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Presenting . . . OMEGA’s Temperature
Measurement and Control Glossary
( Printable Version )
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A comprehensive glossary of terms used in the field of temperature measurement and control. A helpful reference
tool for scientists, engineers, and technicians!
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A
Absolute Zero: Temperature at which thermal energy is at a minimum.
Defined as 0 Kelvin, calculated to be –273.15°C or –459.67°F.
AC: Alternating current; an electric current that reverses its direction at
regularly recurring intervals.
Accuracy: The closeness of an indication or reading of a measurement
device to the actual value of the quantity being measured. Usually
expressed as ± percent of full scale output or reading.
Adaptor: A mechanism or device for attaching non-mating parts.
ADC: Analog-to-Digital Converter: an electronic device which converts
analog signals to an equivalent digital form, in either a binary code
or a binary-coded decimal code. When used for dynamic
waveforms, the sampling rate must be high to prevent aliasing
errors from occurring.
Address: The label or number identifying the memory location where a
unit of information is stored.
Aliasing: If the sample rate of a function (fs) is less than two times the
highest frequency value of the function, the frequency is
ambiguously presented. The frequencies above (fs/2) will be folded
back into the lower frequencies producing erroneous data.
Alloy 11: A compensating alloy used in conjunction with pure copper
as the negative leg to form extension wire for platinum—platinumrhodium
thermocouples Types R and S.
Alloy 200/226: The combination of compensating alloys used with
tungsten vs. tungsten/26%-rhenium thermocouples as extension
cable for applications under 200°C.
Alloy 203/225: The combination of compensating alloys used with
tungsten/3%-rhenium vs. tungsten/25%-rhenium thermocouples as
extension cable for applications under 200°C.
Alloy 405/426: The combination of compensating alloys used with
tungsten/5%-rhenium vs. tungsten/26%-rhenium thermocouples as
extension cable for applications under 870°C.
ALOMEGA®: An aluminum nickel alloy used in the negative leg of a
type K thermocouple (registered trademark of OMEGA
ENGINEERING, INC.).
Alphanumeric: A character set that contains both letters and digits.
Alumel: An aluminum nickel alloy used in the negative leg of a Type K
thermocouple (Trade name of Hoskins Manufacturing Company).
Ambient Compensation: The design of an instrument such that
changes in ambient temperature do not affect the readings
of the instrument.
Ambient Conditions: The conditions around the transducer
(pressure, temperature, etc.).
Ambient Temperature: The average or mean temperature of the
surrounding air which comes in contact with the equipment and
instruments under test.
Ammeter: An instrument used to measure current.
Ampere (amp): A unit used to define the rate of flow of electricity
(current) in a circuit; units are one coulomb (6.28 x 1018 electrons)
per second.
Amplifier: A device which draws power from a source other than the
input signal and which produces as an output an enlarged
reproduction of the essential features of its input.
Amplitude: A measurement of the distance from the highest to the
lowest excursion of motion, as in the case of mechanical body in
oscillation or the peak-to-peak swing of an electrical waveform.
Analog Output: A voltage or current signal that is a continuous
function of the measured parameter.
Analog-to-Digital Converter (A/D or ADC): A device or circuit that
outputs a binary number corresponding to an analog signal level at
the input.
Angstrom: Ten to the minus tenth (10–10) meters or one millimicron, a
unit used to define the wavelength of light. Designated by the
symbol Å.
ANSI: American National Standards Institute.
Anti-Reset Windup: This is a feature in a three-mode PID controller
which prevents the integral (auto reset) circuit from functioning
when the temperature is outside the proportional band.
Application Program: A computer program that accomplishes specific
tasks, such as word processing.
ASCII: American Standard Code for Information Interchange. A seven
or eight bit code used to represent alphanumeric characters. It is
the standard code used for communications between data
processing systems and associated equipment.
ASME: American Society of Mechanical Engineers.
Assembler: A program that translates assembly language instructions
into machine language instructions.
ASTM: American Society for Testing and Materials.
Asynchronous: A communication method where data is sent when it is
ready without being referenced to a timing clock, rather than
waiting until the receiver signals that it is ready to receive.
ATC: Automatic temperature compensation.
Auto-Zero: An automatic internal correction for offsets and/or drift at
zero voltage input.
Automatic Reset: 1. A feature on a limit controller that automatically
resets the controller when the controlled temperature returns to
within the limit bandwidth set. 2. The integral function on a PID
controller which adjusts the proportional bandwidth with respect to
the set point to compensate for droop in the circuit, i.e., adjusts the
controlled temperature to a set point after the system stabilizes.
AWG: American Wire Gauge.
B
Background Noise: The total noise floor from all sources of
interference in a measurement system, independent of the presence
of a data signal.
Backup: A system, device, file or facility that can be used as an
alternative in case of a malfunction or loss of data.
Bandwidth: A symmetrical region around the set point in which
proportional control occurs.
Basic: A high-level programming language designed at Dartmouth
College as a learning tool. Acronym for Beginner’s All-purpose
Symbolic Instruction Code.
Baud: A unit of data transmission speed equal to the number of bits
(or signal events) per second; 300 baud = 300 bits per second.
BCD, Buffered: Binary-coded decimal output with output drivers, to
increase line-drive capability.
BCD, Parallel: A digital data output format where every decimal digit
is represented by binary signals on four lines and all digits are
presented in parallel. The total number of lines is 4 times the
number of decimal digits.
BCD, Serial: A digital data output format where every decimal digit is
represented by binary signals on four lines and up to five decimal
digits are presented sequentially. The total number of lines is four
data lines plus one strobe line per digit.
BCD, Three-State: An implementation of parallel BCD, which has
0, 1 and high-impedance output states. The high-impedance
state is used when the BCD output is not addressed in parallel
connect applications.
Beryllia: BeO (Beryllium Oxide), a high-temperature mineral insulation
material; toxic when in powder form.
BIAS Current: A very low-level DC current generated by a panel meter
and superimposed on a signal. This current may introduce a
measurable offset across a very high source impedance.
Binary Coded Decimal (BCD): The representation of a decimal number
(base 10, 0 through 9) by means of a 4-bit binary nibble.
Binary: Refers to the base 2 numbering system, in which the only
allowable digits are 0 and 1. Pertaining to a condition that has only
two possible values or states.
Bipolar: The ability of a panel meter to display both positive and
negative readings.
Bit: Acronym for binary digit. The smallest unit of computer
information, it is either 0 or 1.
Blackbody: A theoretical object that radiates the maximum amount of
energy at a given temperature, and absorbs all the energy incident
upon it. A blackbody is not necessarily black. (The name blackbody
was chosen because the colour black is defined as the total
absorption of light energy.)
BNC: A quick disconnect electrical connector used to interconnect
and/or terminate coaxial cables.
Boiling Point: The temperature at which a substance in the liquid
phase transforms to the gaseous phase; commonly refers to the
boiling point of water which is 100°C (212°F) at sea level.
BPS: Bits per second.
Breakdown Voltage Rating: The dc or ac voltage which can be applied
across insulation portions of a transducer without arcing or
conduction above a specific current value.
