295. A piezoelectric oscillator manometer: D. J. Pacey, Vacuum, 9, 261–263, 1959

Abstract

Long-term maintenance of a low residual pressure is the prime goal of the preparation steps and pump-down procedure in any electron tube. By miniaturization of the volume down to a few cm3, like in miniature cathode ray tubes (CRTs), field emitter displays and channel photo-multipliers, accurate measurements of the pressure in a sealed-off tube is not feasible. The total pressure is frequently estimated on the basis of the pumping speed of the getters. In the case of a CRT, the total pressure could be determined since the electron gun can be operated as an ionization gauge, but the correlation to the absolute pressure must be known. In present experiments, miniature test CRTs were processed by a standard bake-out and pump-down procedure, except that getters were not activated. The total pressure was measured by the electron gun (e-gun) and spinning rotor gauge (SRG), from 10−4 to 10−5 mbar. From SRG measurements extended over several days and subsequent experiments, it was found that the heated e-gun parts released mostly hydrogen. The initial value of the outgassing rate was Q≅1×10−9mbar l H2/s. This adjustable, but a relatively constant Q enabled a small Penning gauge, attached to the CRT to be calibrated. The ratio of Q vs. the discharge current I was Q/I≅0.0785mbar l H2/(s A) for hydrogen. At this stage of the experiment, it was possible to detect changes of Q within the CRT in the order of Q≅1×10−12 mbar l H2/s, at a pressure in the order of 10−11 mbar.