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There are many factors which limit the accuracy of the final figure for the temperature. Absolute accuracy relates to how closely the thermocouple derived temperature mimics the actual temperature. The major errors usually arise from the thermocouple materials themselves. There are international standards that specify the amount by which sensors may deviate from ideal behaviour. These figures are reproduced here. The table shows the maximum permitted error in thermocouples which comply with IEC 584-2 (1982) and in PRTDs (platinum resistance temperature devices) which comply with BS 1904 (1984) Class A. All figures are in º Celsius.
Temperature B type E type J type K type N type R type S type T type PRTD -200 - - - 3.0 3.0 - - 3.0 0.55 -100 - - - 2.5 2.5 - - 1.5 0.35 0 - 1.7 1.5 1.5 1.5 1.0 1.0 0.5 0.15 200 - 1.7 1.5 1.5 1.5 1.0 1.0 0.8 0.55 400 - 2.0 1.6 1.6 1.6 1.0 1.0 - 0.95 600 1.5 3.0 2.4 2.4 2.4 1.0 1.0 - 1.35 800 2.0 4.0 - 3.2 3.2 1.0 1.0 - 4.30 1000 2.5 - - 4.0 4.0 1.0 1.0 - - 1200 3.0 - - 9.0 9.0 1.3 1.3 - - 1400 3.5 - - - - 1.9 1.9 - - 1600 4.0 - - - - 2.5 2.5 - -
Relative accuracy relates to measurement of changes in temperature, which is often the real purpose of thermocouples. Here the effect of noise can be very important.
Last revised 26 January 1996
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