So if the watch does not leak, then there is no build up of pressure inside the watch case. These testers rely on the fact that if the watch leaks while under pressure, it will also leak when the external pressure is removed.
If the watch were to somehow let air in, but not out, you would never see bubbles and would never know if the case leaked.
If you plan to follow this up with a final test with the movement in the case, then I would suggest first running the whole test once, but never putting the watch into the water. - this is more a test of the crystal holding than anything else. Then if the crystal does not pop out, you can run the test again and actually submerge the watch.
That appears to be the same style of Calypso tester I used to have, and I had several crystals blow out - all while the watch was above the water line though...
Cheers, Al
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