Why do you need a watch waterproof to 1220m...
Read and learn (source unknown - I only had a xerox copy)
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In the early 1970's, Rolex supplied Submariner watches for divers working for Comex (Compagnie Maritime d'Expertise), a French company, working out of Marseilles, which specialized in deep diving using pressure chambers. Problems arose when the watches experienced a build up of helium gas from the pressure of the undersea environment. When the pressure chambers were decompressed, the helium could not escape from the innards of the case fast enough, and often times blew the crystal out of the case. An occupational hazard in the cramped confines of the Pressure Chambers.
Comex approached Rolex with a possible solution to this problem by modifying the Submariner case with a helium escape valve on the side of the watch. This proved to be effective for these types of dives, and Rolex decided to make a production version of this watch with the helium valve. This watch was launched as the "Sea-Dweller' in 1971.
As you may already know, the Sea-Dweller is designed to be used in an underwater habitat that has a mixed atmosphere of helium and oxygen. This permits the concentration of oxygen to have a normal partial pressure without the nitrogen which is the major gas found in air along with oxygen. If normal air were used in a deep sea habitat, the pressurized concentration of nitrogen would cause nitrogen narcosis (very undesirable).
With helium replacing the nitrogen, many dive problems are solved, but some others are created. Because the helium molecule is so small (atomic weight: 1; atomic number: 1 - remember?), helium can easily infiltrate through the crystal and O rings of the Submariner or Sea Dweller. Since the heli-ox mixture in the under sea habitat will be so much greater than that of the surface atmospheric pressure, the accumulated helium pressure inside a Submariner would be so great, that on surfacing the waterproof integrity could fail since there is no way for the Submariner to release the gas inside the case in order to equalize with the decreased pressure of the water near the surface. Naturally the crystal is the weakest component of the watch movement's case. As a helium filled toy balloon rises to greater altitudes it encounters lower atmospheric pressure so that the pressure differential inside the balloon becomes so much greater that the atmosphere around it that it expands. The same principle is at play in the ocean except that the effect is greatly exaggerated (water weighs so much more than air). now the helium that was pressurized in the watch at a great depth may be equal to several atmospheres, but as it rises to the surface, unlike with the balloon, there is no place for it to expand. Therefore the gas pushes outward with tremendous pressure. As mentioned above the case is much stronger than the watch crystal and eventually something must give way.
With the Sea Dweller ROLEX has solved this problem by using a one way valve (pressure relief valve). If you are not going to live in an under sea habitat with marine biologists or geologists, then you don't need the sea dweller. Essentially it is the Submariner with a relief valve and designed to take the pressure at a depth of 4000 feet rather than the lesser depths of the Submariner. For any other purpose, the Submariner is just as good as the Sea Dweller and of the same quality.
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So now you know
