[Goodwatch]

When commercial deep-sea divers operate on great depth, they work from a diving bell. A diving bell is a cable suspended watertight chamber used for underwater work. It has a lid in the floor and is supplied with compressed air. Using the diving bell a small number of divers can be lowered into the water. Once they have reached their work site they use the diving bell as a base. While working at the depth they can return to the diving bell to eat and rest.
When the diving bell is lowered into the water and pressure in the bell increases, helium is added to the breathing mix. This is done to remove nitrogen and reduce the percentage of oxygen to below that of air, and thereby allow the mix to be breathed safely at greater depth. It is not safe to breathe atmospheric air (which contains 78% nitrogen and 21% oxygen) at great depth, because the nitrogen becomes narcotic and the relatively high percentage of oxygen becomes toxic as pressure from depth increases.

The divers continue to breathe the helium/oxygen mix while working at the depth. At their returning to the surface, the pressure in the bell is maintained. The diving bell is locked onto a decompression chamber when reaching the surface. The pressure in the decompression chamber is then equalised to the pressure in the bell and the divers move from the bell to the chamber. They then go through a decompression routine that can take several days.

Prior to this decompression routine the divers have been in the highly helium saturated atmosphere for an extended period. Because helium is such a small molecule, it will seep through the watch’s seals under the high pressure in the bell. If the watch stays in the helium-saturated atmosphere for extended period, then helium will continue to seep into the watch until the air pressure inside the watch, equalise to the air pressure in the environment.

While the helium seeped into the watch over an extended period, it cannot seep out any faster. The pressure in the decompression chamber decreases faster than the helium can escape the watch and this can make the crystal on the watch pop out during the return to atmospheric air pressure. To avoid the crystal from popping out the professional diver’s watches have a helium escape valve. The idea is that the helium can escape through the valve as the pressure in the decompression chamber decreases.

Source: watchlife.com