The Ultrasonic flowmeters
Ultrasonic flowmeters are instruments for pipe flow measurement in process control, flow survey, custody transfer etc. They have many advantages over conventional flowmeters. Today, they can be handheld or clamp on, allowing greater ability to check flows in different areas of pipe and they don't have to be cut into the pipe. This keeps the meters from being contaminated. They can not only be used for flow survey, but in HVAC and many other applications where mobility is a concern. They come in two basic types depending on the application.
A transit-time flowmeter is based on the time difference between upstream and downstream sound propagation intervals. It works accurately on clean water and homogeneous liquids such as sea water, process liquids, oil, and chemicals with no air bubbles or contaminates.Doppler ultrasound flowmeters are volumetric and require particles or bubbles in the flow. They will not work in distilled water or drinking water. Aerations would be required in clean liquids. These flowmeters are ideal for applications where low pressure drop, chemical compatibility, and low maintenance are required. Basically, they employ the frequency shift of an ultrasonic signal after being reflected off suspended particles or gas bubbles in motion. See Doppler affect.
If one is considering this type of flowmeter, you need to to find out some basic facts. Does the liquid being measured have particles 100ppm of microns in size? Do you need a handheld or continuous monitoring? Is the pipe always full? Ultrasound does not work well through air. The amount of flow, temperature and pressure are also needed. The size of the pipe is another important consideration. Will you need an analog output? If the pipe wall is made of stainless steel, it might conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading. An ultrasonic flowmeter can also have problems with copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes. They have built in acoustic discontinuities that scatter the transmitted signal enough to render the flowmeter inaccurate.
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