BTU: British thermal unit. The quantity of thermal energy required to
raise one pound of water at its maximum density, 1 degree F. One
BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt
hour is equivalent to 3412 BTU.
Bulb (Liquid-in-Glass Thermometer): The area at the tip of a liquid-inglass
thermometer containing the liquid reservoir.
Burn-In: A long term screening test (either vibration, temperature or
combined test) that is effective in weeding out premature failures
because it simulates actual or worst case operation of the device,
accelerated through a time, power, and temperature relationship.
Burst Proportioning: A fast-cycling output form on a time
proportioning controller (typically adjustable from 2 to 4 seconds)
used in conjunction with a solid state relay to prolong the life of
heaters by minimizing thermal stress.
Bus: Parallel lines used to transfer signals between devices or
components. Computers are often described by their bus structure
(i.e., S-100, IBM PC).
Byte: The representation of a character in binary. Eight bits.
C
Calender-van Dusen Equation: An equation that defines the
resistance-temperature value of any pure metal that takes the form
of (RT = RO) (1 + AT + BT2) for values between the ice point (0°C)
and the freezing point of antimony (630.7°C) and the form RT = RO
[1 + AT + BT2 + C (T–100)T2] between the oxygen point (–183.0°C)
and the ice point (0°C).
Calibration: The process of adjusting an instrument or compiling a
deviation chart so that its reading can be correlated to the actual
value being measured.
Calorie: The quantity of thermal energy required to raise one gram
of water 1°C at 15°C.
Cavitation: The boiling of a liquid caused by a decrease in pressure
rather than an increase in temperature.
Celsius (Centigrade): A temperature scale defined by 0°C at the ice
point and 100°C at the boiling point of water at sea level.
Ceramic Insulation: High-temperature compositions of metal oxides used
to insulate a pair of thermocouple wires. The most common are
Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO). Their application
depends upon temperature and type of thermocouple. High-purity
alumina is required for platinum alloy thermocouples. Ceramic
insulators are available as single and multihole tubes or as beads.
Ceramic: Polycrystalline ferroelectric materials which are used as the
sensing units in piezoelectric accelerometers. There are many
different grades, all of which can be made in various configurations
to satisfy different design requirements.
Character: A letter, digit or other symbol that is used as the
representation of data. A connected sequence of characters is called
a character string.
Chatter: The rapid cycling on and off of a relay in a control process
due to insufficient bandwidth in the controller.
CHROMEGA®: A chromium-nickel alloy which makes up the positive
leg of type K and type E thermocouples (registered trademark of
OMEGA ENGINEERING, INC.).
Clear: To restore a device to a prescribed initial state, usually the zero state.
Clipping: The term applied to the phenomenon which occurs when an
output signal is limited in some way by the full range of an
amplifier, ADC or other device. When this occurs, the signal is
flattened at the peak values, the signal approaches the shape of
a square wave, and high frequency components are introduced.
Clipping may be hard, as is the case when the signal is strictly
limited at some level, or it may be soft, in which case the clipping
signal continues to follow the input at some reduced gain.
Clock: The device that generates periodic signals for synchronization.
Closeness of Control: Total temperature variation from a desired set
point of system. Expressed as “closeness of control” is ±2°C or
a system bandwidth with 4°C, also referred to as “amplitude of
deviation.”
CMR (Common-Mode Rejection): The ability of a panel meter to
eliminate the effect of AC or DC noise between signal and ground.
Normally expressed in dB at dc to 60 Hz. One type of CMR is
specified between SIG LO and PWR GND. In differential meters,
a second type of CMR is specified between SIG LO and ANA GND
(METER GND).
CMV (Common-Mode Voltage): The AC or DC voltage which is
tolerable between signal and ground. One type of CMV is specified
between SIG LO and PWR GND. In differential meters, a second
type of CMV is specified between SIG HI or LO and ANA GND
(METER GND).
Colour Code: The ANSI established colour code for thermocouple
wires in the negative lead is always red. Colour Code for base metal
thermocouples is yellow for Type K, black for Type J, purple for
Type E and blue for Type T.
Common Mode: The output form or type of control action used by
a temperature controller to control temperature, i.e. on/off, time
proportioning, PID.
Common-Mode Rejection Ratio: The ability of an instrument to reject
interference from a common voltage at it’s input terminals with
relation to ground, usually expressed in dB (decibels).
Communication: Transmission and reception of data among data
processing equipment and related peripherals.
Compensated Connector: A connector made of thermocouple alloys
used to connect thermocouple probes and wires.
Compensating Alloys: Alloys used to connect thermocouples to
instrumentation. These alloys are selected to have similar thermal
electric properties as the thermocouple alloys (however, only over
a very limited temperature range).
Compensating Loop: Lead wire resistance compensation for RTD
elements where an extra length of wire is run from the instrument to
the RTD and back to the instrument, with no connection to the RTD.
Compensation: An addition of specific materials or devices to
counteract a known error.
Compiler: A program that translates a high-level language, such
as Basic, into machine language.
Conductance: The measure of the ability of a solution to carry
an electrical current. (See Equivalent Conductance)
Conduction: The conveying of electrical energy or heat through
or by means of a conductor.
Confidence Level: The range (with a specified value of uncertainty,
usually expressed in percent) within which the true value of a
measured quantity exists.
Conformity Error: For thermocouples and RTD’s, the difference
between the actual reading and the temperature shown in published
tables for a specific voltage input.
Connection Head: An enclosure attached to the end of a thermocouple
which can be cast iron, aluminum or plastic within which the
electrical connections are made.
Constantan: A copper-nickel alloy used as the negative lead in Type E,
Type J, and Type T thermocouples.
Continuous Spectrum: A frequency spectrum that is characterized
by non-periodic data. The spectrum is continuous in the frequency
domain and is characterized by an infinite number of frequency
components.
Control Character: A character whose occurrence in a particular
context starts, modifies or stops an operation that affects the
recording, processing, transmission or interpretation of data.
Control Mode: The output form or type of control action used
by a temperature controller to control temperature, i.e., on/off,
time proportioning, PID.
Control Point: The temperature at which a system is to be maintained.
Convection: 1. The circulatory motion that occurs in a fluid at a nonuniform
temperature owing to the variation of its density and the action
of gravity. 2. The transfer of heat by this automatic circulation of fluid.
Counts: The number of time intervals counted by the dual-slope A/D
converter and displayed as the reading of the panel meter, before
addition of the decimal point.
CPS: Cycles per second; the rate or number of periodic events in one
second, expressed in Hertz (Hz).
CPU: Central processing unit. The part of the computer that contains
the circuits that control and perform the execution of computer
instructions.
Critical Damping: Critical damping is the smallest amount of damping
at which a given system is able to respond to a step function
without overshoot.
Cryogenics: Measurement of temperature at extremely low values,
i.e., below –200°C.
CSA: Canadian Standards Administration.
Current Proportioning: An output form of a temperature
controller which provides a current proportional to the amount
of control required. Normally, a 4 to 20 milliamp current
proportioning band.
