Re-evaluation of sensitivity limit in a 10 N·m deadweight torque standard machine

We have been disseminating the national torque standard via calibration of torque measuring devices (TMDs) to the industry in the range from 0.1 Nm to 10 Nm by using a 10 Nm deadweight torque standard machine developed at NMIJ/AIST (10-Nm-DWTSM). In the previous study, the sensitivity limit of the fulcrum of the 10-Nm-DWTSM had been 2.5 μN·m, and the relative combined standard uncertainty due to the sensitivity limit of the fulcrum was 0.0025 %. In order to expand the range of a national torque standard down to 0.01 Nm in a certain precise level, we had to improve the sensitivity limit of the fulcrum. In this study, we re-evaluated the sensitivity limit of the fulcrum of the 10-Nm-DWTSM by using smaller weights and a lower nominal capacity TMD than the previous experiment. Mass of small weights were 1 mg, 0.5 mg and 0.1 mg. The high accuracy TMD with nominal capacity of 0.1 Nm was developed as a trial, and it was installed on the 10-Nm-DWTSM. The output was measured after the small weight was loaded or unloaded automatically by using the weight loading components of the 10-Nm-DWTSM. As a result, it was confirmed that the sensitivity limit of the fulcrum of the 10-Nm-DWTSM was sufficiently smaller than at least 0.1 mg. It was approximately equivalent to 0.5 μN·m as the torque unit, and the relative combined standard uncertainty limit of the fulcrum was 0.0052 % in the range from 0.01 Nm to 0.1 Nm.