A Method for the Determination of AC-DC Transfer Errors in Thermoelements

A method is presented for the determination of ac-dc transfer error as a function of current in thermoelements. To test the method the transfer error variation has been determined for two 10 mA thermoelements at an ac excitation frequency of 57 Hz, and the results compared. On the basis of one of these determinations a dual multijunction thermal converter has been calibrated to provide a check. A number of single junction thermoelements have also been calibrated, and results indicate significantly larger errors as I ¿ 0 than expected on theoretical grounds. Also, for all thermoelements with Evanohm heaters, the error variation follows a square law of opposite sign to that predicted by theory. An explanation is offered for this behavior. Results indicate that changes in transfer error with current level can be determined to ~1 ppm. Absolute transfer errors for the thermoelements tested however, are probably not known to better than a few ppm.