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There are two main types of ultrasonic flowmeters – transit-time and Doppler -- but much of the research and development work on ultrasonic flowmeters  today is directed toward  transit time meters, specifically multipath transit time meters. These meters send multiple signals across a pipe at different locations to achieve higher accuracy. 

Ultrasonic flowmeter transducers apply voltage to  piezoelectric crystals to generate an ultrasonic signal. The same piezoelectric crystals can also be used to detect the presence of an ultrasonic signal. In this way, an ultrasonic flowmeter transducer can be both a sender and a receiver of signals.

A transit-time ultrasonic flowmeter has both a sender and a receiver. It sends two ultrasonic signals across a pipe at an angle – one with the flow and one against the flow. The meter then measures the “transit time” of each signal. When the ultrasonic signal travels with the flow, it travels faster than when it travels against the flow. The difference between the two transit times is proportional to flowrate. 

Some ultrasonic meters have a single path, requiring one pair of transducers, and some have dual paths, requiring two transducer pairs. An important group of ultrasonic flowmeters has three or more paths; these meters are called multipath. 

Multipath transit time flowmeters use more than one ultrasonic signal, or “path,” in calculating flowrate. Each path requires a pair of sending and receiving transducers. By using more than one path, the flowmeter measures flow at more than one location in the flowstream, leading to greater accuracy.  Multipath flowmeters have been particularly important for measuring natural gas flow, and have become many companies' meter of choice for custody transfer of natural gas.

Info on Doppler meters:

In addition, because of improvements in electronic processing technology, transit-time meters are better able to handle fluids that are not completely clean. This has enabled transit-time flowmeters to be used for applications that could previously only be handled by Doppler flowmeters. These improvements have also increased the accuracy of ultrasonic meters, which has led to broader use of these meters in a wider variety of conditions.

Learn more about New Technology Flowmeters:

Chord vs path

In flowmeter terminology, a path is defined as the route of travel between two ultrasonic transducers. Another term now in common use is the term “chord.” Mathematically speaking, a chord is a straight line within a circle whose endpoints lie on the circumference. However, the term “chord” is also used by some ultrasonic manufacturers to refer to the route of travel between two transducers. In this way, a chord is like a path.

However, a chord is considered to be the route of travel between a transducer and the pipe wall or reflector when the signal is bounced off a wall or a reflector. So in this sense, an ultrasonic signal that bounces off a wall or reflector to a receiving transducer has one path and two chords. One chord is the path of the signal from Transducer A to the pipe wall or reflector, and the second chord is the path of the signal from the pipe wall or reflector to transducer B.

In a direct path, the signal goes from one transducer to another without being reflected. In a reflected or bounced path, the signal from one transducer is reflected or bounced off a reflector or the pipe wall on the way to the second transducer. Unlike a reflected path, a direct path is considered to have only one chord.

Value of bounced or reflected paths

Manufacturers agree about the definitions of path and chord: 

  • A path requires two transducers, and it represents the path of an ultrasonic signal traveling back and forth between the two transducers. 

  • A chord represents the path of an ultrasonic signal going between two transducers.

  • If the path is direct, then the transducers have one path and one chord going between them. 

  • If the path is reflected, or bounced, then there is one path but two or more chords between the two transducers, depending on the number of reflections or bounces.

However, some have different views about the value of bounced or reflected paths. 

Some suppliers believe that the added length of the chords in bounced or reflected paths provides additional diagnostic information and enhanced performance -- and also can result in greater measurement accuracy and repeatability.

Others maintain that suppliers who emphasize the number of chords over the number of paths are engaged in “marketingspeak” and that end-users may interpret the number of chords to be the same as the number of paths.

For further information, please see www.FlowUltrasonic.com and www.UltrasonicFlows.com.

Flow Research, Inc. | 27 Water Street | Wakefield, MA 01880 | (781) 245-3200 | (781) 224-7552 (fax) | (800) 245-1799 (from the USA) | info@flowresearch.com

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