The electromagnetic flowmeter is similar in principle to the generator. The rotor of the generator is replaced by a pipe placed between the poles of a magnet so that the flow of the fluid in the pipe is normal to the magnetic field. As the fluid flows through this magnetic field, an electromotive force is induced in it that will be mutually normal (perpendicular) to both the magnetic field and the motion of the fluid. This electromotive force may be measured with the aid of electrodes attached to the pipe and connected to a galvanometer or an equivalent. For a given magnetic field, the induced voltage will be proportional to the average velocity of the fluid. However, the fluid should have some degree of electrical conductivity.

Ultrasonic Flow Equipment

Devices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signals reflected from discontinuities in the fluid stream to obtain flow measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam from a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow stream. Signals reflected off flow disturbances are detected by a second piezoelectric crystal located in the same sensor. Transmitted and reflected signals are compared in an electrical circuit, and the corresponding frequency shift is proportional to the flow velocity.

Summary

The basic construction and theory of operation of rotameters, nutating disks, anemometers, electromagnetic flow meters, and ultrasonic flow equipment are summarized below.

Other Flow Meters Summary

Rotameter

Consists of a metal float and a conical glass tube Tube diameter increases with height

High density float will remain on the bottom of tube with no flow Space between the float and the tube allows for flow past the float As flow increases, the pressure drop increases, when the pressure drop is sufficient, the float rises to indicate the amount of flow

Nutating Disc

Circular disk which is attached to a central ball

A shaft is fastened to the ball and held in an inclined position by a cam, or roller Fluid enters an opening in the spherical wall on one side of the partition and leaves through the other side

As the fluid flows through the chamber, the disk wobbles, or executes a nutating motion

Hot-Wire Anemometer

Electrically heated, fine platinum wire immersed in flow Wire is cooled as flow is increased

Measure either change in wire resistance or heating current to determine flow

Electromagnetic Flowmeter

Magnetic field established around system pipe

Electromotive force induced in fluid as it flows through magnetic field Electromotive force measured with electrodes and is proportional to flow rate

Ultrasonic Flow equipment

Uses Doppler frequency shift of ultrasonic signals reflected off discontinuities in fluid