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Sinn's aviation watches are not only water-resistant against pressure from outside, they also have a crystal secured against the sudden loss of outside pressure, as e.g. when cabin pressure in an aircraft is suddenly lost. In this case, the pressure inside the watch will be higher than outside, and a crystal could be pushed/sucked out of the watchcase. In fact, this was a common complaint about aviation watches with unarmored acrylic crystals (like the British 6B/159) at the time that pressurized cabins were introduced towards the end of WWII.

I was curious to learn just how the crystal is protected against the eventuality of a sudden drop of outside pressure in Sinn's aviation watches.

So I asked Sinn whether there are mechanical elements at play (e.g. heating the case before inserting the crystal for a firm "shrink-to-fit" of the crystal inside the case, or mounting the crystal from the movement side into the case, or the bezel screwing down the crystal). The answer from Sinn, however, is that the crystal is pressed into the case from the top in the conventional way, and that the dimensions of the case opening and the crystal are such that the crystal will not be pushed out at an outside pressure of 0.2 bar (i.e. -0.8 bar below normal surface pressure, and equivalent to 12,000 meters = 40,000 feet AGL).

In other words, the quality of "unterdrucksicherer Glassitz" (low-pressure resistant crystal seating) derives from having passed a low-pressure test rather than from technical or mechanical features.
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