Abstract: A leak detector is proposed for fluid systems serving intermittent loads. The device generates a signal representative of the minimum level of flow occurring from time to time, this level of flow being taken as the rate of leakage. On the basis of this signal, the device generates outputs indicative of the presence and relative magnitude of leakage. A preferred form of the device monitors flow with a thermal fluid flow detector which is adapted to be removably mounted on a distribution system pipe and to be sufficiently inexpensive to permit wide application of the device to small fluid distribution systems, such as residential water systems.