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Temprel Thermocouple Glossary

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Alternating current; an electric current that reverses its direction at regularly recurring intervals.


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.


A mechanism or device for attaching non-mating parts.


A character set that contains both letters and digits.


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.


An instrument used to measure current.


American National Standards Institute.


American Society of Mechanical Engineers.


A program that translates assembly language instructions into machine language instructions.


American Society for Testing and Materials.


Automatic temperature compensation.


An automatic internal correction for offsets and/or drift at zero voltage input.


American Wire Gauge.


Background Noise:

The total noise floor from all sources of interference in a measurement system, independent of the presence of a data signal.


BeO (Beryllium Oxide), a high-temperature mineral insulation material; toxic when in powder form.


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.



The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured.

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 multi hole tubes or as beads.


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.


The rapid cycling on and off of a relay in a control process due to insufficient bandwidth in the controller.

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).


An addition of specific materials or devices to counteract a known error.

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.


A copper-nickel alloy used as the negative lead in Type E, Type J, and Type T thermocouples.

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.


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.

Cycle Time:

The time, usually expressed in seconds, for a controller to complete one on/off cycle.


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.


Direct current; an electric current flowing in one direction only and substantially constant in value.


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.


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.

Dual Element Sensor:

A sensor assembly with two independent sensing elements.


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.


Electromotive force. A rise in (electrical) potential energy. The principal unit is the volt.


Electromagnetic interference.


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.


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.

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.



A temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level.


A compressible tubular fitting that is compressed onto a probe inside a compression fitting to form a gas-tight seal.

Freezing Point:

The temperature at which a substance goes from the liquid phase to the solid phase.



The amount of amplification used in an electrical circuit.


An instrument that measures small electrical currents by means of deflecting magnetic coils.


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.


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.


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.


The means by which two systems or devices are connected and interact with each other.



The point in a thermocouple where the two dissimilar metals are joined.


Kilowatt (kw):

Equivalent to 1000 watts.

Kilowatt Hour (kwh):

1000 watt hours.

Kilovolt amperes (kva):

1000 volt amps.


Kilovolt amperes (1000 volt amps).



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.

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).


The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms).


Maximum Operating Temperature:

The maximum temperature at which an instrument or sensor can be safely operated.

Mean Temperature:

The average of the maximum and minimum temperature of a process equilibrium.

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.


A transparent mineral used as window material in high temperature ovens.

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 high temperature and severe-duty applications.



An unwanted electrical interference on the signal wires.


National Pipe Thread.



Outside diameter.

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.


The electrical signal which is produced by an applied input to the transducer.


The number of degrees by which a process exceeds the set point temperature when coming up to the set point temperature.



Proportional, integral, derivative. A three-mode control action where the controller has time proportioning, integral (auto reset) and derivative rate action.


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.



Those values which a transducer is intended to measure, specified by upper and lower limits.

Real Time:

The time interval over which the system temperature is sampled for the derivative function.


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.

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.


Resistance temperature detector.


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.


An electrical transmittance (either input or output) that conveys information.


The difference between the upper and lower limits of a range expressed in the same units as the range.


Temperature Error:

The maximum change in output, at any measured value within a specified range, when the transducer temperature is changed from room temperature to specified temperature extremes.

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.


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
T Copper/Constantan
E Constantan/Chromel


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.


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.


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.


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.


A form of pipe fitting where two extension pipes are joined at a separable coupling.



A pressure less than atmospheric pressure.


An electrical potential which can be measured in volts.


An instrument used to measure voltage.


Watt Density:

The watts emanating from each square inch of heated surface area of a heater. Expressed in units of watts per square inch.

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