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EA EBULLIOSCOPIC CONSTANT ECCENTRICALLY ROTATING POROUS DISK ECKERT ENTHALPY ECKERT NUMBER ECKERT, ERG ECONOMIC PENALTIES OF FOULING EDDIES EDDIES IN TURBULENT FLOW EDDY CORRELATION METHOD OF SURFACE HEAT TRANSFER EDDY VISCOSITY EDF EDGE INSTABILITY EFFECTIVE DIFFUSIVITY METHOD EFFECTIVE THERMAL CONDUCTIVITY OF POWDERS EFFECTIVE THERMAL CONDUCTIVITY OF UNSATURATED POROUS MEDIA EFFECTIVENESS - NTU METHOD EFFECTIVENESS OF HEAT EXCHANGER EFFECTIVENESS OF POROSITY ON STAGNATION POINT FLOW EFFECTS OF CHEMICAL REACTIONS ON NONLINEAR MAGNETOHYDRODYNAMIC BOUNDARY LAYER FLOW EFFERVESCENT ATOMIZATION EFFERVESCENT ATOMIZER EFFERVESCENT SPRAYS EFFICIENCY AND STEAM TURBINES EFFICIENCY OF CYCLES EFFICIENCY OF HEAT EXCHANGERS EFFICIENCY OF POWER CYCLES EFFICIENCY OF PROCESSES EFFICIENCY, IN TURBINES EFFICIENT WAVY FIN SURFACE EFFLUENT TREATMENT EFFLUX EIGENFUNCTIONS EIGENVALUES EINSTEIN EQUATION FOR MIXTURE VELOCITY EKMAN-LAYERS ELASTIC WAVES AT POROUS/POROUS ELASTIC HALF-SPACES ELASTICOVISCOUS FLUIDS ELBOW FLOW METERS ELECTRET ELECTRIC (JOULE) HEATERS ELECTRIC ARC ELECTRIC ARC HEATER ELECTRIC CONTACT METHOD, FOR FILM THICKNESS MEASUREMENT ELECTRIC FURNACES ELECTRIC POWER RESEARCH INSTITUTE, EPRI ELECTRICAL COALESCERS ELECTRICAL CONDUCTIVITY ELECTRICAL CONDUCTIVITY OF BUBBLY MIXTURES ELECTRICAL CONTACT METHOD ELECTRICAL POWER GENERATION FROM GEOTHERMAL ENERGY ELECTRICAL RESISTANCE STRAIN GAUGES ELECTRICAL RESISTIVITY OF PARTICLES ELECTRICAL SEPARATION ELECTRICALLY CHARGED PARTICLES ELECTRICALLY CONDUCTING VISCOELASTIC FLUID ELECTRICALLY DRIVEN SHOCK TUBES Electricite de France, EDF ELECTRICITY ASSOCIATION, EA ELECTROCATALYSTS ELECTROCHEMICAL CELLS ELECTROCHEMICAL METHODS ELECTROCHEMICAL THEORY OF CORROSION ELECTROCHEMISTRY ELECTRODE ELECTRODEPOSITION ELECTRODIALYSIS ELECTRODIFFUSION METHOD ELECTRODYNAMIC MODEL IN PLASMA PHYSICS ELECTROFLOTATION, EF ELECTROHYDRODYNAMIC AUGMENTATION ELECTROLYSIS ELECTROLYTE ELECTROLYTE FLOW MEASUREMENT ELECTROLYTE SOLUTION, DIFFUSION IN ELECTROLYTE SOLUTIONS, ADSORPTION FROM ELECTROLYTIC CELL ELECTROLYTIC REACTIONS ELECTROMAGNETIC FLOWMETERS ELECTROMAGNETIC HYBRID MODELS ELECTROMAGNETIC RADIATION ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES, ABSORPTION AND SCATTERING ELECTROMAGNETISM ELECTRON ENERGY LEVELS ELECTRON GAS ELECTRON SPIN RESONANCE SPECTROSCOPY ELECTRON VOLT ELECTRONIC SYSTEMS ELECTRONIC THEORIES, FOR CATALYSIS ELECTRONS ELECTROOSMOSIS ELECTROPHORETIC FORCES ELECTROPLATING ELECTROSPRAYS ELECTROSTATIC ATOMIZERS ELECTROSTATIC CHARGE ELECTROSTATIC EFFECTS ELECTROSTATIC EXTRACTION ELECTROSTATIC FIELDS ELECTROSTATIC PRECIPITATION ELECTROSTATIC PRECIPITATORS ELECTROSTATIC SEPARATION Elementary processes Elementary processes in weakly ionized gases ELLIPSOIDS ELLIPTIC DIFFERENTIAL EQUATIONS ELLIPTIC EQUATIONS ELUTION ELUTION CHROMATOGRAPHY ELUTRIATION EMERGENCY CORE COOLING SYSTEM, ECCS EMISSIONS EMISSIVE POWER Emissivity EMISSIVITY MEASUREMENTS OF POWDERS