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3.2
Direct Flow & Reverse Flow Modes
The MS-40 is capable of detecting leaks in three separate modes: gross
mode, reverse flow mode and direct flow mode. All three flow modes
are controlled directly and automatically by the MS-40's computer.
Reverse Flow:
The Reverse Flow Mode is particularly useful for detecting and measuring
leaks in test objects that have a high degree of out-gassing or when a fast
crossover at a high test port pressure is desired. In this mode, testing for
"Gross" leaks is accomplished by connecting the external roughing pump
to the vacuum system, opening the external rough valve, the fore-line valve
and the gross valve. All other valves are closed. The test port is connected to
the fore-line of the turbo-molecular pump through a restriction. This allows
the sample helium to back-diffuse through the pump. When testing in reverse
mode, the fore-line pressure is usually in the 10-3 Torr range or greater, the
turbo-molecular pump actually acts as a buffer against pressure bursts into the
high vacuum section (the area containing the mass spectrometer). Therefore,
crossover can occur at much higher pressures in reverse flow mode than in
direct flow mode. For the MS-40, the Gross Mode Leak measurement range is
-3
as 10 to 1 x10
std cc/sec.
Without an external pump, Gross Mode is not available. If the unit is
unable to pump down the test object to a sufficient level to begin testing in
reverse flow mode, actual leak testing will not begin. The unit will
remain in its roughing state until the operator stops the test cycle. Since
the maximum crossover for the MS-40 is quite high, this may be an
indication that the test object has a very large leak.
When the test port pressure reaches the pre-designated "crossover 1"
set point (9.6 Torr maximum), the unit will "crossover" into reverse flow
mode testing (the actual point of crossover may be set by the system
operator). At crossover, the fore-line valve opens, the internal rough valve
remains open, while all other valves are closed. Gases from the test object
are introduced into the fore-line of the turbo-molecular pump, and "back-
diffuse" through the pump. As in gross mode testing, the turbo-molecular
pump acts as a buffer against pressure bursts into the high vacuum section
(the area containing the mass spectrometer). For the MS-40, the
-4
-10
Reverse Flow mode leak measurement range is as 10 x 10
to 6 x 10
std
cc/sec.
Vacuum Instrument Corp.
2099 Ninth Avenue
Ronkonkoma, NY 11779
631.737.0900
1109-186-00 Rev. C
3-4
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