Electromagnetic Flowmeter

The electromagnetic flow sensors are useful for measuring the movement of conductive liquids. They are true noninvasive measurements. The operating principle is based on Faraday’s Law of electromagnetic induction.

“If a conductor of length L (m) is moving with a velocity v (m/s), perpendicular to
a magnetic field of flux density B (Tesla), then the induced voltage e across the ends of conductor can be expressed as:

e = BLv

The magnetic field, the direction of the movement of the conductor, and the induced emf are all
perpendicular to each other.”

In the case of electromagnetic flowmeters, the conductor is the liquid flowing through the pipe, and the length of the conductor is the distance between the two electrodes, which is equal to the tube diameter. The velocity of the conductor is proportional to the mean flow velocity of the liquid.

The generated voltage does not depend on parameters such as pressure, temperature, viscosity, conductivity, etc. Only a minimal level of conductivity is required to give this signal a (very small) minimal power.

The accuracy of these meters can be as low as 0.25% and, in most applications, an accuracy of 1% is used.

The traditional magnetic flow meter consists of two units: the measuring probe and electronic converter-amplifier that transforms this mV signal into one or more standard analog or digital signals.

The electrodes are placed at positions where maximum potential differences occur. The electrodes are electrically isolated from the pipe walls by nonconductive liners to prevent short-circuiting of electrode signals. The liner also serves as protection to the flow tube to eliminate galvanic action and possible corrosion due to metal contacts.

The main body of a flowmeter and electrodes can be manufactured from stainless steel, tantalum, titanium, and various other alloys.

The measuring probe consists of two electrodes made of a non-magnetic material
and is positioned at the inside covered with an electric insulating layer. Commonly used insulating layer are Natural rubber or neoprene, PTFE (Teflon), PFA (Per Fluor Alkoxy), Ceramic.

Coil excitation

The magnet coils generate a magnetic field that depends on the form of the field
excitation signal. There are several types of flow meters coil excitation.

(a)    DC excitation: It is only applicable with liquid metals. DC excitation results in electrolysis and hence they are hardly ever used.

(b)   AC or Sinusoidal excitation: An alternating current of 50 Hz to 60 Hz in coils creates
the magnetic field to excite the liquid flowing within the pipe. The disadvantage of these applications is that the electronic noise causes the zero point to drift after a certain time. It is essential to manually readjust the zero point at regular time intervals.

(c)    Pulsed DC field: here the converter feeds the magnetic coils with a pulsed DC (low frequency square wave) current. Because the converter is provided with the necessary intelligence, it can independently control the zero point, so that the zero point is stable. This method has lower power consumption (5-25W).

(d)   Capacitive detection: Here the electrodes, which are in contact with the medium are replaced by capacitive plates and that function as the electrodes of a capacitor. For this model the minimum required conductivity is 100 times lower than for the model with the contact electrodes.

The installation of the flow sensor may occur in every position, as long as the measuring instrument is completely filled with the fluid. If the fluid contains solid particles or fats, the magnetic flow meter is best positioned vertically. When placing it horizontally, the heavier particles will precipitate and the lighter particles will come up.

Theoretically the direction of the flow is not important, as long as the correct electric connection is used.

At a low rate of flow (< 1 m/s) the desired accuracy cannot be obtained. The velocity can be increased by reducing the pipe.

The flow profile is not very important for the magnetic flow meter. In practice,

however, a straight pipe segment of 5 times the pipe’s diameter is recommended. The grounding is of vital importance. This is very important because the voltage on the electrodes amounts to only a few mV.

The pipes of electromagnetic flowmeters must be full of liquid at all times for accurate measurement. If the liquid does not make full contact with electrodes, the high impedance prevents the current flow; hence, measurements cannot be taken. Also, if the pipe is not full, even if contact is maintained between the liquid and electrodes, the empty portions of the pipe will lead to miscalculated flow rates.

Important features

·         Completely obstruction-free and hence no pressure loss.

·         Highly accurate, better than 1% FS for higher flow.

·         Wide span, good linearity.

·         Measuring principle not dependent on pressure, temperature or viscosity.

·         Expensive because of electronics.

·         Minimum conductivity required (5µS/cm).

·         Not dependent on the flow profile

·         No mechanically moving components, maintenance-free.

·         Ideal for contaminated liquids.