EMISSIVITY OF PLANTS AND ANIMALS EMISSIVITY OF TWO-PHASE COMBUSTION PRODUCTS IN A SOLID-PROPELLANT ROCKET ENGINE EMULSIFYING AGENT Emulsions ENDOTHELIAL SURFACE ENDOTHERMIC REACTIONS ENEA ENEL ENERGY ACCOMMODATION COEFFICIENT ENERGY BANDS ENERGY CONSERVATION ENERGY EFFICIENCY ENERGY EFFICIENCY BEST PRACTICE PROGRAMME ENERGY FRACTURE CRITERION ENERGY SPECTRUM OF TURBULENCE ENERGY STORAGE ENERGY SUPPLY ENERGY TECHNOLOGY SUPPORT UNIT, ETSU ENERGY, RENEWABLE ENGINEERING SCIENCES DATA UNIT, ESDU ENHANCED AIR COOLERS ENHANCED OIL RECOVERY ENHANCEMENT OF FILM CONDENSATION HEAT TRANSFER ENHANCEMENT OF HEAT TRANSFER ENHANCEMENT OF MASS TRANSFER ENIAC Enlargement, Flow and Pressure Change in ENSEMBLE AVERAGES ENTE NAZIONALE PER I'ENERGIA ELETTRICA, ENEL ENTHALPY ENTHALPY METHOD ENTHALPY OF VAPORIZATION ENTHALPY, EFFECTIVE FOR SURFACE DESTRUCTION ENTRAINMENT OF DROPLETS ENTRAINMENT OF DROPS, IN ANNULAR FLOW ENTRANCE LENGTH EFFECTS ENTRANCE REGION HEAT TRANSFER, IN TUBES Entrance region. Entry region ENTRAPMENT PUMPS ENTROPY ENTROPY GENERATION ENTROPY OF VAPORIZATION ENVIRONMENTAL CONCERNS ENVIRONMENTAL HEAT TRANSFER ENVIRONMENTAL POLLUTION ENZYMATIC REACTION KINETICS EPRI EQUATION OF MOTION EQUATION OF STATE EQUILIBRIUM EQUILIBRIUM STATE EQUILIBRIUM STATES, THERMODYNAMIC EQUILIBRIUM TEMPERATURE EQUILIBRIUM VAPOR PRESSURE, CHANGE WITH TEMPERATURE ERGODIC PROCESSES ERGODICITY EROSION ERROR FUNCTION ESA ESDU Estimate of P1 error for optically inhomogeneous media ETHANE ETHANOL ETHANOLAMINES ETHENE, SEPARATION OF ETHYLENE ETHYLENE GLYCOL ETSU EUCLIDEAN SPACE EULER CORRELATION EULER EFFICIENCY EULER EQUATION EULER FORMULA EULER NUMBER EULERIAN APPROACH EULERIAN BALANCES EULERIAN DESCRIPTION EULERIAN DESCRIPTION OF MOTION EULERIAN INTEGRAL SCALES EULERIAN SPECIFICATION EURATOM EUROPEAN ATOMIC ENERGY AGENCY, EURATOM EUROPEAN SPACE AGENCY, ESA EVAPORATING SPRAY EVAPORATION EVAPORATION COEFFICIENT EVAPORATION ENHANCEMENT EVAPORATION FROM EARTH'S SURFACE EVAPORATION OF DROPLETS EVAPORATION OF DROPS EVAPORATIVE COOLING EVAPORATORS EVENT TREES Examples of CO2 and H2O EXERGY EXHAUST EMISSION LEVELS EXOSPHERE EXOTHERMIC REACTIONS EXPANSION BELLOWS EXPANSION JOINTS EXPANSION, FLOW THROUGH AND PRESSURE DROP EXPERIMENTAL CHARACTERIZATION OF AN ANODE MATERIAL - HEAT FLUX AND TEMPERATURE FIELD Experimental investigations of turbulence-radiation interaction (TRI) Experimental Methods in Fluid Mechanics Experimental study and theoretical modeling of spectral radiative properties of dispersed materials Experimental techniques EXPLOSION PHENOMENA EXPONENTIAL DECAYING PRESSURE GRADIENT EXPONENTIAL FUNCTION EXPONENTIAL SUMS EXTENDED SURFACE HEAT TRANSFER EXTENDED SURFACES EXTENSIONAL FLOW EXTENSIONAL VISCOSITY External Flows: Overview EXTERNAL JACKET EXTINCTION, PHOTO EXTRA STRESS EXTRACT PHASE EXTRACTION, LIQUID-LIQUID EXTRACTORS EXTRUDATE SWELL EXTRUSION PLASTICS LAGRANGE'S INTERPOLATION FORMULA
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ELECTROMAGNETIC FLOWMETERS

