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Session Topic: 1C, Pressure Session

   

Paper Title: Determination of the Pressure Distortion Coefficient of Pressure Balances using a Modified Experimental Method

   

Date & Time: MONDAY, August 5, 2002

 

10:45am - 12:15pm PARALLEL SESSIONS - SESSION 1

   

Speaker: Wladimir Sabuga PTB

Email: wladimir.sabuga@ptb.de

   

The accuracy of pressure measurements above approximately 50 MPa using pressure balances is limited by the knowledge of their pressure distortion coefficient (l). Numerical methods, with the Finite Element Analysis (FEA) as the most efficient one, allow the radial elastic distortions, the pressure distribution in the gap and, thus, l to be calculated if the properties of the piston-cylinder assembly and of the pressure-transmitting medium are known. In contrast to the theoretical methods, the experimental methods applied to assemblies operated in the controlled clearance mode additionally utilize measured dependencies of effective area and fall rate on the jacket pressure (pj). They require, however, a lot of assumptions being made whose relevance is questionable. In this work, the experimental method by Legras, Huot and Delajoud is modified by the application of FEA to manage without the assumptions made so far, such as the independence of piston distortion from pj, the fluid incompressibility, the distortion of piston and lower cylinder parts in accordance with the Lamé equations and a predefined law of pressure distribution along the piston-cylinder clearance. In addition, the method is extended to the gap of real shape between undistorted piston and cylinder. Starting from Dadson’s equation for the effective area, three different working equations to determine l are derived. Their performance is analyzed using an ideal data set prepared with FEA. Finally, the method is applied to a piston-cylinder assembly which is used at PTB as a primary pressure standard for the 1 GPa range. The results and uncertainties obtained by FEA and the experimental method are compared.

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