Abstract: An instrument for measuring flow rates in systems where contaminants in the moving fluids can accumulate on normal measuring devices and render them inoperative. This instrument consists of pitot-static tubes inserted into the conduit, one upstream and one downstream, the nozzles of said tubes being fed with a purge gas at a pressure higher than that of the vapors in the conduit. The nozzles are enclosed by a capsule with an opening in the side to allow passage of the appropriate gas lines and release of the purge gas into the conduit, said capsule being separately supported on a flexible rod and serves as a flapper valve, altering the pressure on the pitot tube nozzle as the moving fluids push against the upstream end of the cylinder. The change in relative purge gas pressure in the pitot-static arrangement is then monitored as a function of flow rate in the conduit.