Flow rate detector for determining the amount of a liquid flowing through a line, comprising: a connecting piece with an inlet connector and an outlet connector which can be connected to the line; a measuring capsule with a round measuring capsule housing made from metal and a measuring channel, wherein the inlet of the measuring channel is connected to the inlet connector and the outlet of the measuring channel is connected to the outlet connector, so that the liquid volume to be measured flows through the measuring channel; an ultrasonic measuring section which extends through the measuring channel and has two ultrasonic transducers (50a, 50b) that are arranged at the beginning and the end of the measuring channel and send and/or receive ultrasonic signals into the ultrasonic measuring section; deflecting mirrors arranged along the measuring channel that are inclined at an angle to each other in such manner that the ultrasonic signals emitted by the first ultrasonic transducer are reflected towards the second ultrasonic transducer via the deflecting mirrors and/or in the opposite direction back towards the first ultrasonic transducer; characterized in that a measuring chamber insert made from a plastic that absorbs ultrasound is arranged coaxially inside the measuring capsule housing; the measuring channel is conformed inside the measuring chamber insert; the outer wall of the measuring chamber insert has radial openings (46a, 46b, 46c); flat reflecting surfaces (32a, 32b, 32c) are conformed on the inner wall of the measuring capsule housing, wherein the flat reflecting surfaces close the radial openings of the measuring chamber insert. a connecting piece with an inlet connector and an outlet connector which can be connected to the line; a measuring capsule with a round measuring capsule housing made from metal and a measuring channel, wherein the inlet of the measuring channel is connected to the inlet connector and the outlet of the measuring channel is connected to the outlet connector, so that the liquid volume to be measured flows through the measuring channel; an ultrasonic measuring section which extends through the measuring channel and has two ultrasonic transducers (50a, 50b) that are arranged at the beginning and the end of the measuring channel and send and/or receive ultrasonic signals into the ultrasonic measuring section; deflecting mirrors arranged along the measuring channel that are inclined at an angle to each other in such manner that the ultrasonic signals emitted by the first ultrasonic transducer are reflected towards the second ultrasonic transducer via the deflecting mirrors and/or in the opposite direction back towards the first ultrasonic transducer; characterized in that a measuring chamber insert made from a plastic that absorbs ultrasound is arranged coaxially inside the measuring capsule housing; the measuring channel is conformed inside the measuring chamber insert; the outer wall of the measuring chamber insert has radial openings (46a, 46b, 46c); flat reflecting surfaces (32a, 32b, 32c) are conformed on the inner wall of the measuring capsule housing, wherein the flat reflecting surfaces close the radial openings of the measuring chamber insert.