Comparison of methods for calculating distortion in pressure balances up to 400 MPa - EUROMET Project 256

Abstract

EUROMET project #256 was established to compare agreement between laboratory calculations of the elastic distortion that piston-cylinder assemblies of pressure balances, the primary standard for pressure measurements, undergo when subjected to pressure applied via liquid media. The following piston-cylinder assemblies were selected for comparison of the calculations: (i) PTB, 400 MPa, free-deformation type; and (ii) BNM-LNE, 200 MPa, free-deformation and controlled-clearance types. Calculations made at the IMGC using an analytical method were compared with those made at the NPL and the PTB, each using a different finite element technique. All three methods couple calculation of the elastic deformation of the piston-cylinder with calculation of the pressure distribution in the piston-cylinder clearance from a flow model. After reviewing the calculation methods, the paper presents results for each of the comparison systems. The calculated piston and cylinder distortion, annular gap profile, annular pressure distribution, piston fall rates, and pressure distortion coefficients are presented and compared with experimental measurements. A discussion is presented of the extent of the agreement between the calculation methods and what this implies for the reliability of calculated distortion coefficients. The comparison also resulted in other useful knowledge being obtained, for example on the influence of the cylinder boundary conditions and the influence of simplifications adopted in the geometrical model. This information will be of use in the application of such calculations to the design of piston-cylinder assemblies or prediction of the effect of experimentally measurable parameters.