Clamp-on ultrasonic flow meters are portable metering devices that measure volume flow in industrial applications from outside the pipe wall, without breaking the line, using ultrasound. Most flow meters have to be mounted inside the pipe (inline), which takes more time and requires the machinery to be dismounted and the work paused.

Portable flow meters

Portable flow meters can be installed, removed, and transported easily. This makes them an ideal choice for industrial applications that are already running, or that only require occasional flow measurement. In the latter case, an engineer can use the same portable flow meter to measure flow in multiple places by removing and transporting the device after each measurement.

Clamp-on ultrasonic flow meters are among the most popular types of flow measurement today. They are installed on closed pipes, and most often used to measure the flow of liquids.

— IMG of portable EM flow meter

Doppler vs Transit time

Ultrasonic flow meters calculate the flow rate from the frequency shift of the ultrasound waves that result from travelling through the measured material. There are two main ways types of ultrasonic flow meters: transit time and doppler flow meters. While they both use ultrasound, there are some crucial differences in their working principles, which make them ideally suited for measuring different types of materials.

Doppler clamp-on ultrasonic flow meter

Doppler clamp-on ultrasonic flow meters use the doppler effect, a frequency shift in sound waves emitted by a moving source.

The transducer creating the ultrasound waves is mounted on one side of the pipe, and the sound waves are reflected back to the sensor from particles or bubbles present in the flowing material. The flow meter then measures the doppler effect in the returning sound waves and calculates the flow rate.

Doppler flow meters only work on materials that contain particles or bubbles that will reflect sound waves, like sludge or wastewater. They also require the material to flow fast enough for the particles or bubbles to be suspended.

Doppler ultrasonic flow meter

Transit time clamp-on ultrasonic flow meter

Transit time clamp-on ultrasonic flow meters use two transducers (usually on the same side in the case of clamp-on flow meters): one sends out the ultrasound waves, which bounce back from the opposite wall of the pipe, and are intercepted by the second transducer.

When the material is flowing, the sound will travel more quickly in the direction of the flow and slowly against the flow. Flow rate is calculated from this shift in the transit time of the ultrasound waves, hence the name.

Transit time ultrasonic flow meters work best on clear materials, such as water or gas, which let the waves travel unobstructed in both directions.

If you would like to know more about the difference between transit time and doppler ultrasonic flow meter, take a look at our article on ultrasonic flow meters.

Transit time ultrasonic flow meter

The benefits of clamp-on ultrasonic flow meters

Clamp-on ultrasonic flow meters are versatile devices with many advantages:

  • Accuracy: Some clamp-on ultrasonic flow meters are very accurate, measuring flow from as low as 0.03 ft/second. This is enough to detect leaks.
  • Easy installation: Clamp-on flow meters are easy to install and very portable, which can save work hours and resources.
  • Less maintenance: Clamp-on devices are not in contact with the measured material and have no moving parts, which greatly increases their lifetime. They are also immune to sensor buildup and can be used on hazardous or corrosive materials without any extra precautions.
  • Easy calibration: Because of how easy it is to remove them, clamp-on devices can be recalibrated relatively quickly. This leads to even lower maintenance costs and more reliable measurements.
  • Portability: Clamp-on devices can be moved around in a factory, so they can replace multiple flow meters in some cases and even used to validate other flow measurements.
  • No pressure loss: Installing the meter on the outside of the pipe wall eliminates the problem of pressure loss, which can occur with inline measurements.

Clamp-on ultrasonic flow meter installation

Clamp-on ultrasonic flow meters are installed using metal chains, straps, or mounting rails to fix the device to the pipe. Usually, a coupling paste is applied underneath to ensure an acoustically conductive connection.

If the surface of the pipe is very rough, some filing or cleaning may also be required. In some cases, multiple clamp-on flow meters are installed on a single pipe in order to ensure higher measurement accuracy.

Installing the meter should be quick and easy for a trained engineer.


Clamp-on ultrasonic flow meters can be used in any industrial application where a regular ultrasonic flow meter would do. However, they are especially suited for the following materials:


Clamp-on ultrasonic flow meters are versatile devices with many advantages. They can save a lot of work hours and resources if used well, and often replace multiple devices.

If you are having trouble choosing the right flow meter for your application, take a look at our flow meter smart assistant.

If you would like to know more or need some help figuring out which device you need, give our engineers a call and they will be happy to help.

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