Current: The rate of flow of electricity. The unit is the ampere (a)
defined as 1 ampere = 1 coulomb per second.
Curve Fitting: Curve fitting is the process of computing the coefficients
of a function to approximate the values of a given data set within
that function. The approximation is called a “fit”. A mathematical
function, such as a least squares regression, is used to judge the
accuracy of the fit.
Cycle Time: The time, usually expressed in seconds, for a controller
to complete one on/off cycle.
D
Damping: The reduction of vibratory movement through dissipation
of energy. Types include viscous, coulomb, and solid.
Data Base: A large amount of data stored in a well-organized manner.
A data base management system (DBMS) is a program that allows
access to the information.
dB (Decibel): 20 times the log to the base 10 of the ratio of two
voltages. Every 20 dB’s correspond to a voltage ratio of 10, every
10 dB’s to a voltage ratio of 3.162. For instance, a CMR of 120 dB
provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB
provides voltage noise rejection of 3,162/1.
DC: Direct current; an electric current flowing in one direction only
and substantially constant in value.
Deadband: 1. For chart records: the minimum change of input signal
required to cause a deflection in the pen position. 2. For temperature
controllers: the temperature band where heat is turned off upon rising
temperature and turned on upon falling temperature expressed in
degrees. The area where no heating (or cooling) takes place.
Debug: To find and correct mistakes in a program.
Decimal: Refers to a base ten number system using the characters
0 through 9 to represent values.
Default: The value(s) or option(s) that are assumed during operation
when not specified.
Degree: An incremental value in the temperature scale, i.e., there are
100 degrees between the ice point and the boiling point of water
in the Celsius scale and 180°F between the same two points in the
Fahrenheit scale.
Density: Mass per unit of volume of a substance, i.e.: grams/cu.cm.
or pounds/cu.ft.
Deviation: The difference between the value of the controlled variable
and the value at which it is being controlled.
Differential Input: A signal-input circuit where SIG LO and SIG HI are
electrically floating with respect to ANALOG GND (METER GND,
which is normally tied to DIG GND). This allows the measurement
of the voltage difference between two signals tied to the same
ground and provides superior common-mode noise rejection.
Differential: For an on/off controller, it refers to the temperature
difference between the temperature at which the controller turns
heat off and the temperature at which the heat is turned back on.
It is expressed in degrees.
Digit: A measure of the display span of a panel meter. By convention,
a full digit can assume any value from 0 through 9, a 1/2-digit will
display a 1 and overload at 2, a 3/4-digit will display digits up to 3
and overload at 4, etc. For example, a meter with a display span of
±3999 counts is said to be a 3 3/4 digit meter.
Digital Output: An output signal which represents the size of an input
in the form of a series of discrete quantities.
Digital-to-Analog Converter (D/A or DAC): A device or circuit
to convert a digital value to an analog signal level.
DIN (Deutsche Industrial Norm): A set of German standards
recognized throughout the world. The 1/8 DIN standard for panel
meters specifies an outer bezel dimension of 96 x 48 mm and a
panel cutout of 92 x 45 mm.
DIN 43760: The standard that defines the characteristics of a 100 ohm
platinum RTD having a resistance vs. temperature curve specified
by a = 0.00385 ohms per degree.
Discharge Time Constant: The time required for the output-voltage
from a sensor or system to discharge 37% of its original value in
response to a zero rise time step function input. This parameter
determines a low frequency response.
Disk Operating System (DOS): Program used to control the transfer of
information to and from a disk, such as MS DOS.
Displacement: The measured distance traveled by a point from its
position at rest. Peak to peak displacement is the total measured
movement of a vibrating point between its positive and negative
extremes. Measurement units expressed as inches or milli-inches.
Dissipation Constant: The ratio for a thermistor which relates a change
in internal power dissipation to a resultant change of body
temperature.
Drift: A change of a reading or a set point value over long periods due
to several factors including change in ambient temperature, time,
and line voltage.
Droop: A common occurrence in time-proportional controllers. It
refers to the difference in temperature between the set point and
where the system temperature actually stabilizes due to the timeproportioning
action of the controller.
Dual Element Sensor: A sensor assembly with two independent
sensing elements.
Dual-Slope A/D Converter: An analog-to-digital converter which
integrates the signal for a specific time, then counts time intervals
for a reference voltage to bring the integrated signal back to zero.
Such converters provide high resolution at low cost, excellent
normal-mode noise rejection, and minimal dependence on circuit
elements.
Duplex: Pertaining to simultaneous two-way independent data
communication transmission in both directions. Same as
“full duplex”.
Duplex Wire: A pair of wires insulated from each other and with
an outer jacket of insulation around the inner insulated pair.
Duty Cycle: The total time to one on/off cycle. Usually refers to the
on/off cycle time of a temperature controller.
Dynamic Calibration: Calibration in which the input varies over a
specific length of time and the output is recorded vs. time.
E
Echo: To reflect received data to the sender. For example, keys
depressed on a keyboard are usually echoed as characters
displayed on the screen.
Electrical Interference: Electrical noise induced upon the signal wires
that obscures the wanted information signal.
Electromotive Force (emf): The potential difference between the two
electrodes in a cell. The cell emf is the cell voltage measured when
no current is flowing through the cell. It can be measured by means
of a pH meter with high input impedance.
Electronic Industries Association (EIA): A standards organization
specializing in the electrical and functional characteristics of
interface equipment.
EMF: Electromotive force. A rise in (electrical) potential energy.
The principal unit is the volt.
EMI: Electromagnetic interference.
Emissivity: The ratio of energy emitted by an object to the energy
emitted by a blackbody at the same temperature. The emissivity of
an object depends upon its material and surface texture; a polished
metal surface can have an emissivity around 0.2 and a piece of
wood can have an emissivity around 0.95.
Endothermic: A process is said to be endothermic when it absorbs
heat.
End Point (Potentiometric): The apparent equivalence point of a
titration at which a relatively large potential change is observed.
Enthalpy: The sum of the internal energy of a body and the product
of its volume multiplied by the pressure.
Environmental Conditions: All conditions to which a transducer
may be exposed during shipping, storage, handling, and operation.
Eprom: Erasable Programmable Read-Only Memory. The PROM
can be erased by ultraviolet light or electricity.
Error: The difference between the value indicated by the transducer
and the true value of the measured value being sensed. Usually
expressed in percent of full scale output.
Error Band: The allowable deviations to output from a specific
reference norm. Usually expressed as a percentage of full scale.
Eutectic Temperature: The lowest possible melting point of a mixture
of alloys.
Excitation: The external application of electrical voltage current applied
to a transducer for normal operation.
Exothermic: A process is said to be exothermic when it releases heat.
Expansion Factor: Correction factor for the change in density between
two pressure measurement areas in a constricted flow.
Explosion-Proof Enclosure: An enclosure that can withstand an
explosion of gases within it and prevent the explosion of gases
surrounding it due to sparks, flashes or the explosion of the
container itself, and maintain an external temperature which will
not ignite the surrounding gases.
