Flowmeters and flow measurement

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

The pressure at level a in the manometer is (Pi + pAzmg) in arm 1 and (P2 + pmkzmg) in arm 2 where p and pm are the densities of the flowing fluid and immiscible liquid respectively. These two pressures are equal since the two arms of the manometer are connected by a continuous column of stationary liquid. Therefore 

Other flowmeters are in common use which operate on principles differing from head flowmeters. Mechanical flowmeters have primary elements which contain moving parts. These flowmeters include rotameters, positive displacement meters and velocity meters. Electromagne- 

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