Ultrasonic flow meters use sound waves to measure fluid velocity. They are available in both clamp-on (non-invasive) and inline configurations, making them one of the most versatile flow measurement technologies.
Two Main Technologies
Transit-Time Ultrasonic
Transit-time meters work by sending ultrasonic pulses between two transducers, one upstream and one downstream. The pulse traveling with the flow arrives slightly faster than the pulse traveling against the flow. The difference in transit times is proportional to the fluid velocity.
This method requires a relatively clean fluid with few particles or bubbles, as these can scatter the ultrasonic signal.
Doppler Ultrasonic
Doppler meters work by transmitting an ultrasonic signal into the flow and measuring the frequency shift of the signal reflected by particles or bubbles in the fluid. The frequency shift is proportional to the fluid velocity.
This method requires particles or bubbles in the fluid to reflect the signal. It is less accurate than transit-time but works in dirty or aerated fluids.
Clamp-On vs Inline
Clamp-On (Non-Invasive)
Clamp-on ultrasonic meters mount on the outside of the pipe. The transducers transmit sound through the pipe wall and into the fluid.
Advantages include no pipe cutting or process shutdown required for installation, no pressure drop, no contact with process fluid, and the ability to measure on pipes of virtually any size. Limitations include lower accuracy than inline meters (typically ±1-2%), sensitivity to pipe wall condition and lining, and the need for adequate pipe wall thickness information.
Inline (Wetted)
Inline ultrasonic meters are installed directly in the pipe like traditional flow meters. They provide higher accuracy (±0.5% or better), are used for custody transfer applications, and are available in multi-path configurations with 4 or more paths for the highest accuracy.
Multi-Path Ultrasonic Meters
For custody transfer and high-accuracy applications, multi-path ultrasonic meters use 4 or more pairs of transducers positioned at different chord positions across the pipe. This provides a much more accurate measurement of the average fluid velocity across the full pipe cross-section. Multi-path meters can achieve accuracy of ±0.1% to ±0.2%.
Advantages of Ultrasonic Flow Meters
- Available in non-invasive clamp-on configuration
- No pressure drop (clamp-on and inline)
- Works with large pipe sizes (up to several meters in diameter)
- No moving parts
- Bi-directional measurement
- Can measure clean liquids and gases
Limitations
- Transit-time requires clean fluids
- Clamp-on accuracy depends on pipe wall condition
- Straight-run requirements can be significant (10-30D upstream)
- Performance affected by flow profile disturbances
- Temperature limitations for clamp-on transducers
Common Applications
Ultrasonic flow meters are widely used in oil and gas custody transfer using multi-path inline meters, water distribution systems, HVAC energy metering, chemical processing, temporary flow measurement using clamp-on meters, and large pipe applications where other technologies are impractical or too expensive.
Selection Tips
Choose transit-time for clean fluids where accuracy is important. Choose Doppler for dirty fluids with particles or bubbles where indicative measurement is acceptable. Choose clamp-on when you cannot shut down the process for installation or need a temporary measurement. Choose multi-path inline for custody transfer and the highest accuracy requirements.