Comparison of DC current comparator bridges for resistance metrology

Direct-current comparator bridges (DCC) are the working horse of primary resistance metrology in the intermediate resistance range. Having a ratio accuracy reaching 10−7 or better, they allow the realisation 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 decadal resistors (1 ohm to 10 kohm) of very high stability in a thermostated environment. The results show that the three bridges give mutually compatible results within the manufacturer specifications, therefore mutually validating the bridges; nevertheless, the readings time series show quite different statistical behavior, with internal correlations, making an evaluation on the Type A measurement uncertainty not trivial.