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Flow meters are classed as volumetric or inferential, the latter term referring to meters that determine velocity from other variables such as pressure differences across a device such as an orifice plate. There is a large variety of flow measurement device, using numerous physical principles. Full discussion of the whole range of flow measurement device is out of the scope of this book but the reader will find a comprehensive reference in the Flow Measurement Handbook (Baker, 2000). Table 18.4 gives typical information on some of the flow meters usually encountered in the water industry.
Mass magnetic flowmeter such as the Coriolis meter provide a more sophisticated metering device. Sometimes configured in a distinctive U-tube shape, an internal tube is set oscillating using an electric current supplied to coils at either end of the tube. The flow of liquid through the tube sets up a twisting force on the inner tube due to the naturally occurring Coriolis Effect. Sensors fitted along the length of the tube detect and measure the twisting force, which is a function of the mass flow rate; the processed data provides production and fluid density data.
The principles of orifice and venturi meters are discussed in Section 14.16. Two other kinds of inferential (or momentum) meter are the Dall tube and the V cone venturi. In both, flow accelerates through a constriction and leads to a pressure drop. The pressure difference is measured in the Dall tube and the V cone venturi as an indicator of velocity (and so flow) in the same way as for an orifice. The V cone venturi design is claimed to have a turn-down ratio of 25:1 and to be less affected by conditions upstream and downstream and can be fitted into shorter lengths of straight pipe than is recommended for other meter types. Further types of momentum meter are indicated in Table 18.5.
An ultrasonic flow meter as shown in Fig. 16.11 measures the velocity of a fluid to calculate volume flow. The vortex flowmeter can measure the average velocity along the path of an emitted beam of ultrasound by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates. They are often inexpensive to use and maintain because they do not use moving parts, unlike mechanical flow meters.
Insertion probe flow meters are installed for temporary measurement of flow for consumption surveys or for distribution networks analyses. These instruments are either the turbine or electromagnetic (EM) type, the latter becoming more common. Both are inserted into the pipe where flow measurement is required. The turbine type uses a small rotating vane at the end of a probe to record flow velocity. The vane is susceptible to damage, in which case the instrument has to be returned to the manufacturer for repair and recalibration. The turbine meter is inserted through a 40 mm diameter tapping in the pipe which has to be of at least 200 mm diameter. The EM probe (Plate 30(c)) uses an electromagnet at its end to apply a magnetic field to the water. Electrodes either side of the probe pick up the induced EMF in the water which is proportional to the velocity past the electrodes. The tapping for an EM insertion probe is 20 mm diameter and can usually be installed in pipes of diameter 150 mm and greater. EM probes are made up to 1 m long; therefore, they cannot be used for pipes of diameter greater than 900 mm and are restricted to flow with velocity less than about 1.75 to 2.0 m/s due to the flexibility of the probe.
