Welch DUOSEAL 1399B-01, Owner'S Manual

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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 flammable!

2.233  Fine Leaks

Locating very fine leaks requires a helium-sensitive, mass-spectrometer leak detector.  

This instrument will locate leaks which cannot be detected by any other method.  

Numerous fine 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, first 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 refill with a fresh charge.