E-Zine May 2008
The flowmeters to be discussed use ultrasonic energy or correlation as their primary flow measurement technique. Therefore, a common class of "ultrasonic flowmeters" comprised of an open channel flowmeter (such as a flume or weir) that incorporates an ultrasonic level measurement is specifically excluded from this discussion.
Ultrasonic flowmeters use sensors to generate ultrasonic waves and direct them into the flowing stream. Information from the remnants of these sound waves is used to determine the flow of fluid passing through the flowmeter. Ultrasonic flowmeters have no moving parts.
Focusing ultrasonic energy into the flowing fluid and detection of its remnants is predicated upon a complete ultrasonic circuit. The ultrasonic circuit can consist of the transmitting device, entry pipe wall, entry pipe liner, fluid, reflections off pipe walls, exit pipe liner, exit pipe wall, and receiving device. For the flowmeter to function properly, all parts of the ultrasonic circuit and their interfaces must allow the passage of sufficient ultrasonic energy. If a part of the circuit attenuates ultrasonic energy excessively, the ultrasonic signal at the receiving device may weaken. A weak received signal may cause the flowmeter to be erratic or cease to function.
For example, an ultrasonic circuit could be compromised by paint on the outside of the pipe, drying of the ultrasonic coupling compound, pipe material, coating or corrosion on the inside of the pipe, and a poorly bonded pipe liner. Tuberculation, or the growth of barnacles on the inside pipe wall, can also compromise the ultrasonic circuit.
Most designs utilize Doppler, transit time (time-of-flight), pulse repetition (sing-around), or phase shift sensing techniques. While dependent upon signal processing techniques, many designs are capable of measuring flow in both the forward and reverse directions.
It should be noted that some designs are dependent on the speed of sound in the fluid. Therefore, changing composition, temperature, and/or (gas) pressure can change the speed of sound in the fluid, and hence affect flowmeter performance. Some ultrasonic flowmeters measure the speed of sound of the fluid and correct for this effect. Other designs avoid this issue by using flow equations in which the speed of sound is not needed.
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Excerpted from Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters