Condenser Performance

why not air air test? that works for leaks as well.

I don't understand the question. For instance what kind of condenser, how is it used, and what exactly are you trying to improve?

However, a helium leak test (where the device being tested is pressurized with helium) is routinely used for sealed devices such as condensers, pressure or vacuum vessels, etc., because the HE molecule being the smallest, passes most easily through any leak and can be detected in very small amounts by a helium detector which typically can also be used to "sniff out" the location of the leak.

You don't have to wait around for a pressure gauge to register a drop. With a very small leak and using air, that could take quite a while; many times longer than the He test, which is almost instant. Where He tests are not possible or practical, air is often used but then left for from some number of hours (typically 24) up to a week or even longer.

The other down side to pressure-decay tests is that it is easily impacted by temperature. Flow metering is much more accurate.