Uncertainty evaluation of a 10N·m dead weight torque standard machine and comparison with a 1kN·m dead weight torque standard machine

Abstract

A 10N·m dead weight torque standard machine (10-N·m-DWTSM) has been developed and evaluated since 2006 at the National Metrology Institute of Japan (NMIJ), a part of the National Institute of Advanced Industrial Science and Technology (AIST). Previously, the lengths of a moment arm, made of a low-thermal-expansion alloy (Super Invar), and the sensitivity limit of the fulcrum were evaluated. However, it is known that mechanical parts made of Super Invar vary in size with time. Therefore, the sensitivity limit of the fulcrum should be investigated under real calibration conditions. In this study, the moment arm lengths and the sensitivity limit of the fulcrum were re-evaluated. The moment arm lengths were found to have increased by an average of 6.3μm in five years. The relative combined standard uncertainty of the moment arm length, warm, was re-evaluated in consideration of the uncertainty of the secular length change and was found to be 1.8×10−5. The sensitivity limit of the fulcrum was investigated by using a highly accurate, small-rated-capacity torque measuring device. The relative combined standard uncertainty due to the sensitivity limit of the fulcrum was 2.5×10−5 in the 0.1–10N·m torque range. The uncertainty budget table of the 10-N·m-DWTSM was completed. The relative expanded uncertainty of torque realized by the 10-N·m-DWTSM, Wtsm, was evaluated in the 0.1–10N·m torque range and was found to be 6.6×10−5, with a coverage factor, k, being equal to 2. In addition, the 10-N·m-DWTSM was compared with the existing 1-kN·m-DWTSM at NMIJ by using small-rated-capacity torque measuring devices at 5N·m and 10N·m torque steps. Two loading conditions were adopted in this comparison. The comparison results showed good agreement within the uncertainties in all cases. Thus, the torque realized by the 10-N·m-DWTSM was shown to be equivalent to that achieved by the 1-kN·m-DWTSM.