Also, rotometers do not require straight pipe runs for installation so they can be installed just about anywhere. Velocity Devices Probably the most common velocity device used for flow measurement is the magnetic flowmeter. Magnetic flowmeters cause no head loss and they can easily measure liquids with solids in suspension. By their design, they produce an electrical signal ideal for plant transmission. In a magnetic flowmeter, the pipe is lined with a nonconducting material and at least two electrodes are mounted flush with the nonconducting wall.
Electromagnetic coils surround the flow path with a uniform magnetic field.
Faraday's Law dictates that the voltage produced by a conducting fluid flowing through a magnetic field is directly proportional to the velocity of the fluid. The major disadvantage of magnetic flowmeters is that they cannot be used for hydrocarbons due to hydrocarbon's low conductivities. Another velocity device, which can be used for hydrocarbons, is called a vortex-shedding meter. You can read more about these devices here . The most common displacement flow-measuring device is the turbine meter.