Temperature measurement: Thermocouples

This Data Item is available as part of the ESDU Sub-series on Heat Transfer. Background information and practical guidance on designing temperature measuring systems using thermocouples is provided. The nominal temperature range covered is -200 degrees C to 2000 degrees C but the comments apply, in general terms, to all thermocouple systems. The information is aimed at the user who wishes to design and install a practical thermocouple system using improved techniques that will allow temperatures to be measured within known tolerances. The selection, preparation, and installation of thermocouples, the use of compensating or extension cables, methods of referencing to a known temperature and measurement system are considered. The requirements for reliable systems operating to commercial tolerances are also described. Various factors that might impair the accuracy and stability of thermocouples are identified together with methods of reducing their effect. A check list for the design of a thermocouple system is given and a flow chart procedure for selecting appropriate thermocouple materials is provided as well. The employment of the techniques described will ensure that the temperature of the measuring junction is within known tolerances.