E-Zine September 2010
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Straight run requirements for ultrasonic flowmeters are not excessive, so this technology can be applied where limited straight run is available. Applications where the velocity profile changes with the flow rate can cause erratic operation of some ultrasonic flowmeters. Some ultrasonic flowmeters have Reynolds number constraints, so they are generally not applied in applications where the liquid exhibits a relatively high viscosity. Other designs correct for Reynolds number, however their performance is usually degraded in the transitional flow regime. Ultrasonic flowmeters generally exhibit their best performance in the laminar and turbulent flow regimes.
Clamp-on or insertion ultrasonic flowmeter sensors are often installed to measure the flow through large pipes to avoid the expense of a (large) full-bore spool piece. The ultrasonic sensors for most pipe sizes are similar, so flowmeter cost is almost independent of pipe size. These attributes of ultrasonic flowmeters often make this technology less expensive and more convenient to install.
Ultrasonic flowmeters measure liquid velocity, from which the volumetric flow rate is inferred. The measurement is linear with liquid velocity and exhibits a relatively large turndown. In addition, the range of flow measurement is relatively large.
Ultrasonic flowmeters can be applied to gas flows, particularly stack gas, flare gas, and natural gas. Some ultrasonic flowmeters can measure the flow of liquids in partially filled pipes.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters