A device for determining and/or monitoring the volume, and/or mass, flow of a medium flowing through a containment in a stream direction. The device includes: At least one ultrasonic transducer, which emits and/or receives ultrasonic measuring signals; and a control/evaluation unit, which determines the volume, and/or mass, flow of the medium in the containment on the basis of the ultrasonic measuring signals according to the travel time difference principle or the Doppler principle. A first clock-pulse generator having a first pulse rate and a second clock-pulse generator having a second pulse rate are provided, with the first pulse rate of the first clock-pulse generator being greater than the second pulse rate of the second clock-pulse generator. The control/evaluation unit operates the first clock-pulse generator intermittently. The control/evaluation unit sizes the duration of a switch-on, or measuring, phase and/or a resting phase as a function of the available energy.