Measurement of the variation of molecular impact rate within a spherical calibration vessel

Abstract

When calibrating high vacuum gauges by the dynamic flow method the calibration chamber is continuously exhausted with gas flowing into the vessel at a controlled rate to obtain a given pressure. The gas flow and pumping speed must be sufficient to minimise the effect of gas desorption. An investigation was made of the departure from molecular chaos within a spherical vessel (61 cm in diameter) under the foregoing conditions. Variations in the molecular impact rate were determined around the circumference of the chamber using a movable ionization gauge fitted with a small spherical receiver with which gas molecules with preferred directions were scattered before entering the ionizing region; the instrument is known as a “pressure transducer”. Gas was admitted to the test vessel from a spherical gas reservoir connected to an argon supply. Measurements were made with several types of apertures and tubulations between the reservoir and test vessel. The effect of the geometry of the pumping port and baffle plates on the gas distribution was also studied. The foregoing studies were also made with an ionization gauge having a tubular envelope terminated with an orifice plate for gas admission. The gauge showed an enhanced response to preferred flow when the molecular beam entered its envelope through the orifice at normal incidence because the incident gas passed directly into the most effective ionizing region. In conclusion, the “pressure” variations measured are considered in relation to inaccuracies that could be encountered in gauge calibration systems.