Correction in Velocity

Correction in Velocity Recalling the fact that the measured value of the piezometric pressure drop for a real fluid is always more due to friction than that assumed in case of an inviscid flow, a coefficient of velocity Cv (always less than 1) has to be introduced to determine the actual velocity Vc when the pressure drop  in Eq. (15.12) is substituted by its measured value in terms of the manometer deflection '∆h' Hence,     (15.13) where '∆h' is the difference in liquid levels in the manometer and ρm is the density of the manometric liquid. Volumetric flow rate            (15.14)       If a coefficient of contraction Cc is defined as, Cc = Ac /A0, where A0 is the area of the orifice, then Eq.(15.14) can be written, with the help of Eq. (15.13),     (15.15)

The value of C depends upon the ratio of orifice to duct area, and the Reynolds number of flow. The main job in measuring the flow rate with the help of an orificemeter, is to find out accurately the value of C at the operating condition. The downstream manometer connection should strictly be made to the section where the vena contracta occurs, but this is not feasible as the vena contracta is somewhat variable in position and is difficult to realize. In practice, various positions are used for the manometer connections and C is thereby affected. Determination of accurate values of C of an orificemeter at different operating conditions is known as calibration of the orifice meter.