Glossary of Thermocouple and RTD Terms

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 at 100°C. It indicates the basic change in resistance in a material for each °C change in temperature.
• 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 concerned.
• Ampere (Amp): The unit defining the quantity of electricity (current) flowing in a circuit; units are coulombs per second.
• Ammeter: Instrument which measures current.
• Annealing: Heat treatment of materials to relieve internal stresses, dislocations, etc.
• 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 that falls on it; also a perfect radiator of energy.
• Boiling point: The temperature at which a substance in the liquid phase transforms into the gas phase; commonly, for water, this is nominally 100°C.
• Bonded hot junction: Refers to a type of mineral insulated thermocouple where the measuring junction is integral with the tip of the sheath, thus electrically grounded to it.

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 and as beads.
• Chromel: Trade name of nickel-based, high temperature positive thermocouple material used in Type K thermocouples with Alumel.
• Cold junction: Original name for the reference junction of a thermocouple - now implying floating temperature.
• Cold seal: The enclosure surrounding the flexible tail connections on a mineral insulated thermocouple unit.
• Colour codes: The 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 with relation to local ground - expressed in dB.
• Compensating cable: Cable for extending thermocouple circuits more cheaply. It has conductors that, in combination, match the thermocouple characteristics over a 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.
• Compensator: An externally powered electrical network containing a temperature sensitive element connected to a thermocouple to provide an equivalent 0°C reference voltage.
• Connection head: A cover or enclosure fitted over a thermocouple or RTD protection tube at the end remote from the sensor. It contains electrical terminals to allow convenient connection of the sensor to the rest of the circuit.
• Constantan: A copper-nickel alloy originally developed for electrical resistance purposes, now forming the negative leg of Type J and Type T thermocouples.
• Curie point: The temperature at which a magnetic material becomes substantially non-magnetic.

D

• DC: Direct Current
• Drift: Shift of a signal or reading over long periods due to factors like ambient temperature change, hysteresis of the sensor and other physical, chemical and electromagnetic effects.
• Duplex: A name commonly given when two thermocouples or RTD’s are housed in the same probe.

E

• EMF: Abbreviation for electromotive force - the electrical potential difference developed by sources of energy in electrical circuits.
• EMI: Electromagnetic interference.
• Emissivity: The ratio of emitted energy from a surface compared with a black body at the same temperature.
• Error: The difference between the correct or desired value and the real value.
• Exposed junction: A form of faster response thermocouple probe constructed such that the measuring junction protrudes beyond the sheath, and is thus fully exposed to the medium being measured.
• Extension cable: A method of extending thermocouple circuits with lower cost or more suitable types of wiring cable - contains similar materials to those of the thermocouple itself.

F

• Fahrenheit scale: A temperature scale still in use for historical reasons where degrees Fahrenheit (°F) designate a point on the temperature scale and the magnitude of a temperature interval. 32°F is the ice point; 212°F is the boiling point of water.
• Fixed Point: A measure of the voltage that an insulating material can withstand before an electrical breakdown occurs.
• Flexible tails: The stranded flexible insulated wires attached to mineral insulated thermocouples for connection purposes.
• Freezing point: The temperature at which a substance changes state from the liquid phase to the solid phase - in water 0°C.
• Gain: The degree of amplification in an electrical circuit.

G

• Galvanometer: An instrument designed to measure small currents via deflecting magnetic coils.
• Ground: The reference point for an electrical system.
• Grounded junction: Construction of a thermocouple probe where the measuring junction is in electrical contact with the sheath so that sheath and thermocouple are at the same potential.

H

• Heat transfer: Thermal energy flow from one body having higher energy to a body of lower energy - by conduction, convection and radiation.
• Hot junction: Original and commonly used name for the measuring junction of the thermocouple - see Measuring junction.

I

• Ice point: The temperature established by ice melting at a pressure of 1 standard atmosphere. 0°C on ITS-90.
• Impedance: Total resistance to electrical current flow (resistive plus reactive).
• Insulated hot junction: A form of construction adopted primarily for mineral insulated thermocouple units where the measuring junction is separated and electrically insulated from the cable sheath.
• Insulation resistance: The value of the electrical resistance existing between conductors, or between the conductors and the outer casing of an electrical system when the conductors are not connected.
• Intrinsically safe: An instrument which is designated as intrinsically safe will not pass, contain or generate sufficient electrical energy under any conditions that could lead to the ignition of a hazardous gas mixture.
• ISA: Instrument Society of America.
• Isothermal: The condition of uniform, constant temperature.
• ITS-90: The International Temperature Scale of 1990 defines procedures by which thermometry systems can be calibrated such that the values of temperature obtained are concise and consistent instrument-to-instrument and sensor-to-sensor - while approximating to thermodynamic values within the limits of the technology currently available.

