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  • Type of flow Meter

    Measuring the flow of liquids is a critical need in many industrial plants. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements or failure to take measurements can cause serious (or even disastrous) results.

    With most liquid flow measurement instruments, the flow rate is determined inferential by measuring the liquid's velocity or the change in kinetic en

    ergy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe's cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid's flow rate in such cases is:

    Q = V x A

    where

    Q = liquid flow through the pipe



    Turbine Flowmeter
    There is many different manufacturing design of turbine flow meters, but in general they are all based on the same simple principle:

    If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow.
    The turn down ratios may be more than 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. Accuracy may be better than +/-0,1%.



    Electromagnetic Flowmeter
    An electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube.

    The voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element.

    Electromagnetic flowmeters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy.

    Electromagnetic flowmeters have a relatively high power consumption and can only be used for electrical conductive fluids as water.



    Ultrasonic flowmeters can be divided into Doppler meters and time-of-travel (or transit) meters.

    Doppler meters measure the frequency shifts caused by liquid flow. Two transducers(one to transmit and the other to receive signal) are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element, Fig. 10. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.
    A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed. The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used. Total weight, including the case, is 22 lb. A set of 4 to 20 mA output terminals permits the unit to be connected to a strip chart recorder or other remote device.
    Because solids particles or enterained gases are required for measurement, Doppler meters are not appropriate for clean liquids. In general, Doppler flow meters are less accurate than TOF flow meters, however, they are less expensive. 

    Time-of-travel(Transit-Time) meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions.

    A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption. (Back to Meter Types Table)
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