engleski

Determination of DMM error using modified Hamon device

An accredited (National) laboratory for electrical measurements in Croatia was established at the Faculty of Electrical Engineering and Computing (FER, previously ETF - Faculty of Electrical Engineering) University of Zagreb in 1994. Operating within Department of Electrical Engineering Fundamentals and Measurements of the Faculty, its main task was to ensure a system of international traceability for the basic electromagnetic standards, usable for calibration of maintaining, secondary and working standards in other calibration and testing laboratories all-around the country. Together with the Voltage Balance ETF-84 as the primary voltage standard and Cs-clock as the frequency and time standard, it was necessary to build up a group of accurate resistors as a primary Ohm standard. The methods of maintaining and calibrating these resistors in a specially constructed ultrathermostat as well as the results of their first intercomparisons using two digital multimeters have been already published [4]. Since our laboratory does not belong to those laboratories around a world that have Josephson Voltage Standard, we have designed a method of calibration of DMM suitable for our purposes using specially built Hammon divider. We are comparing resistors with ratio 1:1 and 10:1. When comparing 1:1 there are no systematic errors, but when comparing 10:1 there is a cumulative systematic error of DMMs. First, we calculated the cumulative error of DMMs that we want to measure. Then, we measured the contact and interconnection resistance and analysed their influence on calculation of error. We were then able to modify Hammon divider in such a way that contact and interconnection resistance do not have effect on calculation of error and (or) measuring process.

Resistance measurement; Hamon device; DMM

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