17 Appendix

17.2.Accuracy of the thermocouple measurements

The measurement inaccuracies thereafter are given as maximum values: typical values are 2 to 3 times less.

The measurement accuracy in temperature is the sum of several possible sources of inaccuracy: Pl: accuracy of standardisation

Ps: cold weld accuracy

Pm: accuracy of the measurement of the equivalent voltage

Pd: accuracy of the measurement offsets

The total accuracy is then: Pt = Pl + Ps + Pm + Pd

For the recorder:

Pl = 0.25 C for all the thermocouples

Ps = 1.25 C for all the thermocouples

Pd = 0.15 % of the offset

Pm = given in the tables

Measurement accuracy: Pm

The measurement accuracy Pm depends on the voltage range used by the appliance. The measurement error in degrees will then be the measurement error in voltage divided by the slope in V/C of the thermocouple. For each thermocouple, an illustrative value of the slope is given at a given temperature. This slope value is relevant for the thermocouple and allows you to calculate the temperature error. If you want to have even further accuracy, you have to read the slope in the thermocouple reference tables (because it changes with the temperature).

EXAMPLE OF ACCURACY CALCULATION

A measurement is made with the range 50C centred on 25C with a J thermocouple without cold weld compensation.

Pt = Pl + Ps + Pm + Pd

Pl = 0.25 C (accuracy of standardisation)

Ps = 0C (no cold weld compensation)

Pd = 0.04C (+ 0.15 % of the temperature, paper medium)

Pm = 0.20C (extracted from the table at range 50C)

Max. total accuracy:

Pt = 0.25 + 0.04 +0.20 = 0.49C

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Escort 3016B, 3008B, 3004B user manual Accuracy of the thermocouple measurements, Example of Accuracy Calculation