VACUUM GAUGE CALIBRATION AT THE NPL (INDIA) USING ORIFICE FLOW METHOD

This paper describes the primary high vacuum standard established at t he National Physical Laboratory (New Delhi) for calibration of gauges in the molecular flow range. The system, based on the orifice flow met hod, employs a fixed orifice of 8 l/s argon conductance. A diffusion p ump-Ti sublimation pump combination gives base pressure of 10(-8) Pa. The gas from the flow meter enters the calibration sphere through a ca pillary 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 d own to 2 x 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 t he flow ratio of the system. Results on the determination of flow rati o for nitrogen are presented. Also discussed are the uncertainties ass ociated with the measurement of the flow ratio and the generated press ure. (C) 1998 Elsevier Science Ltd. All,rights reserved.