Metrological assessment of DC current comparator resistance bridges

Direct-current comparator bridges (DCC) are the working horses of primary resistance metrology in the intermediate resistance range. Having a ratio accuracy reaching 10-7 or better, they allow the realization of resistance scales and the calibration of artifact standard resistors for customers. In this paper we compare the performances of three commercial DCC bridges, by performing measurements on resistors in decadal ratios (1 omega to 10 k omega) of very high stability in a thermostated environment. The results show that the three bridges give mutually compatible results within the manufacturer's specifications, therefore mutually validating the bridges; nevertheless, the readings time series show quite different statistical behavior, with internal correlations, making an evaluation of the Type A measurement uncertainty not trivial.