The moment, and the pressure, at which the roughing valve closes and the high vacuum valve opens. Crossing over too high hits the high vacuum pump with a slug of gas it cannot handle: a cryopump overloads and warms, a diffusion pump stalls and backstreams, a turbo is braked hard. Cryopumps carry a crossover rating in mbar·L or Torr·L, so the highest allowed crossover pressure is that figure divided by the chamber volume.
Cryo's rated for 150 torr-litres and the chamber's 500 litres, so cross over at 0.3 torr, not a hair above.
If the chamber volume changes, for example after adding a large fixture or a new port, recalculate the crossover pressure rather than copying the old recipe.
Also heard as
- crossover pressure
- cross over
Related terms
Rough out
To pump a vented chamber from atmosphere to crossover with the roughing pump, before the high vacuum valve opens.
Cryopump
Capture pump that freezes and adsorbs gas onto surfaces cooled to about 10 to 20 K by a closed-cycle refrigerator.
Gate valve
Full-bore valve with a sliding plate that isolates a high vacuum pump or a load lock from the chamber.
Roughing pump
Pump that takes a chamber from atmosphere down to the pressure where the high vacuum pump can take over.
Pump down
To evacuate a chamber from atmosphere to its working pressure, through roughing, crossover and high vacuum.