Glossary of Temperature Terminology
Accuracy - Preciseness of the sensor's reading in relation
to the actual value being measured.
Annealing - This process is a treatment consisting of
heating to and holding at a suitable temperature followed by
cooling at a suitable rate used primarily to soften metallic
materials, also to produce desired changes in other properties.
Alpha - The average percent change in resistance per degree
of a resistance temperature device (RTD) between 0° and 100°C. For
a 100 ohm platinum RTD, 0.00385 ohm/ohm/ °C is the most common
alpha. Alpha is also referred to as the temperature coefficient.
Alumel - The aluminum nickel alloy used for the negative
leg of a Type K thermocouple. Alumel is a trade name of Hoskins
A.N.S.I. - American National Standards Institute.
A.S.T.M. - American Society for Testing and Materials.
AWG - American Wire Gauge. Same as Brown and Sharpe (B & S)
Bell Spring - A strain relief spring used at the transition
of the tube and lead wire to eliminate stress on wires.
Calendar-Van Dusen Equation - Temperature resistance
formula which accounts for the small deviation from linearity.
R/Ro = 1 x alpha [T - delta (T/100 - 1)-beta (T/100 - 1) (T/100)3
Calibration - 1 The combination of metals used in manufacturing
thermocouples (ie. chromel-alumel). 2 This term can also refer to
the process of adjusting an instrument or preparing a deviation
chart for a sensor so that an exact error can be determined at any
Calibration Point - The temperature at which a deviation from
actual temperatures can be determined for a specific probe.
Celsius (Centigrade) - The thermometer scale with an interval of
100 degrees between the ice point of water (0°C) and boiling point
of water (100°C).
Cold Junction - The reference junction of a thermocouple which is
held at a constant known temperature.
Cold Side - The end of the sensor which connects to the instrument
used to read or transmit the sensor signal.
Code - Color code for thermocouple wires established by ANSI. The
negative lead wire is red for all thermocouples. For base metal
thermocouples the positive leg is as follows: Type J-white, Type
K-yellow, Type T-blue, Type E-purple, Type N-orange. For nobel
metal thermocouples, the positive leg is green.
Compensated Connector - Used with thermocouples, a connector is
made with thermocouple alloys and must be matched with the
Compensating Loop - Used for measuring the lead wire resistance of
RTD's. An additional wire is run from the instrument to the
and then back to the instrument with no direct connection to the
Connection Head - A housing for termination wires which is
attached to the end of a thermocouple or RTD. These are most
commonly available in aluminum, cast iron, black nylon, or white
plastic, sometimes referred to as "pecker head".
Constantan - The copper -nickel alloy used for the negative leg in
Type J, Type T, and Type E thermocouples.
Cryogenics - Of or relating to extremely low temperature (ie below
0°C). A type T thermocouple is commonly used to measure these
Degree - An incremental value in the temperature scale. For
example, there are 180° between the ice point and boiling point of
water in the Fahrenheit scale.
DIN - Deutsche Industrial Norms. The German agency responsible for
setting the world wide recognized alpha of 0.00385 ohm/ohm/°C (DIN
43760). Has been superceded by IEC 751.
Drift - A change in the sensor's indication of temperature over a
long period of time attributed to such elements as thermal
cycling, cold working, and corrosive contamination.
Dual Element Sensor - An assembly with two separate sensing
Duplex Wires - A single pair of wires insulated from each other
and covered with an overall insulating jacket.
Electrical Interference - Electrical noise that produces
undesirable responses in the signal being emitted from the
Electroetch - Electrochemical metal marking to identify probes. A
controlled low-voltage current etches out the metal in the
presence of electrically-conductive electrolyte fluid. Permanent
marks are made in seconds.
Electroplating - Electrodepositing a metal or alloy in an adherent
form on an object serving as a cathode.
Electropolishing - A technique in which a high polish is produced
by making the specimen the anode in an electrolytic cell, where
preferential dissolution at high points smooths the surface.
Element - any basic thermocouple or RTD without hardware.
EMF - Electromotive force. Used in reference to the millivolt
output of a thermocouple. It is the potential energy of a
EMI - Electromagnetic interference.
Error - The difference between the correct value and the actual
Explosion Proof Housing - 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 - Style of thermocouple in which the hot
junctions protrudes beyond the sheath material and is fully
Extension Grade Wire - This grade of wire is used between the
thermocouple and the reference junction and has thermoelectric
properties similar to the thermocouple wires it is used with. The
major advantage of extension grade wire is its lower cost in
comparison to thermocouple grade wire. This is especially
important for noble metal thermocouples which have notably less
expensive compatible base metal extension wires.
