there are two common signs that a cold cathode is contaminated. the first is when you see discoloration (such as a rainbow effect) on the ionization chamber or when looking into the flange. the second is when your gauge measures lower than it should (i.e. base pressure is usually 5e-7 torr but the gauge creeps down into the 10-8 range or lower).
Category: Instrumentation Gauging
Sub-Category: Cold Cathode Ion Gauges