Uncertainty Propogation for Force Calibration Systems

Abstract

There are several labs operating throughout the world, which does not follow a designated guideline for calculating measurement uncertainty for force calibrations done in accordance with the ASTM E74 standard. Realizing the need for a guidance document, Morehouse decided to draft this document explaining how to calculate measurement uncertainty and how uncertainty propagation for force calibration systems works. The document examines uncertainty contributors for different tiers in the calibration hierarchy. We start with tier one laboratories using primary standards which are dead weight machines and work through the uncertainty propagation through tier two or secondary laboratories and then tier three laboratories. Calibrations, repeatability studies, and other tests were performed at each tier using different types of force calibration equipment. The paper follows the uncertainty progression and answers a question of what type of calibration standard and Calibration and Measurement Capability (CMC) is needed to achieve a specific Calibration and Measurement Capability at the next tier. Through examining the various uncertainty contributors we arrive at a conclusion that several force scopes may not be realistic in their CMC claims which means they may not be able to make statements of conformance. The testing proved the importance of the reference standard in relation to overall expanded uncertainty. Deadweight primary standards are predictably the best possible reference standard. A laboratory using secondary standards—those standards calibrated by deadweight—can achieve CMC’s as low as 0.02 % of applied force if they are using several standards. Nonetheless, the downside of using several standards is that this method involves standards to be changed at least once during the calibration which often further impacts test results. Failing to account for all the uncertainty contributors at any tier and not calculating Calibration and Measurement Capability properly will influence the Unit Under Test (UUT) in several ways resulting in lower combined uncertainties and raising measurement risk levels on all instruments in the entire measurement chain.