Machine Design

# Sensor Sense: Magnetic inductive flowmeters

Faraday's law of induction states that a conductor moving through magnetic flux induces an electromotive force in the conductor.

In layman’s terms, a wire generates a voltage as it moves through a magnetic field. The amount of voltage generated depends on the strength of the magnetic field, the length of the wire in the field, and the speed that the wire moves through the field.

Magnetic inductive flowmeters work on the same principles. An electrically conductive fluid flowing through a magnetic field acts just like the moving wire. A voltage is generated through the fluid at right angles to the magnetic field. The amount of voltage generated is directly proportional to the speed of the fluid flow. Speed of flow combined with the pipe diameter determines rate of flow.

It’s important to note that the fluid must be electrically conductive. Just as copper is a better conductor than iron, some fluids conduct better than others. For example, tap water is a great conductor; however, deionized water is an insulator and so cannot work with a magnetic flowmeter. Other liquids that have little to no conductivity are hydrocarbons, oils, and nonaqueous solutions.

Conductance is the reciprocal of resistance and is measured in Siemens (S). The way of stating this mathematically is to say S = 1/Ω, where Ω = the resistance in Ohms. An earlier term for the measure of conductance was the mho — that’s Ohm spelled backwards. Fluids used with magnetic flowmeters must meet a minimum value of conductance for rates to register properly.

Conductivity varies with temperature, so the conductivity of the liquid must remain adequate for measurement over the entire operating temperature range. Likewise, concentrations of total dissolved solids, acids, and caustics also affect conductivity. In general, conductivity rises with concentrations, but only up to a point. Many acid and caustic solutions drop in conductivity when concentrations exceed 20%.

Viscosity typically does not affect magnetic flowmeter readings. However, flow rates for high-viscosity fluids should be kept high to prevent buildup along the inside of the pipe. The stationary coating on the sides effectively shrinks the pipe diameter which makes flow readings higher than actual flow rates.

Magnetic inductive flowmeters are not affected by fluids containing suspended debris and solids. This gives them a decided advantage over paddle wheel, vortex, and variable-area-tube flowmeters where debris can clog or jam their operation. Flow rate measures correctly as the solids are moving at the same rate as the liquid and there’s no obstruction in the path to block passage.

Turck Inc. (turck.com) provided information for this column.

In the magnetic inductive flowmeter, a solenoid coil produces a magnetic field (B) through the pipe. The velocity of a conductive fluid (V) passing through the magnetic field generates a voltage (E) at right angles to the field. The greater the velocity, the higher the induced voltage. Measuring electrodes read the voltage which is then converted into a flow rate by the meter.