Abstract: A volume flow meter for the measurement of the flow quantity of a liquid, which meter comprises a cylindrical flow channel (3), which has an inlet point (10) and an outlet point (12) for passing the liquid to be measured through the measurement channel. In the measurement channel, there is an axially moving, spring-loaded indicator piston (4), which is shifted by the liquid in the measurement channel into different positions depending on the flow quantities. In the wall of the measurement channel, there is an outflow slot (27) for the liquid, which slot becomes deeper from the inlet point to the outlet point. The meter is provided with a reversing valve (17), so the flow of the liquid to be measured can be guided through the by-passing channel (26) to the opposite side (3') of the indicator piston (4), as seen from the inlet point (10), so as to shift the indicator piston towards the inlet point. This movement of the indicator piston wipes the wall of the measurement channel clean from impurities.