The bypass flow meter operates on the differential-pressure-bypass method. A ring with an orifice plate is installed in the pipeline between flanges. The measuring orifice leads to a constriction and causes a differential pressure. As compensation a volume flow occurs in the bypass which is displayed by a variable area flow meter. This partial flow is proportional to the flow rate in the main pipeline.The unimpeded, straight tube length has to be 6DN before and 4 DN behind the mounting position. The partial flow can be turned on and off, as required, by installing two ball valves in the bypass line..