Performance of a rotating disc vacuum gauge in transitional and viscous flow regimes

Abstract

The performance of a rotating disc vacuum gauge has been investigated in the transitional flow regime between molecular and viscous flow. This regime is usually described as extending from Knudsen numbers unity to approximately 0.01. A complete knowledge of the behaviour of the gauge under molecular flow conditions assists the interpretation of its behaviour and characteristic edge effects in the transitional flow regime as a combination of viscous and molecular contributions. The results suggest that the transitional regime extends from Knudsen numbers of approximately 1.0 to 0.02. The molecular-viscous intersection point is indicated to be at a Knudsen number of 0.30. The measured value of the viscosity of air at room temperature was 1.74 × 10 −5 Pa s, within 4% of the accepted value. Pressures measured utilising a semiempirical formula show limited agreement with a spinning rotor gauge in the transitional regime.