In key-comparisons of force standards among National Metrology Institutes, it is a common practice to circulate only strain-gauge force transducers and a common bridge calibration unit and to use each participating laboratory's own precision strain-gauge amplifiers. In this scenario, the amplifiers should be compensated by referring to the common bridge calibration unit; however, in some combinations of force transducers and amplifiers, undesirable differences in the indications were observed even after compensation. This paper reports on experiments to examine the remaining individual differences among six precision amplifiers, revealing that considerable differences remained in some cases. The maximum difference exceeded both the uncertainty of the reference voltage ratio signal from the bridge calibration unit and the instability in the sensitivities of the force transducer and the amplifier. One measure to cope with this problem would be to make an intra-laboratory comparison among multiple amplifiers within the laboratory, prior to conducting inter-laboratory comparisons, if possible. (C) 2016 Elsevier Ltd. All rights reserved.
