Interpreting Signals from Differential Temperature Transducer
Thermocouples, while maintaining fixed calibration, have non-linear characteristics (the proportionality factor between differential temperature and electrical output is not constant). There are several ways to obtain accurate values of temperature differential from the measured millivolt signals.
Where the differential temperature is small (~5°C or less), a single value of sensitivity (millivolts/degree) can be used, based on the average temperature (Tin + Tout)/2. The NIST tables for thermocouple characteristics contain values for sensitivity at each temperature. See Figure A.
Where the temperature differential is large, an algorithm is available which is based on an 8th order polynomial curve fit to the thermocouple characteristics. It requires, as input, an approximate value of inlet temperature of the fluid. The procedure is described in "Differential Temperature Transducer - an iterative method of evaluating the signal", published in Measurements & Data, October 1981.