Ionization gauge filaments

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. 

However, most ionization gauges are thermionicgaugesihai obtain positive ions through bombardment of the gas molecules with electrons produced thermionically from a heated filament and accelerated by a grid biased 100 to 200 V positive with respect to the filament. The ions are collected by a cathode that is typically about 20 to 50 V negative with respect to the filament, and the ion current in the cathode circuit is measured with a sensitive electrometer. 

Electrons produced by a heated filament can be accelerated under an appropriate potential to attain sufficient energy to ionize any gas or vapor molecules in their path. In 1957, the early days of gas chromatography, Ryce and Bryce [8,9] modified a standard vacuum ionization gauge to examine its possibilities as a GC detector. A diagram of the device is shown in figure 8. 

Figure 2 H2O flux from the filament and the corresponding temperature of the filament. The H2O flux was measured with the fast ionization gauge (FIG), positioned 6 cm away from the filament.

A steady flow of hydrogen was established over the unheated filament at a pressure, pg, which was determined by the rate gas entered the reaction cell and the rate gas was removed by pumping due to the ionization gauge, the walls, and the port. When the temperature of the filament was raised rapidly to a value in the range 1100-1500°K, the pressure in the cell fell to a value P, which remained steady for about 30 sec before it began to increase. The new pressure P remained constant as long as the glass walls of the reaction cell, which were kept at... 

Despite its tremendous power, Tonks5 arrangement is still limited to the study of easily ionizable metal atoms, inasmuch as it depends on a surface ionization detector. This limitation was removed by Apker (4) who substituted a hot filament ionization gauge as a detector and was therefore, for the first time, able to measure the evolution of electronega-... 

A fixed electric current flows through the filament of the ionization gauge, heating it until it glows red. At these high temperatures, the metal atoms in the filament emit electrons. Some of these electrons then collide with gas molecules, and the resulting impact can knock an electron off the molecule, producing a... 

Figure 5.11 I Schematic diagram of an ionization gauge. Electrons emitted from the hot filament collide with gas molecules and knock an electron free. The resulting cations are collected at the center of the gauge, and the current they produce is a measure of the gas pressure.

The cold cathode ionization gauge has offsetting advantages relative to the hot cathode type. It is insensitive to mechanical vibrations, is not susceptible to an inrush of air in the event of a vacuum accident (that can destroy the healed filament in the hot cathode type) and is less susceptible to harm from contaminants. 

Ionization gauge (vacuum technology) A vacuum gauge that uses the ion current formed by electron-atom collisions as an indicator of the gas pressure (density). The electrons are formed as secondary electrons from ion bombardment or from a hot thermoelectron-emitting filament. See also Vacuum gauge. 

Bayard-Alpert ionization gauge Figure 3.1.8 Hot-filament ion gauge. 

They should be used in conjunction with a TC gauge the ion gauge filament should not be on until the TC gauge indicates that the pressure is below about 10 pm. When not in use, turn off the filament, or at least set the meter to the least sensitive scale. The ionization and collection process actually removes some gas from the vacuum space, and this pumping can improve the vacuum or maintain a vacuum in the presence of a small leak. Much larger versions of this device are made specifically for pumping. 

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