Instrumentation The magnetic induction flowmeter, an old but compara- tively unknown device, deserves to be better known

HE measurement of flow is a T perennial problem in spite of the fact that many different types of in- struments are available for the pur- pose. Fixed-area orifice type flow- meters are probably the most widely used. There are many varieties of these using different types of orifice and numerous types of differential pressure measuring devices. An- other type widely used is the vari- able-area flowmeter or rotameter. For some applications, displacement type meters using pistons, nutating

Figure 1. Schematic Diagram of 'Induction-Type Flowmeter

disks, or bellows coupled to appro- priate counting mechanisms are the most satisfactory. Inferential meters in which some form of pro- peller or turbine drives a counter are also widely used. There are other less widely used methods such as the thermal method in which the temperature rise downstream from a heating unit is used to indicate flow rate.

The fact that no one type of flowmeter can solve all flowmetering problems accounts for the fact that such a wide variety of types has been developed. The perfect flowmeter would produce no more pressure drop than a piece of pipe the length and diameter of that which it re- places. Its calibration would be independent of the fluid character- istics such as viscosity, specific gravity, suspended solids, and elec-

bg M a l p h W. Munch

is proportional to the speed of rotation; in the flowmeter, it is proportional to

trical or therms1 conductivity. Its speed of response would be greater than that of any change in flow rate to be measured. The response of the meter would be a linear function of flow over the operating range. It should be possible to construct it of materials resistant to the fluid to be metered. Finally, it should be easily installed, easily maintained, and low in first cost. Of course, no one flowmeter can have all these desirable characteristics.

In 1831, Michael Faraday dis- covered that a current is induced in a conductor moving through a mag- netic field. Everyone is familiar with the fact that this discovery is the fundamental principle on which modern electrical generators de- pend; few people realize that it can also be the basis for a fluid flow- meter. The principle is analogous to that of the use of a small gen- erator as a tachometer. In the tachometer, the generated voltage

the linear veloc- ity of the fluid passing through the meter. Fig- ure 1 shows how the principle can be used to measure the linear veloc- ity of a fluid flowing through a pipe or tube. Tube C is arranged to pass through the magnetic field between A and B, the poles of a magnet. The tube must be non- magnetic and must not be an elec- trical conductor. Electrodes D and E are embedded in the walls of the tube diametrically opposite each other and connected to a suitable device to detect the voltage gen- erated.

Faraday's law of induction can be expressed as

E = BlV X lo-* (Continued on page 84 A )

Figure 2. Primary Element of Magnaflow Electromagnetic Flowmeter

January 1952 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y - 83 A

Instrumentation

Here E is the generated voltage in volts, B is the flux density in gauss, I is the inside diameter of the tube in centimeters, and V is the fluid velocity in centimeters per second. B, 1, and V are mutually perpen- dicular. With a nonvarying mag- netic field, the output is a direct current voltage. The early work with this type flowmeter used a non- varying magnetic field. One very serious difficulty was experienced with the method. The flow of direct current through the electrodes pro- duced polarization. To avoid this, nonpolarizable electrodes separated from the flowing solution by suit- able liquid junctions were used. I t was also necessary that the fluid have a rather high electrical conduc- tivity.

TQ avoid polarization effects at the electrodes, the use of an alter- nating magnetic field was intro- duced. The induced voltage pro- duced with this arrangement is an alternating current voltage, so that, if the frequency is high enough, polarization is eliminated. The use of the alternating magnetic field does, however, introduce another difficulty. This is an interference voltage induced in the electrode loop, D-E, acting as a single turn trans- former secondary, by the alternat- ing magnetic field. This voltage is i ndepden t of flow rate and must be balanced out if the voltage out- put of the flowmeter is to be zero at zero flow rate. This effect is lawer at low frequencies than a t higher frequencies, Frequencies from 10 to 5000 cycles have been used by various workers, although 60-cycle excitation is most common. The higher frequencies are desirable only when high resolving power js needed.

In addition to eliminating polari- zation troubles, the use of alternat- ing current excitation has the ad-

can be used to measure the flow of liquids having lower conductivity. It seems practical to meter liquids having conductivity equal to or higher than the average tap water at present.

The Mittelmann Electronics Divi- sion of the Century America Corp., 549 West Washington Blvd., Chi- cago 6, Ill., is now offering magnetic- induction type flowmeters for sale. Figure 2 is a photograph of the pri- mary element of one of their elec- tromagnetic flowmeters. This unit contains the flow tube with the elec- trodes embedded in it, mounted in the gap of an alternating current excited electromagnet. These can be supplied with a variety of indicat- ing, integrating, or control equip- ment depending on the application.

These electromagnetic flowmeters have many of the qualities of an ideal flowmeter. The flow detector has the same inside dimensions as the flow line in which it is in&alled. To all intents and purposes, it is just another section of the flow line, since there are no projections or obstruc- tions within it. This means that pressure drop is insignificant. Cali- bration is independent of the nature of the fluid so long as the conduc- tivity is high enough. The output is a linear function of the flow rate. The sensing tube can be made of Teflon, glass, stainless steel, or plastic.

It is interesting that an idea orig- inated over a hundred years ago and initially developed by biologists for measuring the flow of blood has been made into a practical industrial instrument by the application of modern electronics and electrical engineering . Correspondence concerning this column will be forwarded promptly if addressed to the author, % Editor, INDUSTRIAL AND ENQINEERINQ CHEII- ISTRY, 1155-16th St., N.W,. Washington 6 , D. C.

vantage that the output can be amplified by means of an alternat- ing current amplifier. This makes it possibIe to attain high input im- pedance and good stability. High input impedance means that the flowmeter (Continued on page 86 A )

84 A 86 A