Cold-cathode gauges

Both the thermionic and cold-cathode gauges exert a significant pumping action on a vacuum system due to the breakdown and deposition of molecular ions created by electron impact. These gauges are prone to degas deposited material when turned off, or soon after they are turned on. 

C. Leak Detection Using Vacuum Gauges. Reference has already been made to the use of an electronic vacuum gauge in hunting leaks. A cold-cathode gauge is particularly useful because it has rapid response and operates well in the presence of small leaks. Because of this rapid response, acetone or methanol can be squirted on the suspected area. If a hole is present, a momentary pressure drop will be observed because the acetone or methanol molecules are slower than air to diffuse through the hole. Sometimes a helium stream is directed on the suspected leak in this case a pressure rise is noted when the helium passes over the site of the leak, because helium diffuses more rapidly than air. It also is sometimes possible to use a puttylike material, such as Apiezon Q, to cover temporarily a suspected leak. 

Figure 1. Schematic of apparatus A, calibrated variable-volume mercury burette B, reference volume C, main chamber D, mixing pump E, adsorption chamber F, reference chamber G, constant temperature baths H, mercury manometer J, cold-cathode gauge P, Pirani vacuum gauge R, mercury reservoir...

Hot-cathode gauges are considered fast pumps, but cold-cathode gauges pump 10 times faster. At 10 10 torr there are only 106 molecules per cubic centimeter. If... 

Answer by author Pressures were measured with ion gauges. In some instances cold cathode gauges were also used. 

Vacuum gauge, ionization cold cathode gauge A vacuum gauge that uses an 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. 

Even though hot-cathode gauges are preferred because of their better accuracy, they have disadvantages over cold-cathode gauges because the hot filament causes light, excess electrons, outgassing, and loss of performance over time. 

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