Figure 3 —

Operating Characteristics for Series 274 Ionization Gauge Tubes

Pumping

The ionization gauge exhibits a certain pumping capacity.

This is due to both chemical and electrical effects.

Chemical pumping is due, in general, to the affinity of

gases for very clean surfaces. As the surface becomes

saturated, the pumping action is diminished and reaches a

steady state value. Readings for a typical glass tabulated

gauge will be 20 to 50 percent lower during the period

of chemical pumping. The duration of the pumping may

or ionic pumping saturates after pumping approximately

for pumping effects is to provide a passage of large

conductance between the gauge and the vacuum system.

A third mechanism of pumping in gauges involves chemical

reactions with the hot cathode. A number of gases such as

oxygen, nitrogen, water vapor, and hydrogen have been

shown to react with the carbon present in tungsten. The

ratio of these reactions are dependent on the cathode

temperature, and are low enough that serious errors in

measurement can be avoided when high-conductance

connections are used.

In small systems, however, the change in gas composition

may be significant.

Several gauge configurations are available to cope with the

pumping phenomenon.

The glass envelope gauge equipped with 3/4-inch

tubulation, such as the 274002, has adequate conductance

(274015). To compensate for the high electronic pumping

speed, all internally shielded gauges should be specified

with 1-inch tubulations.

nude gauges are the best solution to severe ionization

gauge pumping problems. With the nude geometry, the

gauge elements can be positioned directly into the vacuum

chamber, thereby eliminating the pressure differential

normally associated with a tubulated gauge. The time

response of this gauge system is greatly reduced as

compared to the tubulated gauge.

Comparing the requirements of a given application to

the items above will permit the selection of the most

appropriate gauge for the purpose.