All About Ultrasonic Flow Meters

Have you ever noticed that when a plane passes overhead, the frequency of the sound as it approaches is different than the frequency once it passes you? The hum it produces seems to change, even though the actual sound produced by the plane has not. The same can be experienced in listening to a car approach and then pass. The sound the car makes has not changed, but what you seem to be hearing does. What is happening that creates this effect? The principle known as “transit time”. This same principle is employed by ultrasonic flow meters to measure the rate of flow.

How do ultrasonic flow meters utilize that phenomenon? They operate by the principle of run-time difference. Ultrasonic waves in the media are influenced by the rate of flow. Two sensors are mounted opposite one another in the pipeline for inline models, or on the pipeline for clamp-on models, which function simultaneously as transmitter and receiver of ultrasonic acoustical signals. If the media isn’t moving, the run-times of both signals are identical. If the media is moving, the run-time of the signal against the flow will be longer than the signal with the flow. This run-time difference is proportional to the flow rate and is processed through a microprocessor to produce a measurement reading.

The modern ultrasonic flow meters that we know today are the evolutionary culmination of the ingenuity of many minds over the last 60 years. The roots of ultrasonic flow meters began back in 1842 when Christian Doppler “discovered” that sounds tend to shift frequencies from when it approaches to when is retreats. The initial principle used in ultrasonic flow meters was Doppler, named after the man who first noticed it as a principle. Ultrasonic flow meters went on to utilize the shift in sound frequency for flow measurement purposes.

Although ultrasonic flow meters have begun to come to the forefront in the last couple decades due to some important advances in the field of ultrasonic instrumentation, the first models were created back in the 1960s. Shigeo Satomura was a Japanese physicist who developed the first ultrasonic flow meter, based on the Doppler principle, for the analysis of the flow of blood.

It wasn’t until a few years later that the principle of operation was developed for use in industrial applications, also based on the Doppler principle of operation. The technology did not catch on widely, due to its initial limited ability to measure some of the more common media of time. By the 1970s, ultrasonic flow meters had become more widely known, but because the technology was misunderstood and misapplied in applications by end users, the technology gained a negative reputation through no fault of its own but through operator error.

In the 1990s, as industry surged, the demand for flow meters to measure various new industrial processes spurred growth and development in the flow meter market. Ultrasonic flow meters also evolved in one key characteristic during this period of development. They began to employ a different principle of ultrasonic measurement, known as transit time, time of flight, or run time difference. This new method of measurement paved the way for the ultrasonic flow meter to take its place among other prominent technologies employed in flow measurement because they could now measure the more common media types.

Since the 1990s, advancements in electronics and processors have allowed it to evolve further alongside similar technologies. All these advancements have brought us to modern day ultrasonic flow meters that offer a lot of advantages for certain application areas that other technologies cannot accommodate.

While there are many models of ultrasonic flow meters that can all offer something slightly different, there are two main types of ultrasonic flowmeters from both an installation aspect and an operating principle aspect.

When it comes to the installation of ultrasonic flow meters, the two different types are inline, where the instrument has a built-in flow body that is fitted directly into the piping systems via connections, or the clamp-on (also known as a portable or strap-on ultrasonic flow meter) where the transmitters are mounted on the outside of the pipe and certain attachment methods can either be more permanent in nature or removable.

From a technology standpoint, there are two types of ultrasonic flow meters. Those that still employ the Doppler principle for measurement, and those that employ the transit time (also known as the time of flight) principle.

Ultrasonic flow meters offer many advantages. They do not have any moving parts that will wear out, shortening the service span of the meter. They require minimal maintenance and have low operating costs. They provide a good dynamic response. They measure non-conductive media and are an excellent alternative for magnetic flow meters that require conductive liquids to operate properly.

They are generally known for high accuracy. They are better at reading low flow rates than vortex flow meters, another flow technology that has no moving parts to wear. They also offer a low pressure drop and can offer many models chemically compatible with chemicals. They can also handle dirty media where certain other technologies cannot.

Ultrasonic flow meters, like any flow technology, also have their limitations as well. They can be higher in initial cost than some other technologies, depending on the exact application demands and variables. They can be harder and more expensive to repair if they do malfunction. They require straight pipe runs to ensure a flow profile that is conducive to the ultrasonic principle of operation. They will not work with liquids that do not pass ultrasonic energy, like slurries. Lastly, the accuracy of ultrasonic flow meters can be compromised with lows flows, like those under 2 ft per second.

Ultrasonic flow meters are useful for a wide variety of application areas. The newer clamp-on models offer an ideal solution to many applications where other technology types aren’t compatible. Common industrial applications for ultrasonic flow meters include power plants, water and wastewater, facility management, chemical industry, and the food and beverage industry.

Ultrasonic flow meters can be used with certain media types that other flow technology may not be compatible with like sanitary liquids, corrosive liquids, and abrasive liquids. Specific models can also be used with cryogenic liquids. They also work with petroleum. Other types of media that are compatible with ultrasonic technology depend on whether the Doppler or Transit-time technology is the operating principle for the meter.

Along with other technologies that do not employ a mechanical principle of operation, ultrasonic flow meters can deliver excellent accuracy. They can also deliver a very high turndown ratio that can handle a wide spectrum of flow rates within a single meter. For example, our DUK Inline Ultrasonic Flow Meter delivers one of the industry's best turndown ratios at 250:1. To learn more about what turndown ratio is, visit our article for an in depth, yet understandable, explanation.

