Bayard gauge

Fig. 14. Ionisation gauges (a) triode head and (b) Bayard-Alpert.

The discovery of the x-ray effect spurred the development of a new generation of hot-cathode gauges designed to minimise this effect. One of the earhest, and commercially the most successful, the Bayard-Alpert gauge shown in Eigure 14b, was developed in 1950 (13). A fine wire, the ion collector, is... 

In practice, it is often necessary to take readings from hot-filament ionization gauges or other devices. Figure 5 gives pump-down curves for six different types of pumping equipment on the same vacuum chamber (23). The shape of curve 1 indicates that a real leak could be responsible for the zero slope demonstrated by the Bayard-Alpert gauge (BAG). The shape of the other curves could result from a combination of real and virtual leaks. 

Fig. 5. Plots of pump-down performance for pumps operating on 0.1-m-dia x 0.43-m-long stainless-steel tubing. Curves 1—4 are sputter-ion pumps of different makes curve 5 is Orbitron type and curve 6, LN-trapped oil DP. Pressure is measured using Bayard-Alpert gauge (BAG) (26). To convert Pa to...

Fig. 6. Bayard-Alpert gauge response vs time upon cutting off power to Orbitron pump (27). To convert Pa to torr, divide by 133.3.

Fig. 13. The introduction of a modulator electrode in a Bayard-Alpert gauge permits the pressure range to be lowered by a factor of 10 to 133 pPa...

In addition to the conventional ionization gauge, whose electrode structure resembles that of a common triode, there are various ionization vacuum gauge systems (Bayard-Alpert system, Bayard-Alpert system with modulator, extractor system) which more or less suppress the two effects, depending on the design, and are therefore used for measurement in the high and ultrahigh vacuum range. Today the Bayard-Alpert system is usually the standard system. 

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

Bayard-Alpert ionization vacuum gauge with modulator... 

Comparison of the Pressure Indication of a Bayard-Alpert and an Extractor Gauge... 

Figure 2. Schematic view of the spectrometer. The components illustrated include M, crystal manipulator Q.M.S., quadrupole mass spectrometer I.G., primary ion source E.S., energy spectrometer G, Bayard-Alpert gauge T, crystal target and G.I., gas inlet. Auxiliary components are omitted for graphical clarity. The SIMS experimental geometry and coordinate systems are defined in the inset. Reproduced with permission from Ref. 4. Copyright 1981, American Institute of Physics.

Details of the collision-induced dissociation (CID) experiments have been described [26]. Argon was used as the collision gas at a total pressure of "4 x 10-t> torr. The collision energy of the ions can be varied (typically between 0 and 100 eV). A Bayard-Alpert ionization gauge was used to monitor static pressures. 

Figure 5.4 Schematic diagram of a hot cathode ionisation gauge of the Bayard-Alpert type...

In the gauge developed by Bayard and Alpert, Ix was decreased by reducing the surface area of the ion collector. In Bayard-Alpert gauges (BAGs), the collector is a thin wire surrounded by a cylindrical, coaxial open anode. With BAGs, pressures down to 10 9 mbar can be measured. 

A Bayard-Alpert gauge has a gauge constant for N2 of 17mbar 1. The emission current can be altered from 0.06 to 0.6 mA. 

Fig. 7.48 The Bayard-Alpert hot-cathode gauge. From Fundamentals of Vacuum Science and Technology, p. 92, by G. Lewin, McGraw-Hill, New York, 1965, reproduced with permission.

Tungsten filaments can create large quantities of CO and C02 during operation, which may, or may not, affect your work. It is possible to obtain Bayard-Alpert gauges that can be preheated, or baked out, prior to use to limit this problem. 

In general, pressure detectors capable of such performance have become available only recently, in the form of the inverted (Bayard-Alpert) ionization gauge (described in Section IV, E, 1). As early as the 1920 s, however, Langmuir and his co-workers (2) did have the means for detecting pressures as low as 10 15 mm for one particular vapor, cesium, and were therefore able to lay the foundations for modem studies of adsorption kinetics. 

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