HELIX CTI-CRYOGENICS CRYO-TORR 100 Installation, Operation And Servicing Instructions page 28

5.7 Determining Crossover
Pressure
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 about 0.5 torr. At crossover
the roughing valve is closed and the high-vacuum valve
opened bringing the vacuum chamber down to a pressure
typically less than 10-6 torr. ntis momentary "pulse" of
gas and water molecules is cryo-condensed on the arrays
of the cryopump.
To determine the maximum permissible CROSSOVER
PRESSURE (CP) perform the following calcu1ation
using the CROSSOVER VALUES (CV) for your Model
cryopump shown in the table below and the actual
VOLUME OF YOUR CHAMBER
(YC).
CROSSOVER VALUES (CV)
CRYO-TORR TORR-LITERS
CT-IOO 40
CT-7 50
CT-8 150
CT-8F 150
Example: (For CT-8/8F)
(Volume of chamber = 100 liters)
CROSSOVER VALUE
cv
CP
= - - - - - - -
=
VOLUME OF CHAMBER VC
150 torr-liters
CP
= - - - - - -
= 1.5 torr
100 liters
For Your Information ••
The calculated crossover pressure may not be opti-
mized for your system. To help prevent any back
streaming during the roughing of the vacuum chamber,
you should stop roughing at as high a pressure as
possible. The optimum crossover pressure for a
vacuum chamber should cause a very slight rise in
temperature with a rapid recovery. Increase the
roughing pressure in small increments (15 to 20%)
until this rise in temperature is noted; then drop the
value by a small amount (10%), this will be the
optimum pressure for that vacuum chamber.
5-4
5.8
ai-CRYOGENICS
Determining Cryopump
Capacity for Condensable
Gases
Cryopump capacity is defined as the total standard liters
of a gas that can be accomodated within a cryopump
prior to regeneration. The number of hours between
regeneration cycles can be easily calculated in the case
of a continuous gas flow of a known gas species:
16.6xC
A
= - - -
B
A = Duration of operation with a continuous gas
flow (hours)
B = Gas Flow (scc/min.)
C = Cryo-Torr capacity for the particular gas
species being flowed (std liters); refer to the
following Tab/e.
CONDENSABLE GASES CAPACITY
(ARGON, NITROGEN, OXYGEN, ETC.)
CRYOPUMP STANDARD LITERS TORR-LITERS
CT-IOO
90 68,400
CT-7
350
266.000
CT-8 1,000 760.000
CT-8F 1,000 760,000
Example: (For CT -8/8F)
For a sputtering application of continuously flowing
argon
gas
at 70 sec/min., the duration of continuous
operation with this gas flow (between regenerations)
would be:
16.6 x 1000 (std liters)
A
= - - - - - - -
=
237
hours
70 (sec/min.)