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Watches may be crafted to become water resistant. These watches are sometimes called diving watches when they are suitable for scuba diving or saturation diving. The International Organization for Standardization issued a standard for water resistant watches which also prohibits the term "waterproof" to be used with watches, which many countries have adopted. Water resistance is achieved by the gaskets which form a watertight seal, used in conjunction with a sealant applied on the case to help keep water out. The material of the case must also be tested in order to pass as water resistant.[17]
The watches are tested in theoretical depths, thus a watch with a 50 meter rating will be water resistant if it is stationary and under 50 meters of still water for a set amount of time. The most commonly used method for testing the water resistance is by depressurizing a small chamber containing the watch. A sensor measures the movement of the case and crystal to gauge how much pressure the watch is losing and how fast. The watch never touches water in this type of machine. Another type of machine is used for very deep measure tests, where the watch is immersed in a small container filled with water, this chamber is then submitted to the pressure the watch is supposed to withstand. In neither case is there any variation in the pressure, nor is the watch submitted to that pressure for an extended period of time(normally only a couple of minutes). These are the only logical ways to test the water resistance of a watch, since adding variations added by time spent underwater or the movement of the wearers hands would simply make this a very intricate and difficult measurement. Although confusing this is the best way of telling the customer what to expect. For normal use, the ratings must therefore be translated from the pressure the watch can withstand to take into account the extra pressure generated by motion and time spent underwater.
Watches are classified by their degree of water resistance, which roughly translates to the following (1 meter = 3.281 feet):[18]
Water resistance rating
Suitability
Remarks
Water Resistant 30 m or 50 m
Suitable for washing hands. 50 m suitable for showering and light swimming.
not suitable for swimming or diving.
Water Resistant 100 m
Suitable for recreational surfing, swimming, snorkeling, sailing and water sports.
not suitable for diving.
Water Resistant 200 m
Suitable for professional marine activity and serious surface water sports.
suitable for diving.
Diver's 100 m
Minimum ISO standard (ISO 6425) for scuba diving at depths NOT requiring helium gas.
Diver's 100 m and 150 m watches are generally old(er) watches.
Diver's 200 m or 300 m
Suitable for scuba diving at depths NOT requiring helium gas.
Typical ratings for contemporary diver's watches.
Diver's 300+ m helium safe
Suitable for saturation diving (helium enriched environment).
Watches designed for helium mixed-gas diving will have additional markings to point this out.
Some watches use bar instead of meters, which may then be multiplied by 10 to be approximately equal to the rating based on meters. Therefore, a 10 bar watch is equivalent to a 100 meter watch.