Glossary of Temperature Sensors


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 Manufacturing Company.

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) wire gauge.

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 temperature.

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.

Color 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 appropriate [thermocouple].

Compensating Loop - Used for measuring the lead wire resistance of [RTD]. An additional wire is run from the instrument to the[RTD] and then back to the instrument with no direct connection to the [RTD] element.

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 temperatures.

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 elements.

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 extension wire.

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 [thermocouple].

EMI - Electromagnetic interference.

Error - The difference between the correct value and the actual value measured.

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 exposed.

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 battery terminals.

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 temperature errors.

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 pronounces “leg”.

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 line.

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] wires.

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 temperature.

Nicrobell - Nicrobell is a Trademark of Incotherm Limited - Hereford, England. It is used as an outer jacket with some sheath material.

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 other metals.

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 temperature.

Power Supply - A separate unit that supplies power to a circuit or system.

Probe - A general term used to describe thermocouples and [RTD].

Protection Tube - A cylindrical tube made of metal or ceramic that is used to strengthen and protect the temperature probes which are inserted inside.

R.F.I. - Radio frequency interference.

Range - The area between two established temperature points. Expressed by stating the lower variable first and the higher variable second.

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 thermocouples whose 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] 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] which check out at 0°C ±.01% of each other, however, any tolerance at any temperature may be specified.

Response Time - The time required for a sensor to show 63.2% of a temperature change.

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] are also made with a sheath design.

Signal - An electrical transmittance that conveys information from the temperature probe.

Span - The difference between the lower and upper limits of a range.

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 Company.

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 cycling.

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.