J

• Joule: A fundamental unit of thermal energy.
• Junction: The point in a thermocouple where the two dissimilar metals are joined - see measuring junction.

K

• Kelvin: The fundamental unit of temperature. 1 Kelvin is equal to 1°C.
• Kelvin Scale: The Kelvin thermodynamic scale of temperature is the fundamental temperature scale. It is determined from absolute zero, and is expressed in Kelvins.

L

• Linearity: The deviation of an instrument’s response from a straight line.
• Loop resistance: The total resistance of a thermocouple caused by the resistance of the thermocouple wire and associated extension or compensating cable making up the circuit.

M

• Maximum operating temperature: The maximum ambient, continuous temperature at which an instrument or sensor can be safely operated.
• Mean temperature: The average of the maximum and minimum temperatures of a process in equilibrium.
• Measuring junction: The electrical connection that comprises one end of the conductors in a thermocouple, and is used as the temperature sensor. See Hot junction.
• Melting point: The temperature at which a substance transforms state from the solid phase to the liquid phase.
• MI (Mineral Insulated) thermocouple: Common name for a thermocouple unit manufactured from metal sheathed, mineral insulated thermocouple 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 RTD’s.

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 the production of Type N thermocouples.
• Noble metals: Metals which tend towards a positive electrochemical potential, are inert and have a high resistance to corrosion and oxidation - gold, silver and platinum.
• Noise: An unwanted electrical interference on the signal wires.

O

• OD: Outer diameter.
• Optical isolation: Two networks connected only via an LED transmitter and photoelectric receiver such that there is no electrical connection.
• Order/disorder transformation: A change in the arrangement of the solute atoms in some alloys induced by certain heating and cooling regimes.

P

• Peltier effect: The absorption or evolution of heat apparent at the junction of two dissimilar conductors when an electric current is flowing.
• Platinum metals: These are platinum, osmium, iridium, palladium, rhodium and ruthenium. Platinum has a melting point of 1773.5°C, and is used for resistance thermometry. Together with iridium and rhodium alloys, it is also used in thermocouple thermometry - see Noble metals.
• Polarity: The sign (positive or negative) of an electrical conductor, etc.
• Positive temperature coefficient: An increase in resistance due to an increase in temperature.
• Primary standard: The standard reference units and physical constants maintained by the National Standards authorities upon which all measurement units are traceable to.
• Probe: Usually refers to a temperature sensor fitted in a rigid or semi-flexible cylindrical protection tube of some kind.
• Protection head: A cover or enclosure fitted over a thermocouple or RTD protection tube at the end remote from the sensor. It contains electrical terminals to allow convenient connection of the sensor to the rest of the circuit - see Connection head.
• Protection tube: A closed end, cylindrical sleeve fitted over a sensor to provide mechanical and environmental protection. Can be made from metal, plastic, ceramic, or refractory material.
• PRT: Platinum Resistance Thermometer.
• Pyrometry: The measurement of temperature.

R

• Range: The area between two limits between 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, and are usually maintained at a known temperature - 0°C - see Measuring junction.
• Refractory metal thermocouple: The class of thermocouples whose materials of construction have melting points above about 1,800°C. Most common are those made from tungsten and tungsten/rhenium alloys, as in Types G, C and D.
• Repeatability: The ability of a sensor or complete sensing system to generate the same output or reading under repeated identical measuring conditions.
• Resistance: The restriction to electrical current flow through a material, measured in Ohms; for a conductor wire, resistance is a function of diameter, length and resistivity (resistance per unit length - a physical material property).
• Resistance thermometer: An instrument or system incorporating a length of wire or film having predictable resistance vs temperature characteristics, forming a temperature sensor. Measurement of the resistance of the device yields its temperature.
• Response time: The time interval between the application of a sudden change of temperature to a sensor and the attainment of a given output. The change is frequently defined as 63.2% of the final value.
• RFI: Radio Frequency Interference.
• RTD: Resistance Thermometer Detector.

