Matt V
3T WIS
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Mixed gas bell divers or saturation divers work or remain ("live") in a pressurized environment (dive bell or chamber) with an athmosphere that contains helium and oxygen (helium has no toxic or narcotic effects, unlike pure oxygen under pressure or nitrogen).
While in this pressurized environment, helium atoms can penetrate into a watch that's otherwise water resistant because the helium atoms are smaller than oxygen atoms or water molecules.
When these guys (wearing a wristwatch that was exposed to the pressurized environment long enough) now go into a decompression chamber to prepare for surfacing, the helium atoms (gas) trapped inside the watch expand as a result of the decompression and can build up enough pressure to "pop" the crystal (keep in mind, the crystal and crystal gasket are designed for the opposite situation: pressure from the outside pushing it deeper into the gasket) if not properly released.
All they'd really have to do is unscrew the crown and the pressure inside the watch would equal that of the outside and allow the helium atoms to expand and escape through the crown tube. Some clever marketeers however came up with the feature of providing an over-pressure valve (HEV / "Helium Escape Valve") that would automatically do the same thing without the need to open the crown (or in case you forget doing that).
And anytime a feature is useful to a certain group of interesting users or application, marketing exploits it as "cool" and "must have" for everyone else. ;)
Of course you could argue that a) a HEV provides an additional potential point of failure and b) if you're never in the situation of going saturation diving, you really don't need it and finally c) that decompression happens in an athmosphere (not inside the water) where you might as well unscrew the crown for the same effect.
But than again, none of us are fighter pilots, saturation divers or secret agents, yet it is kinda cool to know that our watches could do it IF .... :D
Cheers
Matt
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