If so, you must measure the temperature of the cold junction using another sensor, perhaps a thermistor. Your type J thermocouple may be terminated at a cold junction temperature other than 0☌. The graph shows residual errors after calibrating the thermocouple with a rational function of polynomials. The following table contains calibration coefficients for Type J thermocouple wire. The coefficients, fitted data and charts of residual errors may be found in this Excel spreadsheet of type J thermocouple data. The full temperature range is broken into several sub-ranges, and a different set of coefficients used for each. The coefficients, T o, V o, p i and q i were found by performing a least squares curve fit to the NIST data for type J thermocouples. The second form of the equation emphasizes the most efficient order of operations. The function uses a ratio of two polynomials, P/Q, in this case a fourth order to a third order polynomial. Where T is the thermocouple temperature (in ☌), V is the thermocouple voltage (in millivolts), and T o, V o, and the p i and q i are coefficients. Using a least squares curve fitting procedure we fit the National Institute of Standards and Technology (NIST) type J thermocouple data with a rational function of the following form, The calibration equation uses a ratio of two smaller order polynomials rather than one large order polynomial. The rational polynomial coefficients provided here produce an order of magnitude lower errors than the NIST ITS-90 type J thermocouple coefficients for direct and inverse polynomials.Ī rational polynomial function approximation for Type J thermocouples is used for computing temperature from measured thermocouple potential. This page shows you how to convert Type J thermocouple voltage to temperature with rational polynomial functions more efficiently and more accurately than you can using higher order polynomials. However, high order polynomials are often not very accurate – they produce systematic errors as the polynomial oscillates around the fitted data, as shown in these curve fits to Type K thermocouple data.īut there is a better way – using the ratio of two lower order polynomial functions. The National Institute of Standards and Technology ( NIST database of thermocouple values) shows a high order polynomial formula with coefficients curve fitted to Type J thermocouple data they also provide. By choosing the correct thermocouple calibration type, one can better guarantee the life of the sensor, its accuracy requirements, and avoid metallurgy degradation.If you use Mosaic's Thermocouple Wildcard, Type J thermocouple voltage measurements are converted to temperatures for you by the Wildcard's software drivers, and at high accuracy.īut for those of you not using it, but looking for accurate equations for thermocouple voltage to temperature conversion, we hope these thermocouple calibration coefficients are helpful.Įquations for voltage/temperature conversion usually are based on high-order polynomials. Thermocouples are also calibrated according to the application atmosphere, so it is possible to avoid chemical reactions between the operating atmosphere and the thermocouple type. For example, some thermocouples produce significant voltage at low temperatures, while others do not begin to generate voltage until much higher temperatures are reached.Ĭalibration types aim to make it easy for instruments or temperature controllers to correctly match voltage to its corresponding temperature value by creating the straightest possible line for the voltage curve within the prescribed temperature range. In the case of thermocouples, the voltage output is calibrated to account for electromotive force (EMF) and the temperature curve of each thermocouple construction. Thermocouples are analog temperature sensors, which means that their output data must be interpreted and converted.
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