S

• Seebeck coefficient: First derivative of thermal emf with respect to temperature - expressed as mV/°C.
• Seebeck effect: The phenomenon whereby thermal energy produces an emf - forms the basis of thermocouple thermometry.
• Seebeck emf: Open circuit voltage caused by the difference in temperature between the hot and cold junctions of a thermocouple.
• Secondary standard: A standard for parameter measurement itself derived from, and thus traceable to, a primary national standard.
• Sensitivity: The output response - in time or magnitude - 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 of line frequency (50Hz) across its input terminals.
• Sheathed MI cable: Cable comprising one or more conductors embedded in a powdered insulant and surrounded by a metal sheath. Final diameter is produced by drawing or swaging.
• Signal: General term for an electrical current or voltage representing a quantity, event, or whatever.
• Span: The difference between the upper and lower limits of a range.
• Specific heat: The ratio of thermal energy required to raise the temperature of a body 1°C, to the thermal energy required to raise an equal mass of water 1°C.
• Stability: The consistency of output of a sensor to given temperatures.

T

• T/C: Thermocouple.
• Temperature compensator: An externally powered device comprising a temperature sensitive electrical network that can be connected to the thermocouple conductors to provide an equivalent ice point voltage.
• Temperature element: Usually applied to the innermost part of a temperature measuring probe - eg, a ceramic former containing platinum wire, a sleeved thermocouple junction, a glass covered thermistor bead, etc.
• Temperature gradient: The distribution of a temperature interval existing through a body or across a surface.
• Thermal conductivity: The rate at which heat flows through a material for a given temperature difference applied to it, with no gain or loss of heat by the material.
• Thermal emf: Source of electrical energy arising from Seebeck effect. Frequently applied to spurious voltages developed in measuring circuits.
• Thermal expansion: The increase in material size resulting from an increase in temperature - thermal coefficient normally expressed as the length change per degree Centigrade.
• Thermal gradient: The rate of change of temperature as measured through a body or across a surface.
• Thermal radiation: The electromagnetic radiation emitted by any body at a temperature above absolute zero. With two bodies at different temperatures, and in view of each other, there will be a net interchange of heat without the need of an intervening medium.
• Thermistor: A semiconductor which exhibits a large, non-linear change of resistance with temperature - used as a temperature sensor.
• Thermocouple: An electrical circuit comprising two dissimilar materials. A voltage is generated that is dependent on the temperatures at the junctions forming the limits of the dissimilar materials. The reference junction at one end of the conductors is usually maintained at 0°C to allow the measuring junction to be used as a temperature sensor.
• Thermocouple Break Protection: A safety measure for identifying and addressing thermocouple circuit failures.
• Thermocouple Calibrator: An instrument used to validate the accuracy of thermocouples by simulating temperature readings.
• Thermowell: A closed end (re-entrant) metal or ceramic tube, usually fitted permanently in plant to protect a temperature sensor from corrosive environments or mechanical forces.
• Thomson effect: The change of heat content in a single current carrying conductor when situated in a temperature gradient.
• Time constant: See Response time.
• Transducer: A device that converts a physical quantity into a related electrical signal - eg, a resistance thermometer, thermocouple, strain gauge, etc.
• Transistor: A three electrode, solid state, electronic amplifying device - some types can be used as temperature sensors over a limited range.
• Triple point: The temperature at which all three phases of a substance are in equilibrium; the triple point of water is 0.01°C.
• Type J Thermocouple: Iron vs. Constantan thermocouple, suitable for reducing atmospheres, with a range up to 750°C.
• Type K Thermocouple: Comprised of one conductor of Alumel and the other of Chromel, used over a temperature range of 0°C to 1370°C.
• Type T Thermocouple: Copper vs. Constantan thermocouple, ideal for laboratory measurements over the range -250°C to 400°C.
• Type N Thermocouple: Nicrosil vs. Nisil thermocouple, offering greater resistance to oxidation-related drift at high temperatures.

U

• Ungrounded junction: A thermocouple probe constructed such that the measuring junction is fully enclosed by, and insulated from, the sheath material.

V

• Volt: The unit of electrical potential difference between two points in a circuit - derived as work per unit charge, and defined as the potential difference needed to move one coulomb of charge between two points in a circuit using one joule of energy.

W

• Wheatstone bridge: A network of four resistances, an emf source and a null reading instrument (eg: galvanometer) connected such that when the four match there is zero current flow through the instrument.

Z

• Zero offset: The difference between the real measurement zero and the instrument reading.
• Zero power resistance: The resistance of an RTD element dissipating zero power.
• Zero suppression: Where the span of an instrument is offset from zero, allowing it to measure with greater sensitivity in the temperature range of interest.

Further Reading

RTD vs Thermocouple – Choosing the Right Sensor
Explore the features and characteristics of thermocouples and RTDs

RTD Output Tables
View Resistance versus Temperature tables for all Pt100 sensors.

What are the RTD colour codes?
Explore RTD colour codes and wiring configurations.