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Glossary of Thermocouple and RTD Terms

First published 1993. Latest revision 2025.

A

Absolute zero: The lowest possible temperature of a substance. Zero on the Kelvin scale (-273.15°C).
AC (Alternating Current): An electric current that reverses direction periodically.
Accuracy: Nearness of a sensor or indicator measurement reading to the real value of the quantity being measured, usually expressed as a percentage error.
Alpha: The temperature coefficient of resistance of a material, derived from measurements at 0°C and 100°C. Indicates change in resistance per °C.
Alumel: Trade name of nickel-based, high-temperature, negative thermocouple material used with Chromel for Type K thermocouples.
Ambient temperature: The average or mean temperature of the surrounding environment in contact with the sensor or equipment.
Ampere (Amp): Unit defining the quantity of electricity (current) flowing in a circuit; measured in coulombs per second.
Ammeter: Instrument which measures current.
Annealing: Heat treatment of materials to relieve internal stresses or dislocations.
ANSI: Abbreviation for American Society for Testing and Materials.

B

Beryllia: Beryllium oxide – a high-temperature mineral insulation material.
Black body: A body that absorbs all thermal radiation and is a perfect radiator.
Boiling point: The temperature at which a liquid turns to gas, e.g., water boils at 100°C.
Bonded hot junction: A grounded thermocouple junction integral with the sheath tip.

C

Calibration: The process of determining or adjusting values of an instrument by independent measurements of the relevant phenomenon.
Calorie: The quantity of thermal energy required to raise one gram of water 1°C at 15°C.
Celsius scale: The most commonly used temperature scale where degrees Celsius (°C) designate a point on the temperature scale and the magnitude of a temperature interval. 0°C is the ice point; 100°C is the boiling point of water.
Ceramic insulation: High-temperature compositions of metal oxides insulating thermocouple wires—most commonly alumina, beryllia, and magnesium oxide—available as single and multi-hole tubes or beads.
Chromel: Trade name of nickel-based, high-temperature positive thermocouple material used in Type K thermocouples with Alumel.
Cold junction: Original term for the reference junction of a thermocouple—now implies floating temperature.
Cold seal: The enclosure surrounding the flexible tail connections on a mineral insulated thermocouple unit.
Colour codes: IEC standard colour identification and tolerance codes for thermocouple extension and compensating cables, connectors, etc.
Common mode rejection ratio: The ability of an instrument to reject interference from a voltage differential at its input terminals relative to local ground—expressed in dB.
Compensating cable: Extension cable for thermocouple circuits with conductors that match thermocouple characteristics over a limited temperature range, allowing lower-cost extensions.
Compensating loop: A resistance compensation technique for RTDs where an extra wire is run to and from the RTD with no connection to it.
Compensator: An externally powered electrical network containing a temperature-sensitive element connected to a thermocouple to simulate a 0°C reference voltage.
Connection head: An enclosure fitted over the thermocouple or RTD protection tube at the terminal end, containing terminals for electrical connection.
Constantan: A copper-nickel alloy used as the negative leg in Type J and Type T thermocouples, originally developed for resistance applications.
Curie point: The temperature at which a magnetic material becomes substantially non-magnetic.

D

DC: Direct Current.
Dielectric Strength: A measure of the voltage that an insulating material can withstand before an electrical breakdown occurs.
Drift: Gradual shift in signal or measurement over time, often caused by ambient temperature changes, sensor hysteresis, or other physical, chemical, or electromagnetic effects.
Duplex: Term used when two thermocouples or RTDs are housed within the same probe assembly.

E

EMF: Abbreviation for electromotive force—the voltage developed by an energy source in an electrical circuit.
EMI: Electromagnetic interference.
Emissivity: The ratio of radiant energy emitted by a surface to that emitted by a perfect black body at the same temperature.
Error: The difference between the actual value and the correct or intended value.
Exposed junction: A thermocouple design where the sensing junction extends beyond the sheath for direct contact with the measured medium, providing faster response times.
Extension cable: A lower-cost cable used to extend thermocouple circuits, made with materials similar to the thermocouple wires for limited temperature ranges.

F

Fahrenheit scale: A temperature scale where degrees Fahrenheit (°F) define a specific temperature point and interval. Water freezes at 32°F and boils at 212°F under standard conditions.
Fixed point: A precise temperature value used as a reference in calibration.
Flexible tails: Stranded, insulated wires attached to mineral-insulated thermocouples, providing flexible connection points.
Freezing point: The temperature at which a substance changes from liquid to solid. For water, this is 0°C (32°F).
Gain: The amount of amplification provided by an electronic circuit or system.

G

Galvanometer: A sensitive instrument for detecting and measuring small electric currents using magnetic deflection.
Ground: A common electrical reference point for a circuit, typically connected to the earth.
Grounded junction: A thermocouple construction where the sensing junction is electrically connected to the sheath, placing them at the same potential and improving response time.

