overpressure water test vs internal vacuum test

[daniele79]

Hi all,
I'm trying to understand the difference between two leakage test performed in underwater vehicle design.
The first: external overpressure water test
This test is performed with the vehicle inserted into a tank pressurized with water (at the maximum design pressure). verification of tightness is performed measuring the umidity level inside the vehicle.

The second: internal helium vacuum test
normal procedure according to varios regulamentation (EN 1779 for example).

I think that the first test is a tightness test with maximum design deformation (and so it is also a robustness test)
The second is a real leakage test.

And another consideration/question: if I apply the same pressure difference beetween the inner side and the outer side of the vehicle (with external overpressure or internal underpressure) the vehicle structural deformation is the same. Is it correct?

Thanks in advance for your answers

 

[Compositepro]

You cannot change the pressure difference between the inside and outside by more than 15 psi using a vacuum.

 

[bcd]

My guess. High pressure water test is a check for strength. Gas test is a check for porosity or other small leak paths that are too small for water to readily flow through in the duration of the water test.