AVS 51st International Symposium
    Vacuum Technology Wednesday Sessions
       Session VT-WeA

Paper VT-WeA5
Modulated Bayard-Alpert Gauges in a Calibration Laboratory

Wednesday, November 17, 2004, 3:20 pm, Room 303D

Session: Vacuum Measurement, Sensors and Control
Presenter: B.R.F. Kendall, Elvac Laboratories
Authors: B.R.F. Kendall, Elvac Laboratories
E. Drubetsky, The Fredericks Company
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Bayard-Alpert (BA) gauges may have large errors at pressures below about 10@super -9@ Torr because of x-ray and other unwanted effects. One of the simplest ways to overcome these problems is to add a modulator electrode, as described by P.A. Redhead in 1960. At least a factor of ten improvement in minimum detectable pressure is possible. We describe operational experience with both nude and metal-envelope modulated BA gauges on two UHV systems operating in the 10@super -11@ Torr range. In addition to eliminating x-ray errors, these gauges proved to have numerous other advantages which have not been generally recognized. They are unaffected by electrometer zero drift and are capable of discriminating against spurious currents caused by electron-stimulated ion desorption. Standard commercial ion gauge controllers can be used, with the addition of simple external adapters. These can range from elementary manually-operated switches giving single readings of true pressure, up to programmable devices capable of giving continuous readouts of true pressure and the magnitudes of the various error signals. A gauge tube with a modulator electrode can also be used, if necessary, as a conventional BA gauge with a standard controller simply by connecting the modulator to the electron collector (i.e. to the grid). We are replacing most of the conventional BA gauges on our UHV systems with modulated gauges. This has eliminated over-readings which sometimes approached 300 percent of true pressures in the mid 10@super -11@ Torr range.