Effective Area Calculation for Piston-Cylinder Assembly with Large Nominal Diameter

摘要

Two piston-cylinder assemblies with large nominal diameters of 50 mm and 35 mm, which are intended to establish new primary pressure standards, have been set up in the National Metrology Centre, Agency for Science Technology and Research, (NMC, A*Star), the national metrology institute of Singapore. These two assemblies have been calibrated by Geometry Metrology Standards for the properties of straightness, roundness, and diameters. Based on the calibrated data of properties, a known and proven least square method for three-dimensional geometry is applied to determine the effective areas at atmospheric pressure and temperature 20°C for these two assemblies. The present study proposed improved models with higher-order polynomial equation and inclusion of frequency term. By using the present improved models, the residual uncertainty in modelling can be reduced to better fit the roundness and straightness of the cylinder. The determined effective areas from dimensional properties are compared with those by the cross float method based on proven pressure standards. The comparison results show good agreement, validating the traceability of the present data by using least square method. It is also verified that least square method is suitable to be used for the piston-cylinder assemblies with large nominal diameter to calculate the effective area accurately. In addition, cross-float of these two large piston-cylinders with the effective areas determined is applied to calculate the pressure. The deviation of the pressure does not exceed 6 × 10~(-6) over the pressure range (50 to 520) kPa in gauge mode.