Characteristic quantities of a cryopump

The characteristic quantities of a cryopump are as follows (In no particular order)  

Cooldown time The cooldown time of cryopumps is the time span from start-up until the pumping etfect sets in. In the case of refrigerator cryopumps the cooldown time is stated as the time it takes for the second stage of the cold head to cool down from 293 K to 20 K. 

Crossover value The crossover value is a characteristic quantity of an already cold refrigerator cryopump. It is of significance when the pump is connected to a vacuum chamber via an HV / UHV valve. The crossover value is that quantity of gas with respect to T =293 K which the vacuum chamber may maximally contain so that the temperature of the cryopanels does not increase above 20 K due to the gas burst when opening the valve. The crossover value is usually slated as a pV value in in mbar I. 

The crossover value and the chamber volume V result in the crossover pressure p to which the vacuum chamber must be evacuated first before opening the valve leading to the cryopump. The following may serve as a guide  

2(20K) = Net refrigerafing capacity in Watts, available at the second stage of the cold head at 20 K. 

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