H

Heat transfer: Thermal energy flow from a body at higher energy to one at lower energy by conduction, convection, or radiation.
Hot junction: The measuring junction of a thermocouple, also known as the measuring junction.

I

Ice point: The temperature at which ice melts at 1 standard atmosphere, defined as 0°C on the ITS-90 temperature scale.
Impedance: Total resistance to electrical current flow, combining resistive and reactive components.
Insulated hot junction: A thermocouple design where the measuring junction is electrically isolated from the cable sheath, commonly used in mineral insulated units.
Insulation resistance: The electrical resistance between conductors, or between conductors and the outer casing, when circuits are open.
Intrinsically safe: An instrument designed to prevent ignition by limiting electrical energy to levels insufficient to ignite hazardous gases.
ISA: Instrument Society of America, a professional organization for instrumentation and control engineering.
Isothermal: A condition of uniform and constant temperature throughout a system.
ITS-90: The International Temperature Scale of 1990, which defines calibration procedures to ensure consistent and accurate temperature measurement close to thermodynamic values.

J

Joule: A fundamental unit of thermal energy.
Junction: The point in a thermocouple where two dissimilar metals are joined, also known as the measuring junction.

K

Kelvin: The fundamental unit of temperature; 1 Kelvin equals 1°C.
Kelvin Scale: The thermodynamic temperature scale based on absolute zero, expressed in Kelvins.

L

Linearity: The deviation of an instrument’s response from a perfectly straight line.
Loop resistance: The total resistance in a thermocouple circuit, including the thermocouple wire and any extension or compensating cables.

M

Maximum operating temperature: The highest ambient temperature at which an instrument or sensor can operate safely on a continuous basis.
Mean temperature: The average of the maximum and minimum temperatures in a stable process.
Measuring junction: The electrical connection at one end of a thermocouple’s conductors that acts as the temperature sensor; also called the hot junction.
Melting point: The temperature at which a substance changes from solid to liquid.
MI (Mineral Insulated) thermocouple: A thermocouple constructed using metal-sheathed, mineral-insulated cable.
Microamp: One millionth of an ampere.
Microvolt: One millionth of a volt.
Milliamp: One thousandth of an ampere.
Millivolt: One thousandth of a volt.
Mueller bridge: A high-accuracy bridge configuration used to measure three-wire RTDs.

N

Negative temperature coefficient: A decrease in resistance with an increase in temperature.
Nicrosil-Nisil: An advanced nickel/chrome vs. nickel/silicon thermal alloy used in producing Type N thermocouples.
Noble metals: Metals with a positive electrochemical potential that are inert and highly resistant to corrosion and oxidation, such as gold, silver, and platinum.
Noise: Unwanted electrical interference on signal wires.

O

OD: Outer diameter.
Optical isolation: Connection of two networks using an LED transmitter and photoelectric receiver, ensuring no direct electrical connection.
Order/disorder transformation: A change in the arrangement of solute atoms in some alloys induced by specific heating and cooling regimes.

P

Peltier effect: The absorption or release of heat at the junction of two dissimilar conductors when an electric current flows.
Platinum metals: Metals including platinum, osmium, iridium, palladium, rhodium, and ruthenium. Platinum, with a melting point of 1773.5°C, is used for resistance thermometry and, along with iridium and rhodium alloys, for thermocouple thermometry. See Noble metals.
Polarity: The electrical sign (positive or negative) of a conductor or circuit.
Positive temperature coefficient: An increase in resistance with an increase in temperature.
Primary standard: The reference units and physical constants maintained by National Standards authorities, to which all measurement units are traceable.
Probe: A temperature sensor housed in a rigid or semi-flexible cylindrical protective tube.
Protection head: A cover fitted over a thermocouple or RTD protection tube opposite the sensor end, containing terminals for easy electrical connection. See Connection head.
Protection tube: A closed-end cylindrical sleeve providing mechanical and environmental protection for a sensor, made from metal, plastic, ceramic, or refractory materials.
PRT: Platinum Resistance Thermometer.
Pyrometry: The measurement of temperature.

R

Range: The area between two limits within which a quantity is measured, expressed as lower then upper limit.
Reference junction: The electrical connection joining each thermocouple conductor to a copper wire at the ends remote from the measuring junction. These junctions form the reference ends of voltage-generating conductors, usually maintained at a known temperature of 0°C. See Measuring junction.
Refractory metal thermocouple: Thermocouples made from materials with melting points above about 1,800°C, commonly tungsten and tungsten/rhenium alloys, such as Types G, C, and D.
Repeatability: The ability of a sensor or sensing system to generate the same output under repeated identical measuring conditions.
Resistance: The restriction to electrical current flow through a material, measured in Ohms. For a conductor wire, resistance depends on diameter, length, and resistivity (a physical property of the material).
Resistance thermometer: An instrument using a wire or film with predictable resistance-temperature characteristics to measure temperature by resistance.
Response time: The interval between a sudden temperature change applied to a sensor and the sensor output reaching a specified value, often defined as 63.2% of the final value.
RFI: Radio Frequency Interference.
RTD: Resistance Thermometer Detector.

