The continuous flowmeter
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.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.Making tappings and installing insertion probes pose a risk to water quality. Although such risks can be managed, ultrasonic strap-on mass flowmetter are being used increasingly as an alternative and avoid the tedious exercise of velocity profiling. Versions of these meters can be installed on all sizes and materials of pipe used in distribution systems.Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a density meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the Dall tube,47 which is a modification of the venturi, have even been standard instruments. Several other solutions based on plates, rings, or wing-type obstructions are commercially available. This wide variety of devices is not covered here. For further information, the reader should contact the manufacturers of such instruments.