Measurements of the Substitution Sensitivity Factor to Propane, and Helium Gases for the Leak Hunting

Abstract

The substitute sensitivity factor (φ) for the single ionization gauge method was obtained experimentally for propane and helium gas which had been used for the leak hunting at the laboratory with three fixed leaks and an oil diffusion pump system.A series of fixed leak was made by flattening an annealed copper tube with inner diameter of about 1 mm and a flattened length of all the leaks was in 14 mm. The flow rates of those leaks were stably 3.0×10-3, 2.1×10-4, and 1.3×10-5 mm.Hg·1·sec-1 for air from an atmosphere to vacuum respectively.When a probe gas was covering over the leak or removing from it, transitional changes of an ionization gauge, which electron currents was controlled automatically at a constant value by electric circuit, were recorded on an electrical self-recorder.As to the φ values of the probe gases on three fixed leaks fixed values were not obtained just as the reference showed for other gases, but these values spreaded in a wide range as follows : Probe gas leak rates of fixed leaks for air in mm Hg·l·sec-13.0×10-3 2.1×10-4 1.3×10-5Propane 2613.8 13.64.8 6.83.3Helium - (0.960.92) - (0.930.89) - (0.880.85) In order to anaiyze these experimental data, flow rates of the gases were directly measured by the constant pressure method having a small dead volume. The characteristics of flow ratio to air are given as changing from viscous flow to molecular one as the leak rate of air becomes smaller.The spread of φ is thought to be due to the fact that the pumping speed of oil diffusion pump system is unstable at transitional pressure rather than the ionization cross section of probe gases to electrons varies in the gauge.