Ionisation gauges hot cathode

Hot cathode ionisation gauges. The basic hot cathode ionisation gauge is shown in Figure 5.4. A heated cathode (C) emits a current of electrons (I ) that is accelerated through a voltage between the cathode (Uc) and the anode (UA) at a more positive potential. Electrons acquire energy according to UA - Uc. They collide with gas particles, which may be... 

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

A hot cathode ionisation gauge has a gauge constant for N2 of lOmbar-1. For an electron current of 1mA, what is the ion current at 10-7 mbar ... 

A hot cathode ionisation gauge has a gauge constant of 5mbar J for... 

Note It is usual with hot cathode ionisation gauges to keep /- constant by controlling the cathode otherwise it would change due to surface effects such a gas coverage. 

The ion yield (and hence /+) for hot cathode ionisation gauges depends on the type of gas involved since some gases are easier to ionise than others. Indirect reading gauges are calibrated using N2 as the reference gas. To obtain the actual pressure for gases other than N2, the indicated pressure must be multiplied by a correction factor for the gas involved. Thus ... 

Particularly at low pressures, errors may arise with hot cathode ionisation gauges because of two effects the X-ray effect and gas-ion desorption. 

Early hot cathode ionisation gauges were concentric triode gauges in which a central cathode was surrounded by a coaxial cylindrical ion collector. Because of the high X-ray limit, such gauges could only read down to 10 7mbar. From Equation (5.16), it can be seen that the situation could be improved by lowering /r. 

Values for Kcc are significantly higher than gauge constants for hot cathode ionisation gauges. 

As with the hot cathode ionisation gauge, there is a relationship between I+ and gas pressure (see Equation 5.11) so the RGA sensitivity (Srga) can be expressed as ... 

There are two principal types of ionisation gauge, viz. the hot cathode type in which electrons are emitted by a heated filament, and the cold cathode type in which electrons are released from the cathode by the impact of ions. In both cases the vacuum is measured in terms of the ion current. The electrons are accelerated by a potential difference (usually about 2000 V) across the ionisation tube (see Fig. 6.22). Positively charged ions are formed by the electrons striking gas molecules. The number of positive ions produced is a function of the gas density (i.e. the pressure) and the electron current ie which is normally held constant. The ions are collected at a negatively charged electrode and the resulting ion current it is a direct measure of the gas pressure. The hot cathode version is the most sensitive of the two and can be used to measure vacua down to about 10 10 torr ( 10 8 N/m2). 

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