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, or 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 if 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.