Miscellaneous Measuring Devices

The preceding section covers the common types of flow meters such as obstruction devices and rotameter. There are few additional devices that are used for specific applications and these devices can be made such that outputs that vary linearly with flow rate. Some of the special classes of these devices are briefly discussed here.

Electromagnetic flow meters

When a conductor is moved in a magnetic field, an electromotive force is developed due to magnetic induction. The voltage induced across the conductor while moving right angles to the magnetic field is proportional to the velocity of the conductor. This principle is known as Faraday's law and stated by the following equation;

(7.4.1)

where, B is the magnetic flux density (gauss), L is the length of the conductor (cm) and V is the velocity of the conductor (cm/s). If the conductor is replaced by a conducting fluid, then V may be replaced by flow velocity.

Fig. 7.4.1: Operating principle of an electromagnetic flow meter.

A full flow electromagnetic flow meter is a non-intrusive device consisting of a magnetic coil that encircles the pipe containing a flowing fluid (conductive), as shown in Fig. 7.4.1. Two electrodes are drilled and flush-mounted into the inner surface of the pipe but do not interfere with the flow. These electrodes are then connected to the voltmeter that measures the electric potential difference due to the flow velocity of the conducting fluid.

Electromagnetic flow meters are best-suited for measuring flow velocities of liquid metal such as mercury, sodium, and potassium and find applications in nuclear reactors. They can also be used for liquids of poor conductors if they contain adequate amount of charged particles. Flow rate measurement of corrosive liquids, slurries and fertilizers are also possible by electromagnetic flow meters. Commercial magnetic flow meters have rated accuracies of 0.5% to 1%.