Exposed Junction: A form of construction of a thermocouple probe
where the hot or measuring junction protrudes beyond the sheath
material so as to be fully exposed to the medium being measured.
This form of construction usually gives the fastest response time.
F
Fahrenheit: A temperature scale defined by 32° at the ice point
and 212° at the boiling point of water at sea level.
Ferrule: A compressible tubular fitting that is compressed onto
a probe inside a compression fitting to form a gas-tight seal.
Field of View: A volume in space defined by an angular cone extending
from the focal plane of an instrument.
File: A set of related records or data treated as a unit.
Firmware: Programs stored in PROM’s.
Flag: Any of various types of indicators used for identification
of a condition or event, for example, a character that signals the
termination of a transmission.
Floppy Disk: A small, flexible disk carrying a magnetic medium
in which digital data is stored for later retrieval and use.
FM: Factory Mutual Research Corporation. An organization which sets
industrial safety standards.
FM Approved: An instrument that meets a specific set of specifications
established by Factory Mutual Research Corporation.
FORTRAN: Formula Translation language. A widely used high-level
programming language well suited to problems that can be
expressed in terms of algebraic formulas. It is generally used
in scientific applications.
reezing Point: The temperature at which a substance goes from
the liquid phase to the solid phase.
Frequency: The number of cycles over a specified time period over
which an event occurs. The reciprocal is called the period.
Frequency Modulated Output: A transducer output which is obtained
in the form of a deviation from a center frequency, where the
deviation is proportional to the applied stimulus.
Frequency, Natural: The frequency of free (not forced) oscillations
of the sensing element of a fully assembled transducer.
Frequency Output: An output in the form of frequency which varies
as a function of the applied input.
Full Scale Output: The algebraic difference between the minimum
output and maximum output.
G
Gain: The amount of amplification used in an electrical circuit.
Galvanometer: An instrument that measures small electrical currents
by means of deflecting magnetic coils.
Ground: 1. The electrical neutral line having the same potential as
the surrounding earth. 2. The negative side of DC power supply.
3. Reference point for an electrical system.
Grounded Junction: A form of construction of a thermocouple probe
where the hot or measuring junction is in electrical contact with the
sheath material so that the sheath and thermocouple will have the
same electrical potential.
H
Half-Duplex: One way at a time data communication; both devices can
transmit and receive data, but only one at a time.
Handshake: An interface procedure that is based on status/data signals
that assure orderly data transfer as opposed to asynchronous
exchange.
Hardcopy: Output in a permanent form (usually a printout) rather than
in temporary form, as on disk or terminal display.
Hardware: The electrical, mechanical and electromechanical
equipment and parts associated with a computing system,
as opposed to its firmware or software.
Heat: Thermal energy. Heat is expressed in units of calories or BTU’s.
Heat Sink: 1. Thermodynamic. A body which can absorb thermal
energy. 2. Practical. A finned piece of metal used to dissipate the
heat of solid state components mounted on it.
Heat Transfer: The process of thermal energy flowing from a body
of high energy to a body of low energy. Means of transfer are:
conduction; the two bodies contact. Convection; a form of
conduction where the two bodies in contact are of different phases,
i.e. solid and gas. Radiation: all bodies emit infrared radiation.
Heat Treating: A process for treating metals where heating to a
specific temperature and cooling at a specific rate changes the
properties of the metal.
Hertz (Hz): Units in which frequency is expressed. Synonymous
with cycles per second.
Hexadecimal: Refers to a base sixteen number system using the
characters 0 through 9 and A through F to represent the values.
Machine language programs are often written in hexadecimal notation.
Hold: Meter HOLD is an external input which is used to stop the
A/D process and freeze the display. BCD HOLD is an external input
used to freeze the BCD output while allowing the A/D process to
continue operation.
Host: The primary or controlling computer in a multiple part system.
Hysteresis: The difference in output when the measurand value is first
approached with increasing and then with decreasing values.
Expressed in percent of full scale during any one calibration cycle.
See Deadband
I
Impedance: The total opposition to electrical flow (resistive
plus reactive).
Infrared: an area in the electromagnetic spectrum extending beyond
red light from 760 nanometers to 1000 microns (106 nm). It is the
form of radiation used for making non-contact temperature
measurements.
Input Impedance: The resistance of a panel meter as seen from the
source. In the case of a voltmeter, this resistance has to be taken
into account when the source impedance is high; in the case of
an ammeter, when the source impedance is low.
Insulated Junction: See Ungrounded Junction
Insulation Resistance: The resistance measured between two
insulated points on a transducer when a specific dc voltage
is applied at room temperature.
Integral: A form of temperature control. See Automatic Reset (2)
Interchangeability Error: A measurement error that can occur if two or
more probes are used to make the same measurement. It is caused
by a slight variation in characteristics of different probes.
Interface: The means by which two systems or devices are connected
and interact with each other.
Interrupt: To stop a process in such a way that it can be resumed.
Intrinsically Safe: An instrument which will not produce any spark
or thermal effects under normal or abnormal conditions that will
ignite a specified gas mixture.
IPTS-48: International Practical Temperature Scale of 1948. Fixed
points in thermometry as specified by the Ninth General Conference
of Weights and Measures which was held in 1948.
IPTS-68: International Practical Temperature Scale of 1968. Fixed
points in thermometry set by the 1968 General Conference of
Weights and Measures.
ISA: Instrument Society of America.
Isolation: The reduction of the capacity of a system to respond
to an external force by use of resilient isolating materials.
Isothermal: A process or area that is a constant temperature.
Joule: The basic unit of thermal energy.
J
Junction: The point in a thermocouple where the two dissimilar metals
are joined.
K
K: When referring to memory capacity, two to the tenth power
(1024 in decimal notation).
Kelvin: Symbol K. The unit of absolute or thermodynamic temperature
scale based upon the Celsius scale with 100 units between the ice
point and boiling point of water. 0°C = 273.15K (there is no degree
(°) symbol used with the Kelvin scale).
Kilowatt (kw): Equivalent to 1000 watts.
Kilowatt Hour (kwh): 1000 watthours.
Kilovolt amperes (kva): 1000 volt amps.
KVA: Kilovolt amperes (1000 volt amps).
L
Lag: 1. A time delay between the output of a signal and the response
of the instrument to which the signal is sent. 2. A time relationship
between two waveforms where a fixed reference point on one wave
occurs after the same point of the reference wave.
Latent Heat: Expressed in BTU per pound. The amount of heat needed
(absorbed) to convert a pound of boiling water to a pound of steam.
Leakage Rate: The maximum rate at which a fluid is permitted or
determined to leak through a seal.
Limits of Error: A tolerance band for the thermal electric response
of thermocouple wire expressed in degrees or percentage defined
by ANSI specification MC-96.1 (1975).
Linearity: The closeness of a calibration curve to a specified straight
line. Linearity is expressed as the maximum deviation of any
calibration point on a specified straight line during any one
calibration cycle.
Load: The electrical demand of a process expressed as power (watts),
current (amps) or resistance (ohms).
Load Impedance: The impedance presented to the output terminals
of a transducer by the associated external circuitry.
