Instrumentation The magnetic induction flowmeter, an old but comparatively unknown device, deserves t o be better known bg M a l p h W. Munch HE measurement of flow is a perennial problem in spite of the fact that many different types of instruments are available for the purpose. Fixed-area orifice type flowmeters 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. Another type widely used is the variable-area flowmeter or rotameter. For some applications, displacement type meters using pistons, nutating
T
Figure 1. Schematic Diagram of 'Induction-Type Flowmeter
disks, or bellows coupled to appropriate counting mechanisms are the most satisfactory. Inferential meters in which some form of propeller 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 replaces. Its calibration would be independent of the fluid characteristics such as viscosity, specific gravity, suspended solids, and elecJanuary 1952
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 discovered that a current is induced in a conductor moving through a magnetic field. Everyone is familiar with the fact that this discovery is the fundamental principle on which modern electrical generators depend; few people realize that it can also be the basis for a fluid flowmeter. The principle is analogous to that of the use of a small generator as a tachometer. In the tachometer, the generated voltage
Figure 2.
is
proportional
to the speed of rotation; in the flowmeter, it is proportional to the linear velocity of the fluid passing through the meter. Figure 1 shows how the principle can be used to measure the linear velocity 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 nonmagnetic and must not be an electrical conductor. Electrodes D and E are embedded in the walls of the tube diametrically opposite each other and connected t o a suitable device to detect the voltage generated. Faraday's law of induction can be expressed as E
=
BlV X
lo-*
(Continued on page 84 A )
Primary Element of Magnaflow Electromagnetic Flowmeter
INDUSTRIAL AND ENGINEERING CHEMISTRY-
83 A
Instrumentation Here E is the generated voltage in
can be used to measure the flow volts, B is the flux density in gauss, of liquids having lower conductivity. I is the inside diameter of the tube It seems practical to meter liquids in centimeters, and V is the fluid having conductivity equal to or velocity in centimeters per second. higher than the average tap water B, 1, and V are mutually perpen- at present. dicular. With a nonvarying magThe Mittelmann Electronics Divinetic field, the output is a direct sion of the Century America Corp., current voltage. The early work 549 West Washington Blvd., Chiwith this type flowmeter used a noncago 6, Ill., is now offering magneticvarying magnetic field. One very induction type flowmeters for sale. serious difficulty was experienced Figure 2 is a photograph of the priwith the method. The flow of direct mary element of one of their eleccurrent through the electrodes pro- tromagnetic flowmeters. This unit duced polarization. To avoid this, contains the flow tube with the elecnonpolarizable electrodes separated trodes embedded in it, mounted in from the flowing solution by suit- the gap of an alternating current able liquid junctions were used. I t excited electromagnet. These can was also necessary that the fluid be supplied with a variety of indicathave a rather high electrical conduc- ing, integrating, or control equiptivity. ment depending on the application. TQ avoid polarization effects at These electromagnetic flowmeters the electrodes, the use of an alter- have many of the qualities of an nating magnetic field was intro- ideal flowmeter. The flow detector duced. The induced voltage pro- has the same inside dimensions as duced with this arrangement is an the flow line in which it is in&alled. alternating current voltage, so that, To all intents and purposes, it is just if the frequency is high enough, another section of the flow line, since polarization is eliminated. The there are no projections or obstrucuse of the alternating magnetic field tions within it. This means that does, however, introduce another pressure drop is insignificant. Calidifficulty. This is an interference bration is independent of the nature voltage induced in the electrode loop, of the fluid so long as the conducD-E,acting as a single turn trans- tivity is high enough. The output former secondary, by the alternat- is a linear function of the flow rate. ing magnetic field. This voltage is The sensing tube can be made of i n d e p d e n t of flow rate and must Teflon, glass, stainless steel, or be balanced out if the voltage out- plastic. put of the flowmeter is to be zero at It is interesting that an idea origzero flow rate. This effect is lawer inated over a hundred years ago at low frequencies than a t higher and initially developed by biologists frequencies, Frequencies from 10 for measuring the flow of blood has to 5000 cycles have been used by been made into a practical industrial various workers, although 60-cycle instrument by the application of excitation is most common. The modern electronics and electrical higher frequencies are desirable only engineering. when high resolving power js needed. Correspondence concerning this column will be In addition to eliminating polari- forwarded promptly if addressed to the author, % Editor, INDUSTRIAL A N D E N Q I N E E R I N Q CHEIIzation troubles, the use of alternat- ISTRY, 1155-16th St., N.W,. Washington 6 , D. C. ing current excitation has the advantage that the output can be amplified by means of an alternating current amplifier. This makes it possibIe to attain high input impedance and good stability. High input impedance means that the flowmeter (Continued on page 86 A ) 84 A
86 A
