"The purpose and function of the helium relief valve is a common point of confusion. It has nothing to do with normal underwater diving. Neither does it have anything to do with the depth rating for a diving watch. Helium does NOT seep into the watch while the watch is in water at any depth!
To put it simply, you can completely ignore the helium relief valve. This feature is not used in any way with any form of normal SCUBA diving or anything else that involves less than a multi-million dollar deep sea exploration project."
This quote is not really accurate. It is true that Helium does NOT seep into the watch while the watch is in water at any depth. Helium gets in when the divers (deep water saturation divers) are in the bell. As explained by Omega:
'When carrying out work at great depths, professional divers stay in a diving bell for several days, breathing a mixture of gases containing a high proportion of helium. The pressure is gradually increased to reach the pressure at the working depth. The divers, still inside the bell, are then lowered to the working site. They leave the bell to carry out their work.
Once their work is complete, they re-enter the bell, which is then raised to the surface. Pressure is then returned to atmospheric levels and this is when the valve must be opened (only for deep-sea dives lasting several days).
The helium molecules diffuse and penetrate the material of the gaskets. The quantity is sufficient to push out the crystal during the return to atmospheric pressure.'
The gas, called Trimix is a mixture of Oxygen, Helium and Nitrogen. This is when the Helium gets into the watch.
A picture of the diving bell and where these divers will work.
A picture, a bad one sadly of a HEV.
For the automatic HEV like those on the SD and DSSD, the valve will open at the preset depth and pressure and no water will not go in as the gas is 'coming' out and thus preventing water from going in (in theory, the pressure inside is higher than outside and it is a spring-loaded one-way valve). However this does not matter as in most cases the HEV will only come in action inside the bell (which is dry).
The HEV design of the Omega is different (?) from the Rolex design and has to be unscrewed manually (of course the question is will the watch 'pop' if you forget).
Of course the most famous watch with the HEV are the COMEX (COMPAGNIE MARITIME D'EXPERTISE) Rolex.
Rolex ad explaining the HEV.
You can see a video of the DSSD in its natural environment.
http://www.youtube.com/watch?gl=GB&hl=en-GB&v=TsE-SFNrFP0 As for why some watches have and some do not have the HEV, the choice is left to the manufacturer. Some opt to go with the HEV, while some has gone with a different case, crystal and gasket design. I believe Seiko uses special gaskets that can prevent the Helium gas from getting into the watch. Sinn I believe glues the Crystal in place or is filled with oil. Other options include screw in crystal retaining rings.
Seiko 'L' shaped gasket on their watches which are rated Helium Safe to 600M and Water Resistant to 750M.
The HEV first came about in the 60s. Rolex and Doxa collaborated to create the first HEV and featured on the Rolex Submariner/ Sea Dweller and Doxa Conquistador.
The HEV is also affectionally called the 'Fart Valve'.
Of course is the HEV a marketing gimmick or not. I do know people doing deep sea saturation dive and their input on this is that:
a. They don't really care as they use Casio G-Shock during their dive.
b. The popular method is to unscrew the crown of the watch when they are in the bell
Of course we are NOT getting into the semantics of HEV and HRV.
