A Model for Drag Forces in the Crevice of Piston Gages in the Viscous and Molecular Flow Regimes

Published

June 1, 1999

Author(s)

James W. Schmidt, Charles D. Ehrlich, S A. Tison

Abstract

A model for drag forces in the crevice of pneumatic piston gages is presented. The model uses an interpolation function for momentum transfer between the piston and the cylinder mediated by the gas flowing in the crevice. The interpolation function bridges the gap between the molecular-flow and viscous-flow regimes, and is then used to develop and expression for the effective area of a piston gage. The deviation of the effective area of this model from the viscous flow result is:(formula)for a floating-piston design, and(formula)for a floating cylinder design, where A_eff is the effective area, h is the crevice width, R₀ is the radius of the piston, R₁ is the radius of the cylinder, Β₂ and Β₃ are constants that depend on the molecular weight and bulk viscosity of the gas and P₀ and P₁ are the absolute pressures above and below the piston. Model results are compared with published measurements of the effective area of several piston gages in which relative changes as large as 30 x 10^-6 (30 ppm) were observed when different pressurizing gases were used.

Citation

Metrologia

Volume

36

Issue

6

Pub Type

Journals

Keywords

effective area, molecular flow, piston gages, pneumatic dead weight testers, viscous flow

Citation

Schmidt, J. , Ehrlich, C. and Tison, S. (1999), A Model for Drag Forces in the Crevice of Piston Gages in the Viscous and Molecular Flow Regimes, Metrologia (Accessed December 13, 2024)