One of the most specialized types of luxury watch is the Diver's Watch. Unlike many other watches, a divers' watch must put function above everything else, particularly if it is to go to a great depth, as watches used by saturation divers are.
There are set criteria issued by the ISO for what can qualify as a genuine Diver's watch. This means that any watch bearing the legend 'DIVER'S WATCH XXXM' or 'DIVER'S XXXM' (where 'XXX' represents the depth) will have undergone a rigorous set of tests to guarantee that it meets the specifications. These tests include a condensation test which is carried out before and after any other test - the watch is placed on a heated plate at 40-45°C and left for 10-20 minutes until it reaches this temperature itself. The a drop of water at room temperature (18-25°C) is applied to the crystal face, and left there for 1 minute before being wiped off. If any condensation forms on the underside of the crystal, the watch fails, and is immediately discounted from any further tests.
In order to become a diver's watch, contenders also have to descend below their rated depth by a quarter again in still water, to allow for the varying densities of depth caused by weather variations and the fact that seawater is between 2% and 5% denser than fresh water. They will also be subjected to a thermal shock test, being placed in water at 40°C for an hour, then immediately into a container of water at 5°C, again, for an hour, then back into a 40°C bath for another hour. The longest test is one in which the watch is held 30cm underwater for 50 hours - more than two days. For this reason, most diver's watches are made from steel or titanium, with ceramics and some plastics now becoming more common.
Other requirements are more to do with the practicalities of taking a watch under water. All diver's watches are required to have a time pre-selecting device; the ability for the diver to see at a glance how long he has been underwater. In an analogue watch, this usually takes the form of a unilateral bezel - a bezel that can only move one way. Bezels such as this must have five minute intervals marked on them clearly, and have a scale of 60 minutes. The 60/0 minute mark must be visible on the bezel at 25m below the surface, as well as the time itself, so digital watches must have a backlit screen, and analogue watches need luminescent hands that are distinct from each other. They also need an indication that they are working, both at 25 metres and in total darkness, so analogue watches will usually have a 'running' second hand which has a luminescent tip.