Uncertainty Analysis when Using Multifunction Calibrators to Calibrate Digital Multimeters

摘要

Multifunction calibrators (MFC's) are widely used by calibration laboratories for the calibration of digital multimeters (DMM's). This paper presents an uncertainty analysis of this calibration process that is in compliance with ISO/IEC 17025 and ANSI/NCSL Z540-2, the Guide to Uncertainties in Measurement. For this analysis, it is assumed that the calibration laboratory relies on the manufacturer's one-year specification of the calibrator to cover most sources of uncertainty associated with this calibration process. Therefore under the assumption that both the environmental conditions and calibration protocols specified by the manufacturer of the calibrator and by the manufacturer of the DMM are followed, the measurement uncertainty of the calibrator should be well represented by the manufacturer's specification. Under these conditions, there are three primary sources of uncertainty associated with the calibrator/DMM calibration process: (1) manufacturer's specification for the calibrator; (2) resolution of the DMM, and (3) the repeatability of the DMM readings when connected to the calibrator. These sources of uncertainty are discussed in detail and a sample uncertainty budget is presented. The calibration of the DMM by the calibrator results in a correction factor (or offset) and its associated uncertainty for each point tested. It is also shown how the results of this calibration can be used to determine if the DMM is within its manufacturer's specifications (or tolerance).