15°C
Set Point Accuracy–the potential error between a measured
value and the control setting
Cascade–Control function where the output of one control loop
provides the set point for a second loop, which determines the
control action.
Alarm–a control condition or function, indicating that the process is a
predetermined amount above or below the set point.
Ambient Compensation–the ability of an instrument to compensate
for changes in the ambient temperature so that the changes do not
affect control accuracy.
cfm–the volumetric flow rate of a liquid or gas in cubic feet per
CE–A mark that designates compliance with European Union (EU)
requirements for products sold in Europe
Celsius–(Centigrade) a temperature scale with 0°C defined as the
ice point and 100°C as the boiling point of water at sea level.
Ambient Temperature–the temperature of the immediate
surroundings in which equipment is to operate.
Chatter–the rapid cycling of a relay due to too narrow a bandwidth in
the control.
Ampere (amp)–the rate of flow of current in a circuit.
Closed Loop Control–a control system in which process
temperature changes are detected by a sensor. The feedback from
the sensor allows the control to make adjustments for accurate
system regulation.
Analog Indication–a meter with graduated scale and a pointer that
moves to indicate process condition.
Analog Output–a voltage or current signal that is a continuous
function of the measure parameter.
Analog Set Point–potentiometer adjustment of the control setting
Anneal-To relieve stress in a metal or glass material by heating to
just below its melting point, then gradually cooling to ambient
temperature. Annealing lowers tensile strength while increasing
flexibility. Tubular heaters are annealed prior to forming.
ANSI–American National Standards Institute
Anti-reset Windup–a feature in 3 mode (PID) controls which
prevents the integral (automatic rest) circuit from functioning when
the temperature is outside the proportional band.
ASME–American Society of Mechanical Engineers.
Cold Junction Compensation–a temperature sensitive device that
prevents changes in the ambient temperature from affecting the cold
junction of a thermocouple.
Common Mode Line Filter–a device to filter noise signals on both
power lines with respect to ground.
Common Mode Rejection Ratio–the ability of an instrument to
reject interference from a common voltage at the input terminals with
relation to ground. Expressed in dB (decibels).
Conduction–the transfer of heat from one material at a given
temperature to another material at a lower temperature while in direct
contact with each other.
ASTM–American Society for Testing and Materials.
Continuity Check–A test that determines whether current flows
throughout the length of a circuit.
Atmospheric Pressure (Standard)–Pressure exerted by the earth’s
atmosphere on the objects within. Measured at 60°F (15°C), at sea
level, standard atmospheric pressure is 14.7 psia.
Control Loop–the basic control loop of any automatic control system
consists of:
1) variable (process)
Automatic Reset (Integral)–the integral function of a control that
automatically compensates for the difference between the set point
and the actual process temperature. Asignal moves the proportioning band up or down to correct for the droop or offset error.
Automatic Tuning (of control parameters)–a control that calculates
the optimum PID parameters with a built-in software algorithm to
eliminate manual tuning efforts.
Auxiliary Output–additional outputs for control of functions other
than the primary control output, such as lights, buzzers, horns or gas
purges that are triggered by the control alarm function.
AWG–American Wire Gauge.
Bandwidth–the total temperature variation measured at some point
in the system, normally the process.
Baud Rate– In serial communications, the rate of information
transfer in bits per second.
Blackbody–a theoretical object that radiates the maximum amount of
energy at a given temperature and absorbs all energy incident upon
Boiling Point–the temperature at which a substance in the liquid
2) sensor
3) error detector (of control)
4) control
5) final control element (relay, SSR, SCR)
6) temperature indication
Control Mode–the method in which the control restores the system
temperature to set point. On/Off, proportioning, and PID are the most
common control modes.
Convection–the transfer of heat from a source or higher temperature
area in a gas or liquid by the movement and mixing of the masses.
CPS–Cycles per Second (See Hertz).
Current Proportioning– a 4-20 milliamp (typical) current output
which provides a current proportional to the amount of control
Cycle Rate–in a time proportioning control, the period (usually in
seconds) of time that is required to complete one on/off cycle once
temperature has settled at the center of the proportioning band.
DC–direct current; an electric current flowing in one direction and
constant in value.
Data Logging–Recording a process variable over an extended
period of time.
Dead Band–the temperature band where no heating or cooling takes
place, expressed in degrees.
Default Parameters–The programming instructions permanently written
in microprocessor software.
Density–mass per unit of volume, such as lbs./cu.ft.
Derivative–(See Rate)
Deviation–the difference between the selected value and the actual
Deviation Alarm–an offset value that follows the set point. If the set
point is 300°F and the Deviation Alarm value is +20°F (or 320°F),
then the set point is changed to 350°F, the Deviation Value alarm
would be 350°F plus 20°F (or 370°F). See Process Alarm.
