During the pre-cool, the OVC pressure should drop to mbar as N condenses. Occassionally, a problem arises where the pressure will bottom out at mbar and sit there indefinitely.
Symptoms: OVC pressure bottoms out at , even while pumping with the pump cart or temperature less than K.
Diagnose: Pump directly on the OVC pressure gauge with the pump cart. Pressure should drop below mbar easily. If pressure bottoms out around mbar, pressure gauge needs cleaning.
Solution: Clean the OVC pressure gauge.