Thermocouples

An introduction to thermocouples and resistance thermometers

Our range provides two alternative types of temperature sensor – thermocouple or platinum resistance thermometer (PRT).

General guidance for choosing between the two:

Thermocouples are less expensive than PRT sensors, more versatile, less accurate, more rugged, more sensitive, have a wider operating range, are tip sensitive and can be smaller. Extension cabling is more expensive. Various calibrations are available for different

applications.

PRT assemblies are much more accurate and stable, permit greater resolution of measurement but over a more limited range, are stem sensitive, larger and more expensive. Extension cabling is by inexpensive copper wire.

Remember the type of sensor chosen must suit the associated instrumentation.

Thermocouples

For practical purposes, thermocouples are divided into two categories; base metal types such as Nickel Chromium v Nickel Aluminium and Iron v Copper Nickel (Constantan) and rare metal types (usually Platinum v Platinum Rhodium). Measuring junctions are formed by a

spot weld of the two dissimilar wires usually located inside a protecting sheath.

A thermocouple does not measure the temperature at the “hot” (measuring) junction but is a differential measurement between the “hot” junction and the “cold” (reference) junction. For accurate temperature measurement, the reference junction must be held at a fixed, known temperature; alternatively, accurate compensation must be made for any temperature variations of the reference junction.

In the majority of industrial applications, instruments configured for thermocouple inputs incorporate some form of automatic cold junction compensation (C.J.C) The temperature of the input termination is sensed and a corrective signal is applied to the measuring

circuit of the instrument. High accuracy automatic CJC is incorporated in all Eurotherm instruments.

For purposes of calibration of a thermocouple or the associated instrument, reference is made to standard tables which indicate thermocouple output versus “hot” junction temperature with the “cold junction” at 0°C. Where it is impractical to maintain a 0°C cold

junction temperature it is necessary to accurately measure the “cold Junction” temperature, say at 20°C and to add the value of thermocouple output at that temperature (obtained from the table) to the observed output from the thermocouple to arrive at an

absolute figure.