System Leak Test Methods - Agilent Technologies G8610A Operation Manual

B.3.2.2.1 Accumulation Testing Method
This method can both locate and quantify leaks. Some type of shroud or hood is placed in
such a manner as to envelop a potential leak site. A certain amount of time is given to allow
leaking helium to accumulate in the shrouded area, increasing the helium concentration.
The leak detector is then valved-in to the shrouded volume. If many potential leak sites exist
in a manifold or if many parts are to be tested at the same time, they can be sequentially
valved-in to determine which site is leaking (Figure B-7).
B.3.3 System Leak Test Methods
Systems, like individual parts, should be tested with the same pressure differential and in the
same direction as in actual use. Therefore, systems that are under vacuum while in
operation should be leak tested under vacuum, while pressurized systems should be
charged with helium to a pressure similar to operational conditions when possible.
B.3.3.1 Vacuum Systems
In general, vacuum systems are tested with a portable leak detector. Typically the leak
detector is connected by means of a tee connected in between the foreline of the high
vacuum pump and the inlet of its backing pump. A system should be capable of
maintaining a foreline pressure low enough to operate the leak detector at this location.
Helium is supplied to potential leak site using a spray probe or bagging suspected areas. If a
leak exists, helium enters the system and rapidly diffuses through it. The leak detector
should respond within several seconds or less. Leak detector sensitivity is diminished in
systems with large backing pumps. If a system is using a cryopump as a high vacuum pump,
it must be valved off before helium is introduced as cryopumps have limited helium
pumping capacity (Figure B-8).
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Agilent Technologies Helium Leak Detector
Figure B-7 Accumulation: Inside Out