Dimensional calibration of small critical flow Venturi nozzles

A set of 14 Critical flow Venturi nozzles (CFVN) with nominal throat diameters of 1 mm and 2 mm but with different shapes and surface roughness were dimensionally calibrated using the METAS Coordinate Measuring Machine (CMM). A detailed analysis method using the raw data from the dimensional calibration was presented and compared to the reference value given by the dimensional calibration experts. Deviations ranging from less than 0.05 % to 0.24 % for throat diameters are observed, indicating that the analysis method affects the end result. Discrepancies are found due to form error. For such small CFVN, form error is the main contributor to the throat diameter uncertainty and therefore directly affects the end result of a dry calibration for such small nozzles. These CFVN were calibrated in nitrogen using two METAS flow standards over a Reynold number range from 6.5 ∙ 10 4 and 4.0 ∙ 10 5 with an expanded uncertainty of 0.15 % and 0.30 %, respectively. The agreement between predicted and measured discharge coefficients depends on the analysis method to determine the throat diameter. A trend favouring the alternative analysis method is observed but further analysis is needed.