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The electromagnetic flowmeter is one of the most successful nonintrusive types of flowmeter. The basic concept that an emf is induced in a conducting liquid moving through a magnetic field, in accordance with Faraday’s Law of electromagnetic induction, has been known since the early part of the last century. However, it was not until the 1950s that industrial applications became a reality. Some of the earliest applications were concerned with measuring the flow rate of blood, Kolin (1960).

The primary device consists of an electrically-insulating metering tube, typically within a nonmagnetic stainless-steel tube. The choice of liner material depends mainly on the liquid, which can range from slurries or molasses to chemically-aggressive mixtures or molten metals. Conductivity thresholds as low as 5 micro-Siemen/cm are common, with some manufacturers claiming considerably lower values. When measuring the flow rate of an electrolyte, a further problem is that because of the relatively low ion mobility, conductivity is likely to be affected by the flow, making it a function of velocity.

One or more pairs of electrodes, diametrically-opposed, are set into the wall such that the diameter on which they lie, the magnetic field and the flow direction are mutually orthogonal. If the typical electrodes, e.g., dome-headed screws, are replaced by relatively large plates, the larger measuring areas can sometimes reduce the effect of nonstandard flow patterns on the meter. Such electrodes, however, are more prone to fouling, introducing additional errors. The secondary device processes the signal from the electrodes, typically millivolts. It provides outputs which can be in volts, milliamps or pulses per second. Usually, a reference signal from the primary device—which is proportional to the magnetic flux—is compared in the secondary device with the flow signal.

The use of direct current as a means of exciting the magnet has been quickly discarded because of the difficulty of electrochemical and polarization emfs appearing at the electrodes, and for many years excitation by alternating current was the norm. However, a ‘transformer’ effect can exist since the conducting liquid, the electrodes and leads effectively form a single-turn loop, giving rise to a quadrature signal which can be of the same order of magnitude as the metering signal. It should be possible to remove the quadrature signal but complications arise when, for example, liquid conductivity is not uniform or earthing difficulties exist. Since the mid-1970s most manufacturers have used square or trapezoidal-wave excitation, operating usually at a few Hertz and minimizing most of the above difficulties. An additional benefit is that the zero is continually reestablished, avoiding the earlier problems associated with zero drift [Baker (1982)]. The impedance of the liquid path should ideally be at least two orders of magnitude less than that of the input of the secondary device. The situation has improved since the days of the voltameter thermionic valve and germanium transistor. Modern electronic components make it possible for input impedance of the secondary device to be several orders of magnitude greater than that of its predecessors. One of the most recent developments is to position the electrodes outside a dielectric liner, the system operating by capacitive coupling, with no contacts within the metering tube.

