Current-Independent AC-DC Transfer Errors in Single-Junction Thermal Converters

AC-DC transfer errors in single-junction thermal converters (TC´s) at midfrequencies, and the effect of thermoelectric nonuniformities in their heaters, are discussed. Details of special TC´s designed to eliminate or minimize the heater nonuniformities are given. AC-DC transfer measurements have been carried out, relative to a reference pair of converters, on 14 special TC´s made to 5 different designs, by 4 different manufacturers. Current-independent ac-dc difference components are determined for each of the TC´s from a weighted least squares fit of data taken over a range of currents. Theoretical values of current-independent errors are also calculated, using thermoelectric and thermal conductance data determined from measurements performed on the special TC´s or similar units, and applied as corrections to the measured values. The weighted mean of the corrected current-independent ac-dc differences for the group gives an estimate of the current-independent error of the reference pair, and is believed to have an uncertainty of <0.5 ppm. AC-DC transfer measurements, relative to the mean of the group, indicate that two multijunction thermal converters (MJTC´s) have current-independent errors exceeding 2 ppm. The current-independent error determined for the reference TC´s is to be used in conjunction with a new current-dependent error determination to improve the accuracy of Australia´s national ac-dc transfer standard.