Abstract
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.
Published Version
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