Fahrenheit - The temperature scale which designates the ice point
of water as 32° and the boiling point of water as 212°. (0° =
frozen sea water, 100° = the underarm of a healthy man).
Ferrule - Used inside a compression fitting to provide a gas-tight
seal. This is a small machined ring which allows the fitting to be
fixed onto the probe when the fitting is tightened. Ferrules are
available in nylon, teflon and lava (adjustable depth) brass and
stainless steel (permanent depth).
Freezing Point - The temperature at which a substance changes from
a liquid to a solid.
Finish #4 - ASTM - Widely used for restaurant and dairy equipment.
Following initial grinding with courser abrasives, sheets are
generally finished last with abrasives with 120-150 mesh. 150 mesh
~ 32 Roughness Average (RA).
"Green Rot" - 1) Preferential oxidation of the chromel leg of a
Type K thermocouple that occurs when the oxygen content of the
atmosphere surrounding the thermocouple is low. This greenish
surface on the chromel leg most commonly develops at temperatures
between 1500° and 1900°F. 2) Green corrosion that occurs on
Ground - The reference point of an electrical system. A common
ground is the electrical neutral line that has the same potential.
Grounded Junction - A measuring junction constructed so that the
thermocouple and sheath are in electrical contact with each other
at the measuring junction.
Heat - Thermal energy expressed in units of BTU's or calories.
Heat Sink - A body which absorbs thermal energy.
Heat Transfer - The movement of thermal energy from a body of high
energy to a body of low energy. This transfer occurs in three
different modes: conduction, convection, and radiation.
Homogeneity - Term used to refer to the material purity or
sameness of the thermocouple wire along its length.
Hot Junction - The measuring junction of the thermocouple.
Hot Side - The end of the sensor which senses the temperature. It
is called the hot side even if it measures coldness.
Hysteresis - A phenomenon that occurs due to a metallurgical
instability that creates a crystalline ordered arrangement of the
metal with time as the metal experiences temperature cycling or
elevated temperatures (ie: chromel wire cycling above and below
1800°F). This creates inhomogenieties in the metals, thus
I.E.C. - International Electrotechnical Commission
ISA - The Society for Measurement and Control. Formerly known as
the Instrument Society of America.
Ice Point - The freezing point of water, 32°F or 0°C.
Impedance - The resistance to electrical flow which includes
resistive plus reactive.
Insulation Resistance - The resistance value of the insulation or
conductor cover designated in ohms.
Interchangeability Error - A difference in measurement between two
probes due to the slight variation of each probe.
Intrinsically Safe - A system which will not produce any spark
that would cause an ignition.
Isothermal - An area that is at a constant temperature.
Junction - The point at which the two dissimilar metals of a
thermocouple are joined.
Lag - 1) The delay of time between the output of the sensor and
the instrument's response to the signal. 2) The extension of a
thermowell above the threads and below the hex also referred to as
the "T" dimension of a thermowell. This extension is used to get
through insulation or get away from high temperatures. 3) Special
protection for shipping spooled cable. 4) The way our receptionist
Limits of Error - Materials' tolerance for
thermocouples as set by
ANSI specification MC-96.1 (1975). These are tolerances within
which various calibrations of thermocouples must conform.
Linearity - The deviation of a sensor's signal from a straight
Magnesium Oxide (MgO) - An insulation of high purity ceramic that
offers dielectric strength and high resistivity.
Measuring Junction - The thermocouple junction at which the two
dissimilar metals are joined to measure temperature.
Melting Point - The temperature at which a substance changes from
a solid to a liquid.
Mineral-insulated Thermocouple - Thermocouple style which has an
outer jacket of metal tubing with tightly compacted magnesium
oxide insulation separating the internal pair of thermocouple
N.I.S.T. - National Institute of Standards and Technology.
Formerly known as National Bureau of Standards (NBS).
N.P.S. - National Pipe Straight.
N.P.T. - National Pipe Tapered.
Negative Temperature Coefficient - The negative slope of a line
that indicates the change in resistance with the increase in
Nicrobell - Nicrobell is a Trademark of Incotherm Limited -
Hereford, England. It is used as an outer jacket with some sheath
Nicrosil - The nickel chrome alloy used for the positive leg of a
type N thermocouple.
Nisil - The nickel silicone alloy used for the negative leg of a
type N thermocouple.
Nobel Metal Thermocouple - Thermocouple made from Platinum,
Rhodium, or Gold.
Noise - Electrical interference.
O.D. - Outside diameter.
