When clamp-on flowmeters were first introduced for industrial use in the 1960s, Doppler ultrasonic flow measurement was a misapplied technology. Ultrasonic flow meters were more identified with the medical field than in industrial settings.
However, there many advantages to using ultrasonic Doppler flow measurement technology for the pipes industry:
- Doppler ultrasonic flow meters clamp on the outside of a pipe so they are non-invasive and don’t have moving parts.
- Clamp on flow meters don’t cause pressure drops, risk no damage from the process liquid, and require little maintenance.
- When properly calibrated, Doppler flow meters can have a ±1% error accuracy margin
Doppler Ultrasonic Flowmeters in Industrial Applications
The misapplication of Doppler ultrasonic flow measurement technology (or clamp-on flow meters) is mainly due to confusion on what flow meter solution to use for what application.
There are two technologies that are used in flow measurement:
- Doppler/Doppler shift
- Transit time
The key is in the type of liquid that’s being measured: dirty or clean liquids
Doppler / Doppler Shift Flow Measurement
Dirty liquids have solids, air bubbles or gaps that affect flow rate or velocity. In this case, the Doppler shift flow measurement technology is the appropriate solution.
Doppler ultrasonic flow meter basics:
- The central principle: flow measurement is done by measuring the frequency shift (Doppler Effect) of an ultrasonic signal that’s bounced off a moving solid or air bubble.
- A clamp-on flow meter with two transducers—one is a transmitter and the other is a receiver - is mounted outside the pipe.
(Flow profile conditions will dictate where the clamp-on flow meter will be placed.) - The transmitter fires off an ultrasonic signal into the pipe to hit a solid particle/air bubble/gap.
- The solids, air bubbles or gaps in the flow reflect back the ultrasonic signal to the receiver.
- Because these particles in the dirty liquid are moving, the frequency that’s reflected back is shifted or moved.
- We get the flow measurement by calculating the time it takes for the ultrasonic signal or beam to hit the moving particle and then reflect back to the receiving sensor.
- The frequency shift is directly proportional to the velocity of reflected particles in the liquid.
Transit Time Flow Measurement
Transit time flow meters are best used for clean liquids that contain only up to 10% solids. That’s because the clamp-on flow meter measures the transmission and reception of ultrasonic signals differently.
Transit time flow meter basics:
- The central principle: flow measurement is based on the theory that sound waves moving along the direction of flow require less travel time than when moving in opposition to the flow.
- Transit time flow meters measure the time difference in diagonal and vertical ultrasonic signal movements, upstream as against downstream, through the pipe.
- A clamp-on flow meter with two transducers—again one is a transmitter and the other is a receiver—is mounted onto the pipe exterior.
- In the Direct Mode, the clamp-on flow meter transducers are placed on opposing sides of the pipe, so that the transmitting and receiving sensors are communicating directly with each other.
- In the Reflect Mode, the clamp-on flow meter is placed on the same side of the pipe, allowing the ultrasonic beam to bounce off the pipe wall and reflect on the receiving sensor.
- Both modes—which essentially define the sound path of the ultrasonic signal—depend on pipe size and transducer frequency.
- Direct mode flow measurement is typically used for larger pipes and Reflect Mode flow measurement is generally utilised on smaller pipes.
- Flow measurement is taken by calculating the time it takes for the ultrasonic beam to move from the transmitter, bounce off the pipe wall, and arrive at the receiving sensor.
A Little Ultrasonic Flow Meter History
The Doppler Effect theory which is at the heart of ultrasonic flow measurement has been around for almost 170 years.
Christian Doppler, an accomplished mathematician and physicist, discovered in 1842 that a stationary observer perceives sound differently depending on the location of the sound transmitter.
As the sound source moves toward an observer, the wavelengths shorten. When the sound source moves away from the observer, the wavelengths lengthen.
This phenomenon became known as the Doppler Effect—which explains why you’ll hear a blowing horn increasingly louder as a car approaches and the sound progressively drops in pitch as the car drives farther away.
Shigeo Satomura, a Japanese physicist, brought the first ultrasonic flow meter into the mainstream with the release of his findings in December 1955 of his measurement of the Doppler shift of ultrasonic signals emitted by a beating heart.
By 1963, the first ultrasonic flow meters were being used for industrial purposes—three years after Satomura’s research confirmed that the Doppler flow meter could measure different types of blood flow.
The Doppler ultrasonic flow measurement technology has come a long way from its dubious status in the 70s and 80s. Today, non-invasive, non-intrusive ultrasonic flow meters are being used across the world in measuring liquids and gases across a wide range of applications.
