ATEQ is the world number one supplier of automatic leak testing machines & leak test systems for assembly lines or laboratories.
40 years after its creation, ATEQ is now a $ 60 million company with offices in 37 countries selling and servicing more than 100,000 leak testers installed in more than 5,000 customers.
This consists of putting a part under pressure and sinking it in a bucket of water. 
The leaks will be visible by the air bubbles that will then escape from it

The drawbacks are: automatization test impossible – human errors – wetting parts – no reject level.
The only advantage is the localization of the leak.

This is usually employed for applications that can not be treated by air/air technology (very slight leaks, important volumes, leak localizations).
The drawbacks are: expensive cost – need a specialized operator.The advantages are: localization – very sensitive – large volume
This is the method best adapted to industrial products. 
It consists of putting the part under pressure and then measuring the variation of pressure after stabilisation 
If the pressure drop is higher than the tolerance level, the part will be thrown out.

The drawbacks are: no localization
The advantages are: automatization – no human error – fast – reject level – traceability…

Yes, of course !
Contrary to commonly held ideas,  using a reference is a gain of time since 
thermal stabilisations are identical and the measurement may begin earlier.