The test results lead to a better understanding of how the validity of the calibration and the long-term accuracy of an USM is influenced by build-up or/and changes in wall roughness and how this can be diagnosed. By comprehensive testing in harsh corrosive atmosphere, which is considered to be more severe than normal practical situations, the effects of a) upstream pipe surface roughness and b) complete corrosion of a meter package (diameter change and/or wall roughness)on the meters accuracy have been shown. The results lead to the conclusion that a calibration respectively recalibration of this type of USM as a package (upstream pipe with meter) is not required, as the change of the inlet piping due to corrosion and different wall roughness's doesn't influence the meters accuracy. This saves handling costs for the initial and the future recalibrations of the meter and is a contribution to the ongoing discussion wheather the originalspoolpieces are needed for calibration or not.
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