Logarithmic Scale: A method of displaying data (in powers of ten) to yield
maximum range while keeping resolution at the low end of the scale.
Loop Resistance: The total resistance of a thermocouple circuit
caused by the resistance of the thermocouple wire. Usually used
in reference to analog pyrometers which have typical loop
resistance requirements of 10 ohms.
LSD (Least-Significant Digit): The rightmost active (non-dummy)
digit of the display.
LS-TTL Compatible: For digital input circuits, a logic 1 is obtained
for inputs of 2.0 to 5.5 V which can source 20 ¦ÌA, and a logic 0
is obtained for inputs of 0 to 0.8 V which can sink 400 ¦ÌA. For
digital output signals, a logic 1 is represented by 2.4 to 5.5 V with
a current source capability of at least 400 ¦ÌA, and a logic 0 is
represented by 0 to 0.6 V with a current sink capability of at least
16 MA. ¡°LS¡± stands for Low-power Schottky.
LS-TTL Unit Load: A load with LS-TTL voltage levels, which will draw
20 ¦ÌA for a logic 1 and ¨C400 ¦ÌA for a logic 0.
M
M: Mega; one million. When referring to memory capacity, two
to the twentieth power (1,048,576 in decimal notation).
Manual Reset (Adjustment): The adjustment on a proportioning
controller which shifts the proportioning band in relationship
to the set point to eliminate droop or offset errors.
Manual Reset (Switch): The switch in a limit controller that manually
resets the controller after the limit has been exceeded.
Maximum Operating Temperature: The maximum temperature
at which an instrument or sensor can be safely operated.
Maximum Power Rating: The maximum power in watts that a device
can safely handle.
Mean Temperature: The average of the maximum and minimum
temperature of a process equilibrium.
Measurand: A physical quantity, property, or condition which
is measured.
Measuring Junction: The thermocouple junction referred to as the hot
junction that is used to measure an unknown temperature.
Melting Point: The temperature at which a substance transforms from
a solid phase to a liquid phase.
Mica: A transparent mineral used as window material in hightemperature
ovens.
Microamp: One millionth of an ampere, 10¨C6 amps.
Microcomputer: A computer which is physically small. It can fit on top
of or under a desk; based on LSI circuitry, computers of this type
are now available with much of the power currently associated with
minicomputer systems.
Micron: One millionth of a meter, 10¨C6 meters.
Microvolt: One millionth of a volt, 10¨C6 volts.
Mil: One thousandth of an inch (0.001¡Ü).
Milliamp: One thousandth of an amp, 10¨C3 amps, symbol mA.
Millimeter: One thousandth of a meter, symbol mm.
Millivolt: Unit of electromotive force. It is the difference in potential
required to make a current of 1 millampere flow through a
resistance of 1 ohm; one thousandth of a volt, symbol mV.
Mineral-insulated Thermocouple: A type of thermocouple cable which
has an outer metal sheath and mineral (magnesium oxide)
insulation inside separating a pair of thermocouple wires from
themselves and from the outer sheath. This cable is usually drawn
down to compact the mineral insulation and is available in
diameters from 0.375 to 0.010 inches. It is ideally suited for hightemperature
and severe-duty applications.
Minor Scale Division: On an analog scale, the smallest indicated
division of units on the scale.
Modem: Modulator/Demodulator. A device that transforms digital
signals into audio tones for transmission over telephone lines,
and does the reverse for reception.
MSD (Most-Significant Digit): The leftmost digit of the display.
Mueller Bridge: A high-accuracy bridge configuration used to measure
three-wire RTD thermometers.
Multiplex: A technique which allows different input (or output) signals
to use the same lines at different times, controlled by an external
signal. Multiplexing is used to save on wiring and I/O ports.
N
N/C (No Connection): A connector point for which there is no internal
connection.
NBS: National Bureau of Standards.
NEC: National Electric Codes.
Negative Temperature Coefficient: A decrease in resistance with
an increase in temperature.
NEMA-4: A standard from the National Electrical Manufacturers
Association, which defines enclosures intended for indoor or
outdoor use primarily to provide a degree of protection against
windblown dust and rain, splashing water, and hose-directed water.
NEMA-7: A standard from the National Electrical Manufacturers
Association, which defines explosion-proof enclosures for use
in locations classified as Class I, Groups A, B, C or D, as specified
in the National Electrical Code.
NEMA-12: A standard from the National Electrical Manufacturers
Association, which defines enclosures with protection against dirt,
dust, splashes by non-corrosive liquids, and salt spray.
NEMA-Size Case: An older US case standard for panel meters, which
requires a panel cutout of 3.93 x 1.69 inches.
Network: A group of computers that are connected to each other
by communications lines to share information and resources.
Nibble: One half of a byte.
Nicrosil/Nisil: A nickel-chrome/nickel-silicone thermal alloy used to
measure high temperatures. Inconsistencies in thermoelectric
voltages exist in these alloys with respect to the wire gauge.
NMR (Normal-Mode Rejection): The ability of a panel meter to filter
out noise superimposed on the signal and applied across the SIG HI
to SIG LO input terminals. Normally expressed in dB at 50/60 Hz.
Noise: An unwanted electrical interference on the signal wires.
Normal-Mode Rejection Ratio: The ability of an instrument to reject
interference usually of line frequency (50–60 Hz) across its
input terminals.
NPT: National Pipe Thread.
Null: A condition, such as balance, which results in a minimum
absolute value of output.
O
Octal: Pertaining to a base 8 number system.
O.D.: Outside diameter.
Offset: The difference in temperature between the set point and
the actual process temperature. Also referred to as droop.
Ohmmeter: An instrument used to measure electrical resistance.
On/off Controller: A controller whose action is fully on or fully off.
Open Circuit: The lack of electrical contact in any part of the
measuring circuit. An open circuit is usually characterized by rapid
large jumps in displayed potential, followed by an off-scale reading.
Operating System: A collection of programs that controls the overall
operation of a computer and performs such tasks as assigning places
in memory to programs and data, processing interrupts, scheduling
jobs and controlling the overall input/output of the system.
Optical Isolation: Two networks which are connected only through
an LED transmitter and photoelectric receiver with no electrical
continuity between the two networks.
Output: The electrical signal which is produced by an applied input
to the transducer.
Output Impedance: The resistance as measured on the output
terminals of a pressure transducer.
Output Noise: The RMS, peak-to-peak (as specified) ac component
of a transducer’s dc output in the absence of a measurand variation.
Overshoot: The number of degrees by which a process exceeds the set
point temperature when coming up to the set point temperature.
P
Parallax: An optical illusion which occurs in analog meters and causes
reading errors. It occurs when the viewing eye is not in the same
plane, perpendicular to the meter face, as the indicating needle.
Parallel Transmission: Sending all data bits simultaneously.
Commonly used for communications between computers
and printer devices.
Parity: A technique for testing transmitting data. Typically, a binary
digit is added to the data to make the sum of all the digits of the
binary data either always even (even parity) or always odd (odd
parity).
Peltier Effect: When a current flows through a thermocouple junction,
heat will either be absorbed or evolved depending on the direction
of current flow. This effect is independent of joule I2 R heating.
