How Ultrasonic Flow Meters Measure The Flow Of Water?

  Jonathon Micah    May 20, 2013    617


Ultrasonic flow meters are devices that help calculate the flow of water by measuring the velocity of the liquid by using ultrasound. Ultrasonic flow meters are extremely useful as they particularly help in measuring the flow of waste water. They are ideal for measuring waste water flow as when distilled water is used on this kind of device, aerations have to be added. These flow meters work because of the acoustic properties found in the water and things like temperature, density viscosity and suspended particulates impact the result.

Different Ultrasonic Flow Meters

There are three kinds of ultrasonic flow meters that can be used to measure the flow of water. The first kind is known as transmission flow meter, which is also known as contrapropagating transit-time. The transmission flow meters are again of two types- in-line also known as intrusive or wetted meters and clamp-on (non-intrusive) meters. The second type of meter is known as the reflection or Doppler flow meter as it uses Doppler Effect to measure the flow of water. The third kind is called the open-channel flow meter.

How Flow of Water is Measured Using Ultrasonic Flow Meters

To measure the flow of waste water using ultrasonic flow meters the basic principle of the Doppler Effect has to be understood. The Doppler Effect comes into play when the metering technique measures the changing frequencies of water flow that occurs due to discontinuities in the water. It uses a technique which is similar to measuring sound waves. The operation thus uses the frequency shift that reflects the suspended gas bubbles or particles that interrupts the motion of water.

To measure the flow of water, ultrasonic sound is transmitted into a pipe. When the ultrasonic sound passes through the discontinuities in the water, a difference in frequency is observed which is measured. This different frequency is directly proportional to the rate of flow of liquid. However, to ensure that the ultrasonic flow meter works effectively, the liquid should contain at least 100 parts per million (PPM) of 100 micron. This essentially means that the large suspended particles or bubbles should be found in water that is to be measured.

Points to Remember before Choosing an Ultrasonic Flow Meter

Since ultrasonic flow meters come in various forms and thus it is very important that a few things are remembered before choosing a flow meter.


  • The liquid that is to be measured should have 100 ppm of 100 microns?


  • A handheld monitor is necessary for repeatedly measuring the output. A continuous process monitor can also be used.


  • Know the minimum and maximum flow rate of the pipe.


  • Note the minimum and maximum process temperature and process pressure.


  • The size of the pipe that is being used. It is very important that an appropriate size is used.


  • Decide whether you would be using a digital output or an analog device.


  • Before the ultrasonic flow meter is used to measure it should be noted whether the pipe will always be full or not.

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