This International Standard provides a method of specifying the performance of hydrometric equipment. It identifies, within a performance framework, the factors that affect the range of uncertainty within which the hydrological quantity is converted into a numerical quantity.This International Standard applies to all equipment used for hydrological measurement, with the exception of equipment used for the determination of water quality. The sensor, the measurement system and the recorder are treated as a unit, whether operated in mechanical, electromechanical or electronic form, or using other physical principles. Because hydrological variables are frequently determined against time, the time measurement device is also considered to be an integral part of the measurement system.