The Basics of Ultrasonic Meters

Ultrasonic meters offer water utilities high accuracy and long-term reliability in an expanding range of applications.
The Basics of Ultrasonic Meters
Ultrasonic meters measure flow using sound waves at frequencies above the range of human hearing. The sound waves are generated by applying a small voltage to a piezoelectric transducer in the meter.

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Today’s business environment demands informed decision-making. It’s essential to provide superior customer service, use water and energy wisely, manage operations efficiently and sustain regulatory compliance, all while continuing to control revenue and resources.

When it comes to water, now more than ever, every drop counts. Ultrasonic flowmeters offer the newest evolution in metering technology for potable, reclaimed and residential fire service applications, truly making water visible to all utilities and end water users.

Today’s technological advancements make the use of ultrasonic technology feasible for small metering applications. In particular, improved high-speed digital signal processing, advanced piezoceramic transducers and sophisticated computer technology make ultrasonic flowmeters a viable, cost-effective option.

Understanding the technology

An ultrasonic flowmeter is an inferential meter that uses ultrasonic technology to measure the velocity of an acoustically conductive liquid or gas moving through it. There are two types of ultrasonic flowmeter technologies: Doppler and transit-time.

Doppler flowmeters

Doppler ultrasonic flowmeters measure the flow of liquids that contain suspended particles (“reflecting particles”). To ensure accurate measurement, particles of a sufficient number and size must be in the medium continuously. Therefore, these meters are normally used with dirty liquids in industrial applications.

Typically, the meters position a pair of transducers opposite each other across the pipe, but the transducers also can be mounted on the same side of the pipe. One transducer sends an acoustic signal or sound wave, which reflects off the particles in the flow stream. As the signal passes through the stream, its frequency shifts in proportion to the mean velocity of the fluid. The other transducer receives the reflected signal and measures its frequency. The meter then calculates flow by comparing the generated and received frequencies.

Transit-time flowmeters

Initially used only for clean liquids, transit-time meters can now accurately measure flow in a wide range of fluids, thanks to dramatic improvements in signal processing technology. Transit-time meters are now among the most universally applied flowmetering methods, and among the most accurate.

Transit-time technology takes advantage of the principle that an acoustic signal travels faster with the flow than against the flow. These meters have a pair of transducers that act as both sender and receiver. The transducer on the upstream side of the meter generates an acoustic signal that travels through the pipe. The downstream transducer receives the signal, and the meter calculates how long the signal took to travel from one transducer to the other.

Next, the downstream transducer generates a signal that travels upstream and is received by the upstream transducer. The meter then calculates how long the signal took to travel in the upstream direction.

Both Doppler and transit-time meters may be designed as insertion meters, with wetted transducers located within the flow tube, or as clamp-on meters, with the transducers mounted on the outside surface of the pipe.

Ultrasonic signals and transducers

Ultrasonic vibrations are sound waves at frequencies above the range of human hearing. In ultrasonic flowmeters, these sound waves are generated by applying a small voltage to the piezoelectric transducer in the meter. The opposite also occurs: An ultrasonic signal received by the transducer is converted to an electrical signal. The transducer is simply a device that converts one type of energy to another.

The ultrasonic signal propagates at the speed of sound, but that speed is variable: It depends on the medium through which the sound travels and the temperature of that medium.

Therefore, ultrasonic meters are calibrated for a specific liquid or gas and for a specific temperature range. Ultrasonic meters that are compatible with a variety of liquids or gases must be adjusted if the medium is changed.

Ultrasonic vs. mechanical

Unlike mechanical water meters, ultrasonic meters use solid-state measurement technology. With no moving parts, ultrasonic meters operate quietly and improve accuracy and long-term reliability. Typical accuracy is within plus or minus 1.5 percent over the normal operating range of the meter and within plus or minus 3 percent at extended low flows.

In the past, water utilities have used ultrasonic meters primarily for large-volume applications including raw water, water treatment and water custody transfer. Now, however, ultrasonic meters have gained acceptance in residential metering because they have several advantages over traditional measurement technologies. These include:

  • Wide measuring range or turndown
  • High accuracy sustained over the meter’s life
  • High repeatability
  • Minimal intrusion in the flow stream
  • Negligible pressure drop
  • Immunity to influence from particles in the stream
  • Bidirectional design
  • Minimal maintenance requirements

In general, these same advantages apply to small ultrasonic water meters when compared to positive displacement meters.

Case in point

The experiences of two water companies illustrate advantages that can lead utilities to choose ultrasonic metering.

Tri County Regional Water

Tri County Regional Water, based in Russellville, Arkansas, had failing meters that were not accurately capturing all of the usage data and were causing loss of revenue. After considering various replacement meters, the utility selected ultrasonic meters for their reduced height, durability and accuracy.

“The size of these meters was a key factor,” says John Choate, regional water manager. “They can be mounted upside down, vertical or horizontal and still be read correctly. I also really liked the stainless steel version and the simplicity of a one-piece electronic meter with no moving parts. Stainless steel gives me peace of mind knowing that these meters will have a long life. They are also in compliance with the latest BPA guidelines.”

In an area with a broad range of elevations, the ability to withstand pressure changes was another critical factor in meter selection. “We are not losing nearly as much water,” says Choate. “With 100 to 125 psi on the line, our previous meters would implode. Our new meters can withstand the maximum stated operating pressure of 175 psi, and in fact we found they will withstand 300 psi and more. Similarly, our former meters only read 20 gpm accurately, whereas the ultrasonic ones will read 25 gpm and more.”

Waterworks District 3

To replace its aging metering system, Waterworks District 3 of Beauregard Parish, Louisiana, chose ultrasonic meters for durability and long-term accuracy. With no moving parts, the meters also improve reliability and have greater extended-low-flow accuracy compared to positive displacement meters. “The ultrasonic meters are catching the lowest flow amounts we’ve ever been able to record,” says Jeremy Joffrion, distribution manager.

Along with capturing more revenue, the meters enable the district to gather and analyze more granular measurement data. “It helps me generate better reports,” says Joffrion. “The numbers are much more accurate than in the past and will continue to get better as we install more meters.” The ultrasonic meters will also help the district record flows at new-meter accuracies throughout the meters’ life. Joffrion estimates that with labor efficiencies and added revenue from more accurate metering, the system will pay for itself quickly.

Wide application range

While mechanical meters are predominant in small water meter applications, recent technological advancements are helping ultrasonic flowmeters make inroads. Specifically, transit-time ultrasonic meters can be an effective solution for any application requiring long-term measurement accuracy.

Historically used for clean, potable water, ultrasonic meters are also well-suited for reclaimed water applications or for less-than-optimum water-quality conditions where small particulates exist. The meters’ wide-range capability and robust operating characteristics make them an attractive choice for utilities’ small-metering needs.

About the author

Jan Boyer ( is the marketing manager with Badger Meter, focusing on utility metering and encoder technologies.


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