The apparatus consists of two pipelines emerging out from a common manifold. One pipeline contain a Venturimeter, second contains an Orifice. The pressure tapings from the Venturimeter and Orificemeter are taken to differential manometer to measure pressure difference. The Venturimeter and Orificemeter are connected in parallel and anyone of them can be put in operation by operating valves provided at the downstream. These valves can also regulate the flow.