Peripheral: A device that is external to the CPU and main memory,
i.e., printer, modem or terminal, but is connected by the appropriate
electrical connections.
Phase: A time-based relationship between a periodic function and
a reference. In electricity, it is expressed in angular degrees to
describe the voltage or current relationship of two alternating
waveforms.
Phase Difference: The time expressed in degrees between the same
reference point on two periodic waveforms.
Phase Proportioning: A form of temperature control where the power
supplied to the process is controlled by limiting the phase angle of
the line voltage.
PID: Proportional, integral, derivative. A three-mode control action
where the controller has time proportioning, integral (auto reset)
and derivative rate action.
Piezoresistance: Resistance that changes with stress.
Pixel: Picture element. Definable locations on a display screen that are
used to form images on the screen. For graphic displays, screens
with more pixels provide higher resolution.
Platinel: A non-standard, high temperature platinum thermocouple
alloy whose thermoelectric voltage nearly matches a Type K
thermocouple (Trademark of Englehard Industries).
Platinum: A noble metal which in its pure form is the negative wire
of Type R and Type S thermocouples.
Platinum 6% Rhodium: The platinum-rhodium alloy used as the
negative wire in conjunction with platinum-30% rhodium to form
a Type B thermocouple.
Platinum 10% Rhodium: The platinum-rhodium alloy used as the
positive wire in conjunction with pure platinum to form a Type S
thermocouple.
Platinum 13% Rhodium: The platinum-rhodium alloy used
as the positive wire in conjunction with pure platinum to form
a Type R thermocouple.
Platinum 30% Rhodium: The platinum-rhodium alloy used
as the positive wire in conjunction with platinum 6% rhodium
to form a Type B thermocouple.
Platinum 67: To develop thermal emf tables for thermocouples, the
National Bureau of Standards paired each thermocouple alloy
against a pure platinum wire (designated Platinum 2 prior to 1973,
and currently Platinum 67). The thermal emf’s of any alloy
combination can be determined by summing the “vs. Pt-67” emf’s
of the alloys, i.e., the emf table for a Type K thermocouple is derived
from the Chromel vs. Pt-67 and the Alumel vs .Pt-67 values.
Polarity: In electricity, the quality of having two oppositely charged
poles, one positive, one negative.
Port: A signal input (access) or output point on a computer.
Positive Temperature Coefficient: An increase in resistance due
to an increase in temperature.
Potential Energy: Energy related to the position or height above
a place to which fluid could possibly flow.
Potentiometer: 1. A variable resistor often used to control a circuit.
2. A balancing bridge used to measure voltage.
Power Supply: A separate unit or part of a circuit that supplies power
to the rest of the circuit or to a system.
PPM: Abbreviation for “parts per million,” sometimes used to express
temperature coefficients. For instance, 100 ppm is identical to 0.01%.
Primary Standard (NBS): The standard reference units and physical
constants maintained by the National Bureau of Standards upon
which all measurement units in the United States are based.
Probe: A generic term that is used to describe many types of
temperature sensor.
Process Meter: A panel meter with sizeable zero and span adjustment
capabilities, which can be scaled for readout in engineering units for
signals such as 4–20 mA, 10–50 mA and 1–5 V.
Program: A list of instructions that a computer follows to perform
a task.
Prom: Programmable read-only memory. A semiconductor memory
whose contents cannot be changed by the computer after it has
been programmed.
Proportioning Band: A temperature band expressed in degrees
within which a temperature controller’s time proportioning function
is active.
Proportioning Control Mode: A time proportioning controller where
the amount of time that the relay is energized is dependent upon the
system’s temperature.
Proportioning Control plus Derivative Function: A time proportioning
controller with a derivative function. The derivative function senses
the rate at which a system’s temperature is either increasing or
decreasing and adjusts the cycle time of the controller to minimize
overshoot or undershoot.
Proportioning Control plus Integral: A two-mode controller with time
proportioning and integral (auto reset) action. The integral function
automatically adjusts the temperature at which a system has
stabilized back to the set point temperature, thereby eliminating
droop in the system.
Proportioning Control with Integral and Derivative Functions: Three
mode PID controller. A time-proportioning controller with integral
and derivative functions. The integral function automatically adjusts
the system temperature to the set point temperature to eliminate
droop due to the time proportioning function. The derivative
function senses the rate of rise or fall of the system temperature
and automatically adjusts the cycle time of the controller to
minimize overshoot or undershoot.
Protection Head: An enclosure usually made out of metal at the end
of a heater or probe where connections are made.
Protection Tube: A metal or ceramic tube, closed at one end, into
which a temperature sensor is inserted. The tube protects the
sensor from the medium into which it is inserted.
Protocol: A formal definition that describes how data is to be
exchanged.
PSIA: Pounds per square inch absolute. Pressure referenced
to a vacuum.
PSID: Pounds per square inch differential. Pressure difference between
two points.
PSIG: Pound per square inch gauge. Pressure referenced to ambient
air pressure.
PSIS: Pounds per square inch standard. Pressure referenced
to a standard atmosphere.
Pulse Width Modulation: An output in the form of duty cycle which
varies as a function of the applied measurand.
R
Radiation: See Infrared
Random Access Memory (RAM): Memory that can be both read and
changed during computer operation. Unlike other semi-conductor
memories, RAM is volatile—if power to the RAM is disrupted or
lost, all the data stored is lost.
Range: Those values which a transducer is intended to measure,
specified by upper and lower limits.
Rangeability: The ratio of the maximum flowrate to the minimum
flowrate of a meter.
Rankine (°R): An absolute temperature scale based upon the
Fahrenheit scale with 180° between the ice point and boiling point
of water. 459.67°R = 0°F.
Rate Action: The derivative function of a temperature controller.
Rate Time: The time interval over which the system temperature
is sampled for the derivative function.
Ratiometric Measurement: A measurement technique where an
external signal is used to provide the voltage reference for the dualslope
A/D converter. The external signal can be derived from the
voltage excitation applied to a bridge circuit or pick-off supply,
thereby eliminating errors due to power supply fluctuations.
Read Only Memory (ROM): Memory that contains fixed data. The
computer can read the data, but cannot change it in any way.
Real Time: The time interval over which the system temperature
is sampled for the derivative function.
Record: A collection of unrelated information that is treated as
a single unit.
Recovery Time: The length of time which it takes a transducer
to return to normal after applying a proof pressure.
Reference Junction: The cold junction in a thermocouple circuit which
is held at a stable, known temperature. The standard reference
temperature is 0°C (32°F). However, other temperatures can be used.
Refractory Metal Thermocouple: A class of thermocouples with
melting points above 3600°F. The most common are made from
tungsten and tungsten/rhenium alloys, Types G and C. They can
be used for measuring high temperatures up to 2200°C (4000°F)
in non-oxidizing, inert, or vacuum environments.
Relay (Mechanical): An electromechanical device that completes
or interrupts a circuit by physically moving electrical contacts into
contact with each other.
