The Hard Vacuum Outside/In technique offers a very effective method of leak detection.  In the case of Outside/In leak testing, the test chamber is filled with Helium, with a vacuum drawn inside of the test part.  Helium is drawn by the vacuum, into the test part and is measured by the mass spec unit.  Here, the mass spec measures the amount of helium that flows/leaks into the test part, thus requiring that the part leak backwards.  This is illustrated by the analogy of a doorway.  It can be both an incoming and outgoing passageway.     

This method offers measurable leak rates as low as 10^-9 std. cc/sec, and as high as 10^-3 in high volume production applications.  This technique features high sensitivity, flexibility, and temperature and test time independency.  The Outside/In technique offers an advantage over Inside/Out in that it requires a smaller (therefore, less expensive) vacuum pump, and shorter pump-down time due to the smaller test part volume.

Disadvantages include:  a higher cost than the Effect of the Leak testing methods, requires consumables, vacuum pump down time dependent, rough vacuum requires more highly skilled maintenance,  test parts must be clean and dry, vacuum pump oil waste, and the location of the leak is not identified.  Also, in the majority of cases the largest pressure difference the part can be subjected to is 14.7 PSI or 100 kPa, and the part must structurally withstand the vacuum being drawn inside it.

The test typically consists of the following steps:

	Evacuating the part to a low enough vacuum
	Pressurizing the chamber with Helium or a Helium-air mixture
	Monitoring Helium levels inside the part

Sensitivity range [std.cc/sec]
Temperature dependence
Operator dependence
Main factors affecting sensitivity	Part volume  Test time
Test pressure range	14.7 PSI / 100 kPa
Maintenance required	Qualified maintenance required for vacuum pumps and Mass Spec.
Cost