Deviation Meter–the display of process temperature on meter that
indicates temperature relative to the set point.
Dielectric–an electrical insulator - a material with low electrical
Dielectric Strength–an amount of voltage that an insulating
material can withstand before an electrical breakdown occurs.
Differential–in an on/off control, the temperature difference
expressed in degrees between where the control switches off and the
control switches on.
Differential Mode Line Filter–a device to filter noise signals
between two power lines.
Digital Indication–the actual process temperature in indicated by
Digital Set Point–the desired temperature value is set by means of a
pushbutton or pushwheel switch.
DIN–Deutsche Industrial Norms, a German agency that sets
engineering standards.
Diode–Device that allows current to flow in only one direction.
Drift–a change in a value over a long period due to changes in
factors such as ambient temperature, time or line voltage.
Droop–in time proportioning controls, the difference in temperature
between the set point and where the system temperature stabilizes.
Corrected by automatic or manual reset.
Dual Output–the primary output will regulate the process
temperature. Asecondary output will be utilized for process cooling
or as an alarm.
Duty Cycle–the time to complete one ON/OFF cycle.
Efficiency–the amount of useful output versus energy input.
Electromagnetic Interference (EMI)–electrical and magnetic “noise”
than can be generated when switching AC power. EMI can interfere
with the operation of microprocessor based controls.
Emissivity–The ratio of radiant energy emitted from a surface compared to the radiant energy emitted from a black body at the same temperature.
Endothermic–a process is endothermic when it absorbs heat.
Enthalpy–the sum of the internal energy of a body and the product of its
volume multiplied by the pressure used to evaluate the energy change
occurring when a vapor or gas is heated. Expressed in units of Btu/lb.
or Joules/gram.
Error–the difference between the correct value and the reading or
display value.
Exothermic-a process is exothermic when it generates heat.
Event–a programmable On/Off output used to signal peripheral
equipment or a process.
Flow Rate–speed or velocity of fluid movement.
Form A Relay–Single pole, single throw relay with Normally Open
(NO) and common contacts. When heat is required for a process,
the contacts will close.
Form B Relay–Single pole, single throw relay with Normally Closed
(NC) and common contacts. Contacts are open when coil is energized.
Form C Relay–Single pole, double throw relay with Normally Open
(NO), Normally Closed (NC) and common contacts. Can be selected
as Form A or Form B contact.
fpm–flow velocity in feet per minute.
fps–flow velocity in feet per second.
Fahrenheit–a temperature scale with 32°F defined as the ice point
and 212°F as the boiling point of water at sea level.
Frequency–the number of event occurrences or cycles over a
specified period of time.
Freezing Point–the temperature where a material changes from a
liquid to a solid.
Fuse–A device that interrupts power in a circuit when an overload
Fuzzy Logic–An artificial intelligence technique that allows control
decisions to be made upon approximate or incomplete information.
Fuzzy Logic is a continuous decision making function that can prevent
initial overshoot and set point differentials.
GIGA–the prefix for one billion (G).
gph–the volumetric flow rate in gallons per hour.
gpm–the volumetric flow rate in gallons per minute.
Ground–the electrical neutral line having the same potential as the
surrounding earth; the negative side of a DC power supply; the
reference point for an electrical system.
Grounded Junction–Athermocouple junction in which the sheath
and conductors are welded together forming a completely sealed
integrated junction.
Heat–thermal energy expressed in Calories, Btu’s or Joules.
Heat Balance–proper sizing of the heat source to the requirements of
the system (including heat losses) (See: “Calculating Heating
Requirements” in the Engineering Section).
Heat of Fusion–the amount of energy required to change one pound
of a material from a solid to a liquid without an increase in
temperature. Expressed in Btu/lb.
Heat of Vaporization–the amount of energy required to change one
pound of a material from a liquid to a vapor without an increase in
temperature. Expressed in Btu/lb.
Heat Sink–heat conducting material used to dissipate heat.
Heat Transfer–a process of thermal energy flowing from one body to
1) Conduction: the transfer of heat from one particle of matter to
2) Convection: the transfer of heat from one part of a particle to
another by the mixing of the warmer particles with the cooler.
3) Radiant: the transfer of heat from one body to another as the
result of the bodies emitting and absorbing radiation energy.
Heat Transfer Medium–a gas, liquid or solid through which heat
flows from the heat source to the work.
Hertz–units of expression for frequency, measured in cycles per
Hi-Pot Test–to apply a high voltage to an electrical conductor to test
the surrounding insulation.
Hysteresis–the temperature sensitivity designed into the on/off control action between the on and off switching points. Expressed in percentage of control range.
Ice Point–the temperature where pure water freezes (0°C or 32°F).
