This International Standard applies to instrument and measurement channels which generate a calculation of the mean square voltage (MSV) of a signal arising from a neutron detector, in order to extract from it information relating to the neutron fluence rate of a nuclear reactor. After calibration, this information can be used to derive the relative power and the time constant, for example expressed in terms of period, doubling time, decades per minute or percent per second. The method used to calculate the mean square voltage of the signal is also known as "fluctuation treatment" or "the Campbell method". Associated with other techniques of measurement, such as pulse rate counting or current measurement, the calculation of the mean square voltage allows the assembly of a series of wide range neutron fluence rate measurements for the simplification of nuclear instrumentation systems in the control of nuclear reactors. This standard describes the principles, the terminology, the characteristics, the requirements and the testing methods related to instrumentation and measurement of the neutron fluence rate using MSV techniques for nuclear reactor control. Typical examples of the application of the MSV techniques are given.