Area-velocity flowmeters make it possible to reliably measure flow in any part-filled pipes or in open channels without weirs/flumes.
Industry is often faced with the problem of measuring flow in an open channel. The conventional solution is a weir or flume, but these are often expensive, time-consuming to build, and disrupt plant operations. An ultrasonic area-velocity flowmeter is a suitable solution. This consists of a streamlined sensor mounted near the bottom of the channel which measures flow velocity and liquid level. These measurements, combined with the description of the geometry of the channel, give volumetric flow rate. The same procedure applies equally in part-filled pipes.
Flow velocity is measured by a probe which transmits ultrasound into the liquid to create a 'zone' of inspection. Bubbles and solids carried through this zone by the liquid flow reflect ultrasound back to the probe. Sophisticated electronics analyse the signals and determine the flow velocity. Typically, velocities from 0,5 to 5 m/s can be measured with a resolution of 1 mm/s.
The liquid level is measured using a miniature pressure transmitter connected to the sensor (or any other level measurement available). The flowmeter contains predefined profiles, eg circular, elliptical or rectangular. In addition, the user can capture the channel cross-section through data entered or diagrammatically. The electronics calculates the cross-sectional area of the liquid from the liquid level measurement and the section profile stored. Volumetric flow rate is calculated using the instantaneous velocity and cross-sectional area.
The system displays flow velocity, flow rate and totalised flow. Analog outputs transmit flow information and opto-isolated switches can be used as alarms or to transmit flow information. The integral datalogger stores all measured and calculated information for downloading onto a PC for graphing or analysis. Reliable and quick to install, these ultrasonic area-velocity flowmeters have found wide application internationally, typically for effluent, wastewater, irrigation channels and flow-balance applications.
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