Relay (Solid State): A solid state switching device which completes
or interrupts a circuit electrically with no moving parts.
Remote: Not hard-wired; communicating via switched lines, such as
telephone lines. Usually refers to peripheral devices that are located
at a site away from the CPU.
Repeatability: The ability of a transducer to reproduce output readings
when the same measurand value is applied to it consecutively,
under the same conditions, and in the same direction. Repeatability
is expressed as the maximum difference between output readings.
Resistance: The resistance to the flow of electric current measured
in ohms (.). For a conductor, resistance is a function of diameter,
resistivity (an intrinsic property of the material) and length.
Resistance Ratio Characteristic: For thermistors, the ratio of the
resistance of the thermistor at 25°C to the resistance at 125°C.
Resistance Temperature Characteristic: A relationship between
a thermistor’s resistance and the temperature.
Resolution: The smallest detectable increment of measurement.
Resolution is usually limited by the number of bits used to quantize
the input signal. For example, a 12-bit A/D can resolve to one part
in 4096 (2 to the 12 power equals 4096).
Resonant Frequency: The measurand frequency at which a transducer
responds with maximum amplitude.
Response Time: The length of time required for the output of
a transducer to rise to a specified percentage of its final value
as a result of a step change of input.
Response Time (time constant): The time required by a sensor
to reach 63.2% of a step change in temperature under a specified
set of conditions. Five time constants are required for the sensor
to stabilize at 100% of the step change value.
RFI: Radio frequency interference.
Rheostat: A variable resistor.
Rise Time: The time required for a sensor or system to respond
to an instantaneous step function, measured from the 10% to 90%
points on the response waveforms.
Room Conditions: Ambient environmental conditions under which
transducers must commonly operate.
Root Mean Square (RMS): Square root of the mean of the square
of the signal taken during one full cycle.
RTD: Resistance temperature detector.
S
SAMA: Scientific Apparatus Makers Association. An association that
has issued standards covering platinum, nickel, and copper
resistance elements (RTD’s).
SCR: Silicon controlled rectifier.
Scroll: To move all or part of the screen material up or down,
left or right, to allow new information to appear.
Seebeck Coefficient: The derivative (rate of change) of thermal EMF with
respect to temperature, normally expressed as millivolts per degree.
Seebeck Effect: When a circuit is formed by a junction of two
dissimilar metals and the junctions are held at different
temperatures, a current will flow in the circuit caused by the
difference in temperature between the two junctions.
Seebeck EMF: The open circuit voltage caused by the difference in
temperature between the hot and cold junctions of a circuit made
from two dissimilar metals.
Self-Heating: Internal heating of a transducer as a result of power
dissipation.
Sensing Element: That part of a transducer which reacts directly
in response to input.
Sensitivity: The minimum change in input signal to which an
instrument can respond.
Sensitivity Shift: A change in slope of the calibration curve due to
a change in sensitivity.
Sequential Access: An access mode in which records are retrieved in
the same order in which they were written. Each successive access
to the file refers to the next record in the file.
Serial Transmission: Sending one bit at a time on a single
transmission line. Compare with Parallel Transmission.
Set Point: The temperature at which a controller is set to control
a system.
Settling Time: The time taken for the display to settle within one digit
final value when a step is applied to the meter input.
SI: System Internationale. The name given to the standard metric
system of units.
Signal: An electrical transmittance (either input or output) that
conveys information.
Signal Conditioner: A circuit module which offsets, attenuates,
amplifies, linearizes and/or filters the signal for input to the A/D
converter. The typical output signal conditioner is +2 V dc.
Signal Conditioning: To process the form or mode of a signal so as to
make it intelligible to, or compatible with, a given device, including
such manipulation as pulse shaping, pulse clipping, compensating,
digitizing, and linearizing.
Single-Ended Input: A signal-input circuit where SIG LO
(or sometimes SIG HI) is tied to METER GND. Ground loops
are normally not a problem in AC-powered meters, since
METER GND is transformer-isolated from AC GND.
Single Precision: The degree of numeric accuracy that requires
the use of one computer word. In single precision, seven digits
are stored, and up to seven digits are printed. Contrast with
Double Precision.
Software: Generally, programs loaded into a computer from external
mass storage but also extended to include operating systems and
documentation.
Source Code: A non-executable program written in a high-level
language. A compiler or assembler must translate the source code
into object code (machine language) that the computer can
understand and process.
Span: The difference between the upper and lower limits of a range
expressed in the same units as the range.
Span Adjustment: The ability to adjust the gain of a process or strain
meter so that a specified display span in engineering units
corresponds to a specified signal span. For instance, a display span
of 200°F may correspond to the 16 mA span of a 4–20 mA
transmitter signal.
Spare: A connector point reserved for options, specials, or other
configurations. The point is identified by an (E#) for location
on the electrical schematic.
Specific Gravity: The ratio of mass of any material to the mass
of the same volume of pure water at 4°C.
Specific Heat: The ratio of thermal energy required to raise the
temperature of a body 1° to the thermal energy required to raise
an equal mass of water 1°.
Spectral Filter: A filter which allows only a specific band width of
the electromagnetic spectrum to pass, i.e., 4 to 8 micron infrared
radiation.
Spectrum: The resolving of overall vibration into amplitude
components as a function of frequency.
Spectrum Analysis: Utilizing frequency components of a vibration
signal to determine the source and cause of vibration.
Spot Size: The diameter of the circle formed by the cross section
of the field of view of an optical instrument at a given distance.
Spurious Error: Random or erratic malfunction.
SSR: Solid state relay. See Relay, Solid State
Stability: The ability of an instrument or sensor to maintain
a consistent output when a constant input is applied.
Stop Bit: A signal following a character or block that prepares
the receiving device to receive the next character or block.
String: A sequence of characters.
Super Cooling: The cooling of a liquid below its freezing temperature
without the formation of the solid phase.
Super Heating: 1. The heating of a liquid above its boiling temperature
without the formation of the gaseous phase. 2. The heating of the
gaseous phase considerably above the boiling-point temperature
to improve the thermodynamic efficiency of a system.
Surge Current: A current of short duration that occurs when power is
first applied to capacitive loads or temperature dependent resistive
loads such as tungsten or molybdenum heaters—usually lasting
not more than several cycles.
Syntax: The rules governing the structure of a language.
T
Tape: A recording medium for data or computer programs. Tape can
be in permanent form, such as perforated paper tape, or erasable,
such as magnetic tape. Generally, tape is used as a mass storage
medium, in magnetic form, and has a much higher storage capacity
than disk storage, but it takes much longer to write or recover data
from tape than from a disk.
PFA: A fluorocarbon polymer used for insulation of electrical wires
(trademark of DuPont).
Telecommunication: Synonym for data communication.
The transmission of information from one point to another.
TEMPCO: Abbreviation for “temperature coefficient”: the error
introduced by a change in temperature. Normally expressed
in %/°C or ppm/°C.
Temperature Error: The maximum change in output, at any measurand
value within a specified range, when the transducer temperature is
changed from room temperature to specified temperature extremes.
