Conversion of an input frequency to a linearly proportional, isolated output current or voltage. The instruments can be used to monitor the speed of rotating machinery and can be followed by a trip amplifier to give alarm, control or shutdown facilities at preset levels of speed. They are ideally suited for use with turbine flow meters to give an analogue measurement of flow rate, and with proximity detectors for non contact speed measurement. The instruments incorporate both a sensitivity adjustment and a reference voltage to allow a variety of sensor types to be used.