Passivation - The changing of a chemically active surface of a
metal to a much less active state. Our use is restricted to
the use of Nitric Acid and Water to treat 300 Series stainless
steel at room temperature. This procedure should not be used with
Platinel - High temperature platinum thermocouple alloy whose emf
output is similar to a Type K thermocouple. Platinel is a
trademark of Englehard Industries.
Platinum - The pure form noble metal which is used for the
negative leg of Type R and Type S thermocouple.
Polarity - A characteristic of electricity in which there are two
oppositely charged poles, one negative and one positive.
Positive Temperature Coefficient - The positive slope of a line
that indicates the change in resistance which the increase in
Power Supply - A separate unit that supplies power to a circuit or
Probe - A general term used to describe
thermocouples and RTD's.
Protection Tube - A cylindrical tube made of metal or ceramic that
is used to strengthen and protect the temperature probes which are
R.F.I. - Radio frequency interference.
Range - The area between two established temperature points.
Expressed by stating the lower variable first and the higher
Reference Junction - The cold junction of a thermocouple which is
either at a constant or at a known temperature.
Refractory Metal Thermocouple - The group of
melting points are above 3600°F. These can be used to temperatures
up to 4000°F in inert, non-oxidizing, or vacuum environments.
Tungsten and tungsten/rhenium are the most common alloys used for
refractory metal thermocouples. of the metal with time as the
metal experiences temperature cycling or elevated temperatures (ie:
chromel wire cycling above and below 1800°F). This creates
inhomogenieties in the metals, thus temperature errors. I.E.C. -
International Electrotechnical Commission.
Repeatability - The ability of a temperature sensor to duplicate
the same output under repeated identical conditions.
Resistance - The opposition to the flow of electric current
measured in ohms.
Resistance Temperature Detector (RTD) - A sensor which measures
temperature by the principle that the resistance of a metal
changes with temperature. Platinum is the most commonly used metal
for RTD's due to its nearly linear temperature vs resistance
relationship and its stability.
Rhodium - The noble metal alloyed with Platinum, used as an
opposing leg in Platinum / Rhodium thermocouples.
RTD's (Matched) - Two or more
RTD's which check out at 0°C ±.01%
of each other, however, any tolerance at any temperature may be
Response Time - The time required for a sensor to show 63.2% of a
SAMA - Scientific Apparatus Makers Association.
Sensor - See "probe".
Sheath - A pair of thermocouple wires which are placed inside a
metal tube then packed with magnesium oxide and swaged.
Thermocouples made from sheath are much more durable than the
hollow tube design thermocouples.
RTD's are also made with a
Signal - An electrical transmittance that conveys information from
the temperature probe.
Span - The difference between the lower and upper limits of a
Stability - The ability of a temperature sensor to maintain a
consistent output at a constant temperature.
Sublime - To change from a solid to a liquid without going through
a gaseous state.
Teflon - Insulation composed of fluorocarbon polymer used on
thermocouples and RTD wires. Teflon is a trademark of Du Pont
Thermal Conductivity - The ability of a material to conduct heat
in the form of thermal energy.
Thermal Expansion - The volumetric increase which occurs to a
material due to the addition of heat.
Thermal Gradient - The temperature differential that may exist in
a single body, environment, or system.
Thermistor - Temperature measurement devices that are
semiconductors with large coefficients of resistance typically
used in the range of -100 to 300°C.
Thermocouple - A sensor which measures temperature by the
principle that the junction of two dissimilar metals will have a
measurable difference in voltage output proportional to the
difference in temperature between the measuring junction and the
cold junction. Standard thermocouple types are recognized by ASTM.
Thermowell - A metal tube which is usually closed at one end made
from solid barstock. It is designed to protect a sensor from flow,
high pressure, and harsh environment.
Transition - 1 n. Normally a hollow metal part attached to the
cold end of a T/C or RTD tube or sheath to facilitate electrical
encapsulation and / or attachment of leadwires or armor. 2 adj.
The location on the sensor where the tube or sheath ends and the
cold side of the sensor begins.
Transmitter - A device used to transmit millivolts from a
thermocouple or ohms from an RTD via a current loop.
Ungrounded Junction - The type construction of a thermocouple
where the measuring junction is recessed and insulated with
magnesium oxide from the sheath material. This style junction
usually is slower to respond to a temperature change than a
grounded junction, but more resistant to drift caused by thermal
Union - A pipe fitting which connects two extension pipes through
a screw connection.
Volt - The work per unit change or potential difference between
two points in a circuit.
Voltage - An electrical potential.
Wheatstone Bridge - A circuit for measuring unknown resistance by
comparing it with known resistances.