Temperature Range, Compensated: The range of ambient
temperatures within which all tolerances specified for
Thermal Zero Shift and Thermal Sensitivity Shift are applicable
(temperature error).
Temperature Range, Operable: The range of ambient temperatures,
given by their extremes, within which a transducer may be
operated. Exceeding compensated range may require recalibration.
Terminal: An input/output device used to enter data into a computer
and record the output.
Thermal Coefficient of Resistance: The change in resistance of a
semiconductor per unit change in temperature over a specific range
of temperature.
Thermal Conductivity: The ability of a material to conduct heat
in the form of thermal energy.
Thermal emf: See Seebeck emf
Thermal Expansion: An increase in size due to an increase
in temperature expressed in units of an increase in length
or increase in size per degree, i.e. inches/inch/degree C.
Thermal Gradient: The distribution of a differential temperature
through a body or across a surface.
Thermal Sensitivity Shift: The sensitivity shift due to changes of the
ambient temperature from room temperature to the specified limits
of the compensated temperature range.
Thermal Zero Shift: An error due to changes in ambient temperature
in which the zero pressure output shifts. Thus, the entire calibration
curve moves in a parallel displacement.
Thermistor: A temperature-sensing element composed of sintered
semiconductor material which exhibits a large change in resistance
proportional to a small change in temperature. Thermistors usually
have negative temperature coefficients.
Thermocouple: The junction of two dissimilar metals which has
a voltage output proportional to the difference in temperature
between the hot junction and the lead wires (cold junction)
(refer to Seebeck emf).
Thermocouple Type Material (ANSI Symbol)
J Iron/Constantan
K CHROMEGA®/ALOMEGA®
T Copper/Constantan
E CHROMEGA/Constantan
R Platinum/Platinum 13% Rhodium
S Platinum/Platinum 10% Rhodium
B Platinum 6% Rhodium/Platinum 30% Rhodium
G* Tungsten/Tungsten 26% Rhenium
C* Tungsten 5% Rhenium/Tungsten 26% Rhenium
D* Tungsten 3% Rhenium/Tungsten 25% Rhenium
*Not ANSI symbols
Thermopile: An arrangement of thermocouples in series such that
alternate junctions are at the measuring temperature and the
reference temperature. This arrangement amplifies the
thermoelectric voltage. Thermopiles are usually used as infrared
detectors in radiation pyrometry.
Thermowell: A closed-end tube designed to protect temperature
sensors from harsh environments, high pressure, and flows. They
can be installed into a system by pipe thread or welded flange and
are usually made of corrosion-resistant metal or ceramic material,
depending upon the application.
Thomson Effect: When current flows through a conductor within a
thermal gradient, a reversible absorption or evolution of heat will
occur in the conductor at the gradient boundaries.
Transducer: A device (or medium) that converts energy from one form
to another. The term is generally applied to devices that take
physical phenomena (pressure, temperature, humidity, flow, etc.)
and convert them to electrical signals.
Transmitter (Two-Wire): A device which is used to transmit temperature
data from either a thermocouple or RTD via a two-wire current loop.
The loop has an external power supply and the transmitter acts as a
variable resistor with respect to its input signal.
Triac: A solid state switching device used to switch alternating current
wave forms.
Triple Point: The temperature and pressure at which solid, liquid, and
gas phases of a given substance are all present simultaneously in
varying amounts.
Triple Point (Water): The thermodynamic state where all three phases,
solid, liquid, and gas, may all be present in equilibrium. The triple
point of water is .01°C.
True RMS: The true root-mean-square value of an AC or AC-plus-DC
signal, often used to determine power of a signal. For a perfect sine
wave, the RMS value is 1.11072 times the rectified average value,
which is utilized for low-cost metering. For significantly nonsinusoidal
signals, a true RMS converter is required.
TTL: Transistor-to-transistor logic. A form of solid state logic which
uses only transistors to form the logic gates.
TTL-Compatible: For digital input circuits, a logic 1 is obtained
for inputs of 2.0 to 5.5 V which can source 40 µA, and a logic 0 is
obtained for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital
output signals, a logic 1 is represented by 2.4 to 5.5 V with a current
source capability of at least 400 µA, and a logic 0 is represented
by 0 to 0.6 V with a current sink capability of at least 16 mA.
TTL Unit Load: A load with TTL voltage levels, which will draw 40 µA
for a logic 1 and –1.6 mA for a logic 0.
Typical: Error within plus or minus one standard deviation (±1%) of
the nominal specified value, as computed from the total population.
U
UL: Underwriters Laboratories, Inc. An independent laboratory that
establishes standards for commercial and industrial products.
Ultraviolet: That portion of the electromagnetic spectrum below blue
light (380 nanometers).
Undershoot: The difference in temperature between the temperature
a process goes to, below the set point, after the cooling cycle
is turned off and the set point temperature.
Ungrounded Junction: A form of construction of a thermocouple
probe where the hot or measuring junction is fully enclosed by
and insulated from the sheath material.
Union: A form of pipe fitting where two extension pipes are joined
at a separable coupling.
V
Vacuum: A pressure less than atmospheric pressure.
Velocity: The time rate of change of displacement; dx/dt.
Vibration Transducer: Generally, any device which converts
movement, either shock or steady state vibration, into an electrical
signal proportional to the movement; a sensor.
Volt: The (electrical) potential difference between two points in a
circuit. The fundamental unit is derived as work per unit charge—
(V = W/Q). One volt is the potential difference required to move one
coulomb of charge between two points in a circuit using one joule
of energy.
Voltage: An electrical potential which can be measured in volts.
Voltmeter: An instrument used to measure voltage.
W
Watt Density: The watts emanating from each square inch of heated
surface area of a heater. Expressed in units of watts per square
inch.
Wheatstone Bridge: A network of four resistances, an emf source,
and a galvanometer connected such that when the four resistances
are matched, the galvanometer will show a zero deflection
or “null” reading.
Window: In computer graphics, a defined area in a system not
bounded by any limits; unlimited “space” in graphics.
Word: Number of bits treated as a single unit by the CPU. In an 8-bit
machine, the word length is 8 bits; in a sixteen-bit machine,
it is 16 bits.
Working Standard: A standard of unit measurement calibrated from
either a primary or secondary standard which is used to calibrate
other devices or make comparison measurements.
Z
Zero Adjustment: The ability to adjust the display of a process or
strain meter so that zero on the display corresponds to a non-zero
signal, such as 4 mA, 10 mA, or 1 V dc. The adjustment range is
normally expressed in counts.
Zero Offset: 1. The difference expressed in degrees between true zero
and an indication given by a measuring instrument. 2. See Zero
Suppression
Zero Power Resistance: The resistance of a thermistor or RTD
element with no power being dissipated.
Zero Suppression: The span of an indicator or chart recorder may
be offset from zero (zero suppressed) such that neither limit of the
span will be zero. For example, a temperature recorder which
records a 100° span from 400° to 500° is said to have 400° zero
suppression.
Zero Voltage Switching: The making or breaking of circuit timed such
that the transition occurs when the voltage wave form crosses zero
voltage; typically only found in solid state switching devices.
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