Abstract: The present invention detects the presence of liquid and it is used to control pumping of a well when the quantity of liquid being discharged by the pump per stroke is much less than the volumetric displacement of the pump.A fluid detector (96) includes a first thermally conducting body (98), a first heater (100), a first temperature detector (108), insulation material (122, 124 and 136) for thermally insulating the first thermally conducting body, a second thermally conducting body (102) exposed to the inside of a conduit (104), a second heater (106) for heating the second thermally conducting body and a second temperature detector (110) for detecting the temperature of the second thermally conducting body or the second heater. One arrangement also includes an electrical bridge network (112) for comparing the relative rates of heat loss from the two thermally conducting bodies.