The Hot-Cathode Ion Gauge

Another common filament material used in hot-cathode ion gauges is thoriated iridium (Th02 on iridium). These filaments are used if there is any likelihood that the system will be exposed to accidental bursts of atmosphere. However, if exposed to hydrocarbons or halocarbons, their emissions can radically change. Use of a silicon oil in the diffusion pump is required with these Aliments. Recalibration of these filaments is constantly required. 

Studies by McCulloh and Hlford57 found that dual tungsten filaments exhibited better linearity and also exhibited sensitivities which were in closer agreement to those stated by the manufacturer. Filaments made out of thoriated iridium demonstrated linearity differences as great as 30%. These differences were evident in standard as well as nude gauges. 

There are several general rules to follow that will make the operation of a hot-cathode ion gauge as trouble-free as possible. Implementation of these rules cannot guarantee success, but ignoring them will ensure problems  

Always connect the gauge as close as possible to the area where measurements need to be made. The farther away the gauge is from the point of measurement, the longer the lag time before the system and the gauge come to equilibrium. The greater the vacuum ( 10 5 torr), the more pronounced this effect. 

On metal systems, you have two choices Connect the ion gauge by connecting tubing (same tubing size rules apply from Point 2), or use a nude gauge. 

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If your gauge is glass-bodied, it cannot be taken apart and you are limited to the chemical cleaning processes mentioned for the hot-cathode ion gauge. 

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