Patent GB-574798-A (GB)
Title: Apparatus for measuring medium and high vacua
IPC: G01L21/00
Filed: 1944-07-12
Abstract: 574,798. Electric control systems. LIEBMANN, G., TOWNSEND, F. H., and CATHODEON, Ltd. July 12, 1944, No. 13300. [Class 40 (i)j [Also in Group XIX] Apparatus for measuring medium and high vacua comprises, in combination, an electrically-operated gauge adapted to measure a medium vacuum either by a change of thermal conductivity or by electric discharge through the vacuum under measurement, a gauge of the ionisation type for measuring high vacua, together with an electric control unit common to both gauges and provided with switching means for connecting the unit to either gauge at will. The gauge for measuring medium vacua may be of the Pirani type in which the temperature of a hot filament in the gas is measured by measuring the filament resistance, the temperature being dependent upon the conductivity of the gas which in turn depends upon the gas pressure. Alternatively, the conductivity and hence the pressure may be measured by measuring the temperature obtained at a thermo-junction. In another form in which the medium vacuum is measured by measuring the current through a gas-discharge tube, the discharge may be assisted by a magnetic field provided by a coil energized from the ionisation gauge filament supply circuit. The ionisation gauge is a triode valve arranged so that the grid current gives a measure of the gas pressure. The electric control unit comprises a transformer 3, 6, 7, 8 connected to mains 1, a rectifier 9 and a switch 14 which connects the unit either to the Pirani gauge or, as shown, to the ionisation gauge. The filament of the latter is connected to plug points 20, the anode to point 21 and the grid to a point 22. The anode current is measured by a milliammeter 17 and the grid current, and thus the gas pressure, by a microammeter 18. A switch 19 short-circuits the microammeter 18 and is mechanically interlocked with the switch 14 so that the latter can be operated only when the switch 19 is closed. A switch 23 can be moved to connect the grid and anode points 21, 22 and to put a resistance in parallel with the anode current milliammeter 17 so that the gauge may be "degassed" by passing an increased filament current. Operation of the switch 14 disconnects the ionisation gauge and energizes the Pirani gauge. The transformer winding 3 formerly supplying the ionisation gauge filament current is now connected to a rectifier 26 supplying current to the Wheatstone bridge circuit 31 to which the Pirani gauge is connected through terminals 35. The milliammeter 17 now measures the bridge current and the microammeter 18 the out-of-balance current and hence the gas pressure. One of the bridge resistances 32 may be replaced by a second Pirani gauge surrounded by gas at a known fixed pressure. The electric control unit may be connected to several Pirani gauges in turn. The readings of the meter 13 may be recorded and the meter may control signalling devices or may control the vacuum system pumps. The two gauges may each be fixed in a separate probe or the filament of the ionisation gauge may be used as the Pirani gauge. Three ways of connecting the two gauge probes into different parts of a vacuum system are described in the Specification.