To measure the volume of fluid that has flowed past a point and to actuate a sampler at certain increments of this volume, a stylus of a flow meter moves to a distance from the center of rotation of a rotating function disc, which distance is related to the height of a float in the fluid stream, with the function disc including a conductive portion having different arcuate lengths at different distances from the center of rotation so that, as the disc rotates, the stylus circumscribes a circle around the function disc at a distance related to the height of the float and is in contact with the conductive portion for a period of time that represents the rate of flow of the fluid that corresponds to the height of the float. Pulses representing in number the rate of flow of the fluid during each revolution are applied to a counter while the stylus contacts the conductive portion which accumulates them to form the integral with respect to the time of the rate of flow, with the output from the counter being used to actuate a liquid sampler at predetermined increments of volume of fluid flowing past the float.