Water clocks are defined as devices to measure time, where water flows in a known volume or flows out of a known volume. The first are called 'inlet clocks', the second 'outlet clocks'. Since ideally the increasing or draining volume remains constant per unit of time, the elapsed time can be determined from the inflow or outflow volume.
Water clocks are rated among the elementary clocks, such as the sundials, but have the advantage that they are independent from the presence of sunshine. A great problem with these watches, however, is their dependance of the temperature, for which reason this should be kept as constant as possible and in any case above 0°C at normal atmospheric pressure. To work around this problem, it is therefore possible to use a medium that has a deeper melting temperature and lower expansion coefficient, for example mercury.