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The machine I use at work (witschi proofmaster s) does a vacuum test before a pressure test only to make sure nothing is seriously wrong before the pressure comes in. So a loose glass or not fully on back stops the pressure crushing a glass in to the watch face for example.
With the standard 100m test it does a vacuum test of 0.7 bar, but obviously the vacuum pressure has nothing to do with the water resistance rating.
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