Water Resistant is a common mark stamped on the back of wrist watches to indicate how well a watch is sealed against the ingress of water. It is usually accompanied by an indication of the static test pressure that a sample of newly manufactured watches were exposed to in a leakage test. The test pressure can be indicated either directly in units of pressure such as bar, atmospheres, or (more commonly) as an equivalent water depth in metres (in the United States sometimes also in feet). An indication of the test pressure in terms of water depth does not mean a water-resistant watch was designed for repeated long-term use in such water depths. For example, a watch marked 30 metres water resistant cannot be expected to withstand activity for longer time periods in a swimming pool, let alone continue to function at 30 metres under water. This is because the test is conducted only once using static pressure on a sample of newly manufactured watches. As only a small sample is tested, there is a small likelihood that any individual watch is not water resistant to the certified depth or even at all. The test for qualifying a diving watch to bear the word "diver's" on the dial is for repeated usage in a given depth and includes safety margins to take factors into account like aging of the seals, the properties of water and seawater, rapidly changing water pressure and temperature, as well as dynamic mechanical stresses encountered by a watch. Every "diver's" badged watch has to be taken through a small but highly specified battery of tests designed to simulate those stresses including being tested for continued water resistance up to 125% of the stated rating (a "200 meter" watch has to be pressured up to 250 meters water depth equivalent and show no signs of intrusion). ISO 2281 water-resistant watches standard The International Organization for Standardization (ISO) issued a standard for water-resistant watches which also prohibits the term waterproof to be used with watches, which many countries have adopted. This standard was introduced in 1990 as the ISO 2281:1990 and only designed for watches intended for ordinary daily use and are resistant to water during exercises such as swimming for a short period. They may be used under conditions where water pressure and temperature vary; German Industrial Norm DIN 8310 is an equivalent standard. However, whether they