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It would appear that a number of folks here have some misconceptions about the HEV.
For dives where you go into the water (no matter how deep) and then re-surface....the HEV does nothing.
The HEV only becomes necessary when a diver does "saturation" dives...meaning you go into the water (DEEP) and rather than coming back to the surface, you go into a HYPERBARIC chamber where the air pressure is EQUAL to the water pressure that you are diving in.....so no decompressing to get "dry".
Now because in hyperbaric environments both Nitrogen AND oxygen become toxic.. N2 becomes an anesthetic and O2 can actually cause seizures and pulmonary edema....the "saturation" diver breathes a mix of LOW percentage oxygen and HIGH concentration helium...to prevent the toxicities of normal air in a hyperbaric environment.
It is during these dives while the dive is "dry" in a hyperbaric chamber that dive watches can become saturated with helium....
Now during decompression...if you do it SLOW enough, you don't need a HEV...but because humans can tolerate a rate of decompression that is faster than helium can seep back out of a watch, you need the HEV to prevent the crystal from blowing off....
So back to my original question....at what pressure differential does the valve open.....I guess someone already answered 2.5 ATM....if that's the case, then watches with HEV should be able to tolerate the Vacuums of space ...one of the requirements for a NASA certified watch.
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