8
2.226 Other Forms of contamination control
The application of the gas ballast is a moderate and very successful method for the removal of
condensable vapors. For very heavily laden systems, other means of removal such as oil separators
may be required. For mild cases of contamination the simple expedient of a cold trap or a change of oil
serve the purpose.
2.23
System Leaks - Impact & Detection
2.231 Large
Leaks
The importance of eliminating all leaks in a vacuum system is obvious when it is realized that a leak
into the system, at atmospheric pressure, expands in volume by a factor of 750,000 to 10,000,000
or more. The pump must remove this added volume to maintain the desired vacuum. Fortunately a
number of effective techniques for leak detection have been developed. Large leaks can be located by
pressurizing the system and painting the suspected area with a thick soap solution. Escaping air will
produce soap bubbles.
2.232 Small
Leaks
Small leaks may also be detected by spraying a suspected area with acetone or gases rich in
hydrogen, and observing a sudden change in pressure on an electrical gauge. The difference in
calibration of these gauges, for air and other gases, will produce a distinct change in the pressure
reading. To use this method of detection, the system must be under vacuum and the gauge sensing
tube must be located between the pump and the area to be probed. Use extreme caution, as these
materials are highly fl ammable!
2.233 Fine
Leaks
Locating
very
fi ne leaks requires a helium-sensitive, mass-spectrometer leak detector. This instrument
will locate leaks which cannot be detected by any other method. Numerous fi ne leaks can have the
total effect of a large leak.
2.30 Shutdown
Procedures
2.301 DuoSeal
Shutdown
A few simple precautions are all that is necessary when shutdown is in order. If a gauge is connected
to the system, fi rst isolate the gauge, then turn off the power and open the system to atmosphere. If
the pump is removed from the system, cover the intake port with a rubber stopper or suitable cover to
protect the pump against contamination and loose particles. If the pump has been contaminated in
service and is going to be shelved for a prolonged period it is best to drain the oil and refi ll with a fresh
charge.
