X-ray equipment, used for radiation therapy at low voltages, is frequently operated in small hospitals which do not have full-time physics staff. Most of the X-ray units involved are not fitted with dose-rate monitors which would allow continuous control of dose so that the output must be checked at frequent intervals by the radiographer using a suitable instrument. The design criteria for such an instrument are discussed and its construction described in detail together with the tests and field trials carried out. The instrument is designed to measure dose rates from 2500 rad min^-1 at 10 kV to 50 rad min^-1 at 250 kV. It has been possible by use of the instrument to determine the 'normal' behaviour of each X-ray unit and to establish criteria by which 'abnormal' behaviour can be determined.