Impedance–the total opposition in a circuit to the flow of alternating
current. Measured in ohms and represented by “Z”.
Infrared–or radiation is the exchange of energy by electromagnetic
waves. The infrared spectrum extends from the deep red end of the
visible spectrum to the microwave region of the radio spectrum, The
portion adjacent to the visible spectrum is of importance to heating.
Radiant heat transfer can be very efficient in directing energy from the
heat source to an object.
Isolation–Electrical Separation
Isothermal–a process or area that maintains a constant
Integral–(See Automatic Reset).
Joule–the basic unit of thermal energy. 1 Joule equals 1 ampere
passed through a resistance of 1 ohm for 1 second.
Junction–Athermocouple junction is the point at which two alloys are
joined. A typical thermocouple circuit would have a measuring and a
refernce junction.
Kelvin–the unit of absolute or thermodynamic temperature scale.
Zero Kelvin is absolute zero, where all molecular activity stops. No °
symbol is used. 0°C = 273.15K; 100°C = 373.15K.
Kilo–the prefix for one thousand (K).
Kilowatt (kw)–1000 watts or 3412 Btu per hour.
Kilowatt Hour–electrical unit of energy expended by one kilowatt in
one hour.
Least Significant Digit–The digit farthest to the right in a display.
Linearity–the deviation of an instrument’s response from a straight
Load–the electrical demand of a process expressed as wattage,
amps or resistance (ohms).
Manual Reset–the adjustment on a proportional control which shifts
the proportioning band in relation to the set point to eliminate droop of
offset errors.
Mass Flow Rate–weight of a substance flowing per unit of time past
a specific cross-sectional area within a system.
Mean Temperature–the maximum and minimum temperature
average of a process at equilibrium.
Measuring Junction–the thermocouple junction at the point of
measurement in the process.
Mega–the prefix for one million (M) (106).
Mechanical Relay–an electromechanical device that completes or
breaks a circuit by opening or closing electrical contacts.
Micro–The prefix for one millionth (10 -6).
Microamp–10-6 amps (one millionth of an amp).
Micron–10-6 meters (one millionth of a meter).
Milli–The prefix for one thousndth (10-3).
Microprocessor–The central processing unit (CPU) that performs the
logic operations in a micro-computer system. The microprocessor in
a process or instrument control decodes instructions from the stored
program, performs algorithmic and logic functions, and produces
signals and commands.
Milliamp–10-3 amps (one thousandth of an amp).
Millivolt–10-3 volts (one thousandth of a volt).
NEC–National Electrical Code
NEMA–National Electrical Manufacturer’s Association
Noise–undesirable electrical interference on the signal wires.
Noise Suppression–a device used to reduce electrical interference.
Normal Mode Rejection Ratio–the ability of an instrument to reject
interference of the line frequency (50-60Hz) across the input
NPT–National Pipe Thread
Offset–the difference in temperature between the set point and the
actual process temperature.
OHM–The unit of electric resistance.
On-Off–a control whose action is full on or full off.
Open Loop Control–a control system with no sensing feedback.
Overshoot–excursion of temperature above the set point.
Phase–time based relationship between an intermittent function and a
reference. Electrically, the expression is in angular degrees to
describe the voltage or current relationship of two alternating
Phase Proportioning–a temperature control form where the power
supplied to the process is controlled by limiting the phase angle of the
line voltage.
PID–three mode temperature control–proportional, integral (automatic
reset), derivative (rate).
Polarity–having two oppositely charged poles; one positive, one
Potting–The sealing of components with a compound such as epoxy
to protect against moisture and other contaminates.
Process Alarm–a fixed alarm or secondary set point value
independent of the primary set point. Should a process value exceed
this value, an alarm condition would register.
Process Variable–the parameter being controlled or measured such
as temperature, relative humidity, flow, level, pressure, etc.
Proportioning Band–a temperature band in degrees within which a
control’s proportioning function is active.
Proportioning Control Mode–when process temperature
approaches set point and enters the proportioning band, the output is
switched on and off at the established cycle time. The change in
power to the load provides a throttling action which results in less
temperature overshoot. This cycling will continue until on and off
times are equal.
psia–pounds per square inch absolute. Pressure reference to a
psig–pound per square inch gage. Pressure reference to ambient air
Quality of Steam–the relative amount of liquid present in saturated
steam as a percent of the total weight. The quality of steam is 100%
less the percent liquid. Dry saturated steam has a quality of 100%.
Ramp–a programmed rise in temperature.
Range–an area between two limits in which a measurement or
control action takes place. Typically expressed in upper and lower
Rankine–an absolute temperature scale based upon the Fahrenheit
scale with 180° between the ice point and boiling point of water.
0°F = 459.67°R.