Although generally similar in pricing, the real advantage of ultrasonic flow meters is the ability to measure non-conductive media. Magnetic flow meters have a minimum amount of conductivity that the media must posses to operate correctly. This means that there are many media types that are not compatible with magnetic flow meter technology that are compatible with ultrasonic technology.

Both ultrasonic and magnetic flow meters are built on principles that do not require moving parts, so they both offer lower maintenance and potentially a longer service life than mechanical flow meters. They both can offer excellent accuracy, however some ultrasonic flow meters can deliver exceptional turndown ratio, like our compact inline DUK Ultrasonic Flow Meter with a turndown ratio of 250:1, which can handle a wide span of different flow rates within a single application. If the flow rate of your application varies widely, an ultrasonic flow meter may be a better option.

Another area where ultrasonic flow meters excel over magnetic flow meters is that they are available in clamp-on versions that can deliver a large cost savings for large bore application installations. They can also handle media characteristics that may not be compatible with inline flow meters or magnetic technology. Clamp-on models also offer the benefit of no media contact and can measure media that other technologies cannot.

Installation of an ultrasonic flow meter depends on the type of ultrasonic flow meter. If the flow meter is an inline version, the meter will need to be fitted into the piping system via well sealed connections. Installation of an inline ultrasonic flow meter requires that you pay attention to the upstream and downstream distance requirements as well. Ultrasonic flow meters should be installed with as much space as possible from any objects that might disturb the flow profile. Although you can install them as close as five times the meter’s length in the pipe, it can result in additional inaccuracies. Always refer to the installation manual for your specific flow meter model as the requirements can vary. For example, our inline DUK Ultrasonic Flow Meter requires 10x both up and downstream. To learn more about straight run requirements and why they exist, visit our article about straight run pipe requirements.

Installing a clamp-on ultrasonic flow meter is significantly easier and is one of the advantages driving its increasing popularity. The transmitters are attached to the external circumference of the pipe via strapping of various types. With clamp-on ultrasonic flow meters, the key to a good installation is making sure that the pipe it is affixed to has acoustical properties that will allow the ultrasonic principle of measurement to function properly.

• Always refer to your product manual for your exact flow meter
• Keep the flow meter free from extreme loads, such as pressure surges with strong dynamic pipe movements, vibrations in the proximity of centrifugal pumps, high temperature media, flooding, etc
• Remove all packing materials and transport retainers and ensure that no such materials remain in the device
• If flow body has an arrow printed on it, orient arrow to flow direction
• Avoid pressure and tensile load
• Avoid valves or large pipe reduction on the inlet side
• Do not exceed torque recommendations when installing the meter

• Always refer to your manual for your exact flow meter
• Make sure that the pipe material, which the transducers will affix to, can carry ultrasonic waves.
• The transducers must be spaced a particular distance away from each other. This can be done via a measurement or a factory-made spacer bar.
• To affix the transducers, it is recommended that you use a small amount of coupling grease in between the pipe and the transducer if the transducer does not already have coupling foil on it.
• Very little contact pressure needs to be applied to the transducer by the attachment straps/chains. Do not overtighten the straps and compromise the transducer.
• Ensure that the area of installation provides enough straight piping run before and after the transducers. These requirements can vary, ours is 10x inlet and 3x outlet.
• For horizontal piping, it is best to mount the transducers at a 10 or 2 o’clock position on the pipe to avoid the influence of gas bubbles at the top of the pipe and the influence of sedimentation at the bottom of the pipe.
• Follow the specific wiring diagram for your meter when commissioning the electronics.

The prices of ultrasonic flow meters depend on a couple variables. One is the installation type, inline or clamp-on, and the other is the characteristics of the specific meter needed. In general, ultrasonic flow meters are similar in price to similar technology types like magnetic flow meters. Ultrasonic flow meters are generally higher in price than mechanical type flow meters that do not employ a sophisticated means of measurement, like the simple well-known variable area flow meters.

For non-conductive liquids, ultrasonic flow meters are a good alternative to a magnetic flow meter in terms of accuracy and pricing because magnetic flow meters require conductive liquids to operate. Depending on certain media characteristics, ultrasonic flow meters can put themselves first in line as a good solution.

Where the price of ultrasonic flow meters really can shine is in large bore applications where clamp-on models are compatible. Because they do not require a significant amount of material to create a giant flow body, they are cheaper to make. They also cost much less to ship as the size of a large bore flow meter usually requires special shipping arrangements on a pallet and clamp-on ultrasonic flow meters fit in a carrier that is roughly the size of a briefcase, like our DUC Clamp-on Ultrasonic Flow Meter. Clamp-on flow meters also deliver value because they do not require system shutdown for installation and do not have large costs acquired with installation as they are simply strapped on the outside.

Ultrasonic flow switches are typically found on ultrasonic flow meters and are rarely stand-alone units that do not include the flow rate monitoring capability that comes from a flow meter. While there are other flow meter technologies that offer flow switches that are not full flow meters, like variable area rotameter technology, the expense to build an ultrasonic instrument is typically not justifiable for just a switch. If you need an ultrasonic flow switch, shop for an ultrasonic flow meter that has an integral flow switch or offers an optional flow switch.

“Ultrasonic flow sensor” is a general term for any flow instrumentation that detects and senses flow. It is commonly used in place of the term “ultrasonic flow meter”, even though “ultrasonic flow meter” is much more commonly used. As most ultrasonic flow switches are found only as an integral part of a flow meter, the term “flow sensor” can also indirectly refer to an integral flow switch. If you are looking for an “ultrasonic flow sensor”, you may be able to find more product options by searching for an “ultrasonic flow meter”.