MKS HPS 903 Series, Operation And Maintenance Manual

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15 Cold Cathode Ionization Vacuum Sensor
1) Pump your vacuum system to a base pressure.
2) Slowly and methodically probe with a small amount of gas.
Flooding the leak or moving quickly past the leak can confuse
the search since time lags may be significant.
3) Note the pressure reading.
The pressure will either rise or fall, depending upon the ionization potential of
the probe gas relative to the system gas. The largest change in value
indicates the probe gas is nearest the leak location.
4) Repeat the test to confirm.