The Helium Accumulation technique measures the Helium leaking from a test part charged with Helium, as it is circulated through a circuit past the mass spec unit.  Here, hard vacuum is not required due to the fact that the test part and chamber are not being evacuated.  The mass spec unit is shielded from the non-evacuated test circuit by a barrier.  Helium is drawn through the barrier by the vacuum from the mass spec unit outside the test circuit.

This method offers measurable leak rates as low as 10^-6 std. cc/sec in high volume production applications.  This technique features a relatively high sensitivity, flexibility, and temperature and test time independency.  The method offers advantages in that it requires no roughing vacuum (lower cost and maintenance), and will therefore tolerate a much more hostile environment (slightly dirty or wet parts).  

Disadvantages include:  a higher cost than the Effect of the Leak testing methods, time dependent, requires more consumables, chamber volume dependent, location of the leak is not identified, and requires a low background of Helium in plant air.  Also, part geometry may induce "blind spots," where helium could get trapped and not get circulated through the test circuit to the mass spec.  The total cycle time of this method is generally longer than the previous methods due to the time involved in allowing the Helium to accumulate in the test circuit from the leaking charged part.  

 

The test typically consists of the following steps:

	Establishing circulation of air in the test circuit
	Pressurizing the part with Helium
	Monitoring Helium levels in the test circuit

Sensitivity range [std.cc/sec]
Temperature dependence
Operator dependence
Main factors affecting sensitivity	Test time Helium background levels
Test pressure range	any
Maintenance required	Qualified maintenance required for Mass Spec.
Cost