Mechanical and electromagnetic flowmeters

Other commonly used mechanical flowmeters are velocity meters. 

These usually consist of a non-magnetic casing, a rotor, and an electromagnetic pickup. The rotor is either a propeller or turbine freely suspended on ball bearings in the path of the flowing fluid with the axis of rotation in line with the flow. The rotor turns in the fluid flow stream at a rate proportional to the flow rate. As the rotor turns it cuts through the lines of force of an electric field produced by an adjacent induction coil. The electrical pulse output from the induction coil pickup is amplified and fed to readout instruments or recorders to give either total flow or flow rate [Holland and Chapman (1966)]. 

Although electromagnetic flowmeters are expensive they are especially suitable for metering liquids containing suspended solids. Furthermore, unlike head flowmeters, they are unaffected by variations in fluid viscosity, density or temperature. Since they are also unaffected by turbulence or variations in velocity profile, they can be installed close to valves, bends, fittings, etc. 

The flowmeters discussed above are used either to measure velocity or volumetric flow rate. They can only be used to measure the mass flow rate if the fluid density is also measured and the volumetric flow rate and density signals are coordinated. 

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