Limitations of Electromagnetic Flow Meter

• The substance being measured must be conductive. Therefore, it can’t be employed for metering the flow rate of gases and steam, petroleum products and similar liquids having very low conductivity.

• To render the meter insensitive to variations in the resistance of liquid, the effective resistance of the liquid between the electrodes should not exceed 1% of the impedance of the external circuit.

• It is a very expensive device.

• As the meter always measures the volume rate, the volume of any suspended matter in the liquid will be included.

• To avoid any trouble which would be caused by entrained air, when the flow tube is installed in a horizontal pipe-line, the electrodes should be on the horizontal diameter.

• As a zero check on the installation can be performed only by stopping the flow, isolating valves are required and a bypass may also be necessary through which the flow may be directed during a zero check.

• The pipe must run full, in case regulating valves are installed upstream of the meter.

Advantages of Electromagnetic Flow Meter

• The obstruction to the flow is almost nil and therefore this type of meters can be used for measuring heavy suspensions, including mud, sewage and wood pulp.

• There is no pressure head loss in this type of flow meter other than that of the length of straight pipe which the meter occupies.

• They are not very much affected by upstream flow disturbances.

• They are practically unaffected by variation in density, viscosity, pressure and temperature.

• Electric power requirements can be low (15 or 20 W), particularly with pulsed DC types.

• These meters can be used as bidirectional meters.

• The meters are suitable for most acids, bases, water and aqueous solutions because the lining materials selected are not only good electrical insulators but also are corrosion resistant.

• The meters are widely used for slurry services not only because they are obstruction less but also because some of the liners such as polyurethane, neoprene and rubber have good abrasion or erosion resistance.

• They are capable of handling extremely low flows.

Disadvantages of Electromagnetic Flow Meter

• These meters can be used only for fluids which have reasonable electrical conductivity.

• Accuracy is only in the range of ± 1% over a flow rate range of 5%.

• The size and cost of the field coils and circuitry do not increase in proportion to their size of pipe bore. Consequently small size meters are bulky and expensive.