Rate (derivative)–a control function that measures the rate of
increase or decrease of the system temperature and brings the
control into an accelerated proportioning action. This mode prevents
an overshoot condition at initial heat-up and with system
Rate Time–the interval over which the system temperature is
sampled for the derivative function.
Repeatability–the ability to give the same output or measurement
under repeated identical conditions.
Resistance–the resistance to the flow of electric current measured in
Resolution Sensitivity–the amount of temperature change that must
occur before the control will actuate. It may be expressed in
temperature or as a percentage of the control’s scale.
Response Time–In analog instruments, the time required for a
change of the measured quantity to change the indication.
In sensors, the time required to reach 63.2% of the step change.
Retransmit Output–analog output scaled to the process or the set
point value.
RS232 or RS485 Output Signal–A serial interface suitable for
connection between a digital control and a personal computer, a host
computer or printer.
RTD–a temperature sensing probe of finely wound platinum wire that
displays a linear resistance change for a corresponding temperature
change. The resistance increases as the temperature rises. A base
resistance of 100 ohms at 32°F is the industry (DIN) standard.
Saturation Temperature–the boiling temperature of a liquid at the
existing pressure.
SCFM–Volumetric flow rate in cubic feet per minute at 60°F (15°C)
and standard atmospheric pressure.
SCR–Silicone Controlled Rectifier
Sensor Breakdown Protection–circuitry which ensures safe process
shut down in the event of sensor failure.
Serial Communications–A method of transmitting data between
Set Point–control setting to achieve or maintain temperature.
Shape Factor–in radiant applications, the amount of energy received
by the target relative to heater rating and distance to the target.
Shield–material surrounding a conductor(s) to prevent electrostatic or
EMI from external sources.
Slide Wire Feedback–Apotentiometer that varies the resistance to
control a valve position.
Soak–To raise the temperature of a metal object in a heated
environment to produce a metallurgical change.
Standard–a reference point from which references or calibrations are
Soft Start–reduces voltage on initial start-up which reduces power to
the heaters. If heater has accumulated moisture internally during a
shut down, soft start will allow heater to dry before full voltage is
applied extending heater life.
Solid State Relay–a solid state switching device which completes or
breaks a circuit electrically with no moving parts.
Span–the difference between the upper and lower limits of a
controller’s range.
Specific Gravity–the ratio of mass of any material to the same
volume of pure water at 4°C.
Specific Heat–the ratio of thermal energy required to raise the
temperature of a particle 1 degree to the thermal energy required to
raise an equal mass of water 1 degree.
Speed of Response–time needed for a temperature change
occurring at the sensor to be translated into a control action.
Stability–the ability of an instrument or sensor to maintain a
constant output when a constant input is applied.
Standard–a reference point from which references or calibrations are
Super Heating–the heating of a liquid above its boiling temperature
without changing to a gaseous state; or the heating of a gas
considerably above the boiling temperature.
Surge Current–a current of short duration occurring when power is
initially applied to capacitive or resistive loads, usually lasting no more
than several cycles.
Temperature Gradient–the range of temperature variations at
various physical locations throughout a thermal system.
Tera–the prefix for one trillion(T).
Thermal Conductivity–the property of a material to conduct heat.
Thermal Expansion–an increase in size due to an increase in
Thermal Lag–the time delay in the distribution of heat throughout a
thermal system.
Thermal System–a series of components arranged and designed to
provide heat. The four elements or components compromising a
Thermal System are:
1) work or load
2) heat source
3) heat transfer medium
4) control system
Thermistor–a temperature sensing probe manufactured of a mixture of
metal oxides then encapsulated in epoxy or glass. A large change in
resistance is exhibited proportional to a change in temperature. The
resistance usually decreases as temperature rises.
Thermocouple–a temperature sensing probe consisting of the
junction of two dissimilar metals which has a millivolt output
proportional to the difference in temperature between the “hot”
junction and the lead wires (cold junction).
Thermowell–a closed-end tube into which a temperature sensor is
inserted to isolate it from the environment.
Transducer–a device that converts a measured variable into another
form which is the transducer ’s output. A thermocouple transforms heat
to a millivolt output.
Transmitter–a device used to transmit temperature data from the sensor.
Undershoot–excursion of temperature below set point.
Ungrounded Junction–Athermocouple junction fully insulated from
the sheath.
Viscosity–the inherent resistance of a substance to flow
Voltage–an electrical potential which is measured in volts.
Wattage–a measurement of electrical power. In a resistive circuit,
VI =W (See Ohms Law formulas).
Watt Density–the rated wattage of an element per unit of surface area.
Usually expressed in watts per square inch.
Zero Voltage Switching–completing or breaking of a circuit when the
voltage wave form crosses zero voltage.
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