Vacuum gauge calibration at the NPL (India) using orifice flow method

Abstract

This paper describes the primary high vacuum standard established at the National Physical Laboratory (New Delhi) for calibration of gauges in the molecular flow range. The system, based on the orifice flow method, employs a fixed orifice of 8 l\\s argon conductance. A diffusion pump—Ti sublimation pump combination gives base pressure of 10 −8 Pa. The gas from the flow meter enters the calibration sphere through a capillary leak with a flow rate of 10 −10 to 10 −8 mol\\s for reservoir pressures of a few Pa to 10 6 Pa, leading to calibration pressures down to 2×10 −5 Pa. The flowrate is measured by monitoring the time rate of rise of pressure using a spinning rotor gauge. Pressures below 10 −5 Pa are generated by flow division technique, in which the gas is diverted in the lower chamber, leading to the gas being introduced in the calibration chamber at a flow rate reduced by a factor equal to the flow ratio of the system. Results on the determination of flow ratio for nitrogen are presented. Also discussed are the uncertainties associated with the measurement of the flow ratio and the generated pressure.