Installation
2-6
P/N 8040613
C
HELIX TECHNOLOGY CORPORATION
-
TI
CRYOGENICS
Crossover Pressure Calculations
Crossover is that point in time when the pumping of a vacuum chamber is
switched from rough pumping to high-vacuum pumping. Rough pumping
brings the vacuum chamber pressure from one atmosphere (760 torr) down
to a pressure of approximately 0.5 torr. At crosover, the roughing valve is
closed and the high vacuum valve is opened bringing the vacuum chamber
down to a pressure typically less than 10
-6
torr. This momentary pulse of
gas and water molecules is cryo-condensed on the arrays of the Cryo-Torr
Cryopump.
To determine the maximum permissible crossover pressure (CP), perform
the following calculation using the crossover values (CV) for your model
Cryo-Torr Cryopump shown in Table 2-1 and the actual volume of your
vacuum chamber (VC).
Where VC = 100 liters:
Table 2-1: Crossover Values
Cryo-Torr Pump
Torr-Liters
4
100
8
150
8F
150
10
300
100
40
250F
300
400
300
500
500
CP =
CROSSOVER VALUE
VACUUM CHAMBER VOLUME
=
CV
VC
CP =
300 TORR-LITERS
100 LITERS
3 TORR
=