Theoretically, a uniform magnetic field can only be achieved by using an infinite magnet. This means that, for practical purposes, certain regions of the cross-section containing the electrodes will produce larger signals than others at the same velocity. Some of the earliest recorded experimental work was that by Williams (1930). He realized that since the velocity profile of a flowing liquid was rarely flat, the emf existing across the pipe diameter deriving from the central velocity would be effectively shunted by those from the slower moving flows near the pipe wall. This would give rise to circulating currents, and the measured signal would be lessened by an Ohmic drop. As it turns out, as Shercliff (1962) explains, because the induced currents flow only in the plane normal to the meter axis, the above effect is self-cancelling if the velocity distribution is axisymmetric. Shercliff (1962) has developed the very important ‘weight-function’ map, Figure 1, which assisted in the prediction of the effects of asymmetry, especially relevant in short-pattern meters. Hartmann (1937) has pointed out that a similar ‘shorting’ takes place at the ends of the necessarily finite meter, where the magnetic intensity falls to zero. This effect has been minimized by the designs of the pole pieces.

Electromagnetic flowmeter.

Figure 1. Electromagnetic flowmeter.

Installation requirements are given in ISO 6817 (1992) and BS 5792 Part 1 (1993) Standards. The meter should be installed in a straight pipe at a distance of at least 10 times the nominal diameter (10 DN) from any upstream disturbance and 5 DN downstream. The flow must be swirl-free and the meter should not be larger or more than 3% smaller than the connecting pipework. To prevent the formation of gas bubbles on the electrodes, these should normally be installed horizontally. In ideal conditions, such meters should be capable of measuring flow rate with an uncertainty of 0.2%.

REFERENCES

Baker, R. C. (1982) Electromagnetic flowmeters. in: Scott, R. W. W. (Ed). London and New Jersey; Applied Science Publishers Ltd.

British Standards Institution (1993) Measurement of conductive liquid flow in closed conduits - Method Using Electromagnetic Flowmeters. BS 5792, Part 1, London.

Hartmann, J. (1937) Hg-dynamics 1, Math-Fys. Medd. 15. No. 6. (Royal Danish Academy of Science and Letters).

International Organization for Standardization (1992) Measurement of conductive liquid flow in closed conduits - method using electromagnetic flowmeters. ISO 6817.

Kolin, A. (1960) Circulatory system; methods; Blood flow determination by the electromagnetic method. in: Glasser, O. (Ed). Medical Physics, 3: 141. Chicago I11: Year Book Medical Publishers.

Shercliff, J. A. (1962) The Theory of Electromagnetic Flow Measurement. Cambridge University Press.

Williams, E. J. (1930) The Induction of emfs in a moving fluid by a magnetic field and its application to an investigation of the flow of liquids. Proc. Phys. Soc., London, 42: 466.

References

  1. Baker, R. C. (1982) Electromagnetic flowmeters. in: Scott, R. W. W. (Ed). London and New Jersey; Applied Science Publishers Ltd.
  2. British Standards Institution (1993) Measurement of conductive liquid flow in closed conduits - Method Using Electromagnetic Flowmeters. BS 5792, Part 1, London.
  3. Hartmann, J. (1937) Hg-dynamics 1, Math-Fys. Medd. 15. No. 6. (Royal Danish Academy of Science and Letters).
  4. International Organization for Standardization (1992) Measurement of conductive liquid flow in closed conduits - method using electromagnetic flowmeters. ISO 6817.
  5. Kolin, A. (1960) Circulatory system; methods; Blood flow determination by the electromagnetic method. in: Glasser, O. (Ed). Medical Physics, 3: 141. Chicago I11: Year Book Medical Publishers.
  6. Shercliff, J. A. (1962) The Theory of Electromagnetic Flow Measurement. Cambridge University Press. DOI: 10.1017/S0022112063210902
  7. Williams, E. J. (1930) The Induction of emfs in a moving fluid by a magnetic field and its application to an investigation of the flow of liquids. Proc. Phys. Soc., London, 42: 466.

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