6 - Leaks, Water Vapor, and Leak Testing

Abstract

There are two types of leaks: real leaks and imaginary ones. In real leak, the system pressure rise and continue rising until it reaches atmospheric pressure when a vacuum system is sealed off. In imaginary leaks initially the pressure rises but then the rate of rise slows and the pressure seems to be at a level off. There are many methods of detecting leaks. It depends upon the pressure of the system as to how sensitive any particular technique is to any particular leak. If the pressure of the vessel is near atmosphere and the leak is huge, then the best method may be as simple as walking around the system and listening carefully. Quite often, it is possible to hear the hissing/sucking sound of air rushing through the gap into the vessel. It is possible to use some of the vacuum gauges to test for leaks, if the system is able to pump down the pressures. The Pirani gauge can be used quite successfully because of the difference in performance of the gauge when measuring different gases. A common leak to identify with a Pirani gauge is of a small water-leak. There are number of dedicated leak detectors available. These are mainly mass spectrometers that have been optimized for the detection of helium gas.