S

Seebeck coefficient: The first derivative of thermal emf with respect to temperature, expressed in mV/°C.
Seebeck effect: The phenomenon where thermal energy produces an emf, forming the basis of thermocouple thermometry.
Seebeck emf: The open circuit voltage generated by the temperature difference between the hot and cold junctions of a thermocouple.
Secondary standard: A measurement standard derived from, and traceable to, a primary national standard.
Sensitivity: The output response magnitude or time developed by a thermocouple or RTD for a given temperature change.
Series mode noise rejection ratio: The ability of an instrument to reject interference, usually at line frequency (50 Hz), across its input terminals.
Sheathed MI cable: Cable with one or more conductors embedded in powdered insulant and surrounded by a metal sheath, the final diameter formed by drawing or swaging.
Signal: A general term for an electrical current or voltage representing a quantity or event.
Span: The difference between the upper and lower limits of a range.
Specific heat: The ratio of thermal energy needed to raise the temperature of a body by 1°C to that needed to raise the temperature of an equal mass of water by 1°C.
Stability: The consistency of a sensor’s output at given temperatures.

T

T/C: Thermocouple.
Temperature compensator: An externally powered device with a temperature-sensitive electrical network that can be connected to thermocouple conductors to provide an equivalent ice point voltage.
Temperature element: The innermost part of a temperature measuring probe, e.g., a ceramic former containing platinum wire, a sleeved thermocouple junction, or a glass-covered thermistor bead.
Temperature gradient: The distribution of temperature across a body or surface.
Thermal conductivity: The rate at which heat flows through a material for a given temperature difference, without heat gain or loss by the material itself.
Thermal emf: Electrical energy generated from the Seebeck effect; often refers to spurious voltages in measuring circuits.
Thermal expansion: Increase in size of a material due to temperature increase, usually expressed as length change per degree Celsius.
Thermal gradient: The rate of temperature change through a body or across a surface.
Thermal radiation: Electromagnetic radiation emitted by any body above absolute zero; heat exchange between bodies at different temperatures without a medium.
Thermistor: A semiconductor device showing a large, nonlinear resistance change with temperature, used as a temperature sensor.
Thermocouple: An electrical circuit made from two dissimilar materials generating a voltage dependent on the temperature difference between junctions; the reference junction is typically maintained at 0°C.
Thermocouple Break Protection: Safety feature to detect and manage thermocouple circuit failures.
Thermocouple Calibrator: Instrument used to verify the accuracy of thermocouples by simulating temperature signals.
Thermowell: A closed-end metal or ceramic tube, usually fixed in plant installations, protecting temperature sensors from corrosion and mechanical damage.
Thomson effect: Change of heat content in a single current-carrying conductor located in a temperature gradient.
Time constant: See Response time.
Transducer: Device converting a physical quantity into an electrical signal, e.g., resistance thermometer, thermocouple, strain gauge.
Transistor: A three-electrode solid-state amplifying device; some types can be used as temperature sensors over limited ranges.
Triple point: Temperature at which all three phases of a substance coexist in equilibrium; for water, 0.01°C.
Type J Thermocouple: Iron vs. Constantan thermocouple, suitable for reducing atmospheres, with a range up to 750°C.
Type K Thermocouple: Alumel vs. Chromel thermocouple, used over 0°C to 1370°C.
Type T Thermocouple: Copper vs. Constantan thermocouple, ideal for lab measurements over -250°C to 400°C.
Type N Thermocouple: Nicrosil vs. Nisil thermocouple, offering improved resistance to oxidation-related drift at high temperatures.

U

Ungrounded junction: A thermocouple probe constructed so the measuring junction is fully enclosed and insulated from the sheath material.

V

Volt: The unit of electrical potential difference between two points in a circuit; defined as the potential difference that moves one coulomb of charge using one joule of energy.

W

Wheatstone bridge: A network of four resistors, an emf source, and a null-reading instrument (e.g., galvanometer) arranged so that when balanced, zero current flows through the instrument.

Z

Zero offset: The difference between the true zero measurement and the instrument reading.
Zero power resistance: The resistance of an RTD element when dissipating zero power.
Zero suppression: Offsetting the instrument span from zero to increase measurement sensitivity within a specific temperature range.

Note: The information in this guide is provided for general informational and educational purposes only. While we aim for accuracy, all data, examples, and recommendations are provided “as is” without warranty of any kind. Standards, specifications, and best practices may change over time, so always confirm current requirements before use.

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Glossary of Thermocouple and RTD Terms

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

R

S

T

U

V

W

Z

Further Reading