Ultrasonic flow meters utilize sound waves to measure the velocity of a fluid from which the volumetric flow rate can be calculated. Unlike most flow meters, ultrasonic meters do not include any moving parts and thus are more reliable, accurate and provide maintenance free operation. Since ultrasonic signals can also penetrate solid materials, the transducers can be mounted onto the outside of the pipe offering completely non-invasive measurement eliminating chemical compatibility issues, pressure restrictions, and pressure loss.

Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. Homogenous fluids, as well as, advanced digital signaling can eliminate many of the problems associated with noise and variations in liquid chemistry.

There are two types of ultrasonic flow meters:

Transit time flow meters measure the travel time of two sound waves. One wave travels the same direction as the flow while the other travels against the flow. At zero flow, sensors receive both waves at the same time, i.e., without transit time delay. As the fluid moves, it takes an increasingly longer time for the downstream wave to reach the upstream sensor. This measured "transit time difference" is directly proportional to the flow velocity and therefore to flow volume. Transit time flow meters require the fluid to be free from suspended solids or gas bubbles and in a closed and full piping system.

Doppler-shift flow meters operate on the principle that the wavelength of an approaching sound source is shorter than the wavelength of that same source as it is moving away. A transducer emits a sound wave which reflects off entrained particles or bubbles back to the transducer. The measured difference in the wavelengths of the transmitted signal versus the reflected signal is proportional to the process' velocity. Doppler flow meters are used for slurries, liquids with bubbles, or gases with sound-reflecting particles. They can also be adapted for use in open channels by integrating with level transmitters.

Selecting a Flow Meter

The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation.

  1. What is the fluid being measured by the flow meter(s) (air, water, etc…)?
  2. Do you require rate measurement and/or totalization from the flow meter?
  3. If the liquid is not water, what viscosity is the liquid?
  4. Is the fluid clean?
  5. Do you require a local display on the flow meter or do you need an electronic signal output?
  6. What is the minimum and maximum flow rate for the flow meter?
  7. What is the minimum and maximum process pressure?
  8. What is the minimum and maximum process temperature?
  9. Is the fluid chemically compatible with the flow meter wetted parts?
  10. If this is a process application, what is the size of the pipe?

If you have any questions or need any help selecting a flow meter, please contact us at sales@instrumart.com or 1-800-884-4967 to speak with an applications engineer.