Measuring flow using an ultrasonic flowmeter may seem like magic, but the process is actually simple in concept. Knowing how these types of flowmeters work, while not essential, will certainly help you become a more effective user, and a true master at measuring flow.

What is meant by Ultrasound?

Ultrasound is the flow of mechanical energy, the same as an ocean wave or human speech. An ocean wave typically originates from the wind, or earth’s tectonic movements, both of which directly apply mechanical forces onto the ocean’s water molecules. These forces push water molecules which collide with neighboring molecules, which again collide with their neighbors and so on. This chain reaction of colliding water molecules causes mechanical energy to transmit through the ocean. This is what we call a wave. With human speech, the sound wave is initiated by our vocal cords, which vibrate causing collisions with surrounding molecules. This time, with air molecules instead of water.

The word ultrasound refers to sound waves at a higher frequency than human speech and hearing. On a larger scale, the same thing happens when playing billiards where a queue ball is struck and collides with other billiard balls. Those balls are then forced to move and collide yet again with others - very similar to mechanical energy transferring as a wave. Ultrasound is the same in concept, but operating at a higher frequency, and at the molecular level.  

Take a look at the animation below. Hover over it and press the play button so you can see the particles and waves. This is a great illustration of what is happening with your flowmeter!

What do we mean by frequency?

The frequency of a wave is the time it takes for a fluid particle to oscillate, or travel back and forth between two points, as the wave passes through it. In the case of an ocean wave, the particles themselves are not actually moving very far distances, and perhaps only a few inches back and forth. We can see the particles of water, or foam on ocean surface move up, then down, as the wave passes. The frequency might all happen in about one second, or said another way, one cycle per second. For ultrasound flow measurement, there would be one-million particle oscillations per second as the wave moves through a fluid. Which is what is meant by a higher frequency.

Humans can hear frequencies up to about 20 kHz, or 20,000 cycles per second. Above this, sound waves are inaudible to humans. If we designed flowmeters to operate at lower frequencies within the human audible range, then you would be able to hear your flowmeter operate. Thankfully, we don’t do that!

About the ultrasonic flowmeter.

With transit time ultrasonic flowmeters, there are typically two ultrasonic transducers which can both transmit and receive ultrasound. They do this by using a special material called a piezoelectric, which is embedded inside the ultrasonic transducer. They are controlled with electronics to physically oscillate at a specific frequency to transmit ultrasound. The flowmeter controls one transducer to oscillate at one million times per second, for just a blip in time. This sends a small sound wave out of the transducer and into the pipe and fluid. The sound wave continues into the fluid, reflects off the backside of the pipe, and heads back to the second transducer where it is picked up by the flowmeter electronics. The transducers can both transmit and listen, so after one is the transmitter and the other listener (receiver), they switch roles. The two ultrasonic transducers do continuous ‘pitch and catch” of ultrasonic bursts through the pipe and fluid. See the image below for an illustration of the ultrasound path from one transducer to the other.

If the fluid is moving, then the soundwave traveling in the same direction as the fluid motion will move faster than in the opposite direction. Hence, the fluid speed is actually slowing down or speeding up the sound transfer between transducers. As the fluid speed changes, so does the time the ultrasound takes to travel from one transducer to the next. This is what the flowmeter measures to infer flow rate or “transit time”. It is the same concept as rowing a boat upstream for 100 yards, versus rowing downstream 100 yards. We all know rowing that same 100 yards upstream will take a lot longer than rowing downstream. In the same way, the flowmeter is sending sound waves upstream and downstream to detect the fluid motion.

Ensuring a proper measurement.

We now know that for the flowmeter to operate it must be able to transmit sound waves out of one transducer, through the pipe, through the fluid, back through the pipe wall, and into the receiving transducer. Throughout this process, we must make sure that we enable this sound transfer to happen efficiently and to avoid things that may impede this process.

The most common impediment to ultrasound transfer is air which will completely stop high frequency sound. Using coupling gel or coupling pads placed between the flowmeter and pipe help fill the air gap so that ultrasound can get out of the transducer and into the pipe. Air may show up in various places: an empty pipe, partially full pipe or under flakey paint chips to name a few. Another form of air is micro bubbles, caused by cavitation, a circulating system, or a deliberately aerated pipe.

SoundWater ultrasonic flowmeters accommodate many difficult applications by pushing more and more ultrasonic power into the pipe, and by intelligently processing the ultrasound signal. In situations where the ultrasound transfer is altogether not possible, we can often make the situation better if we know what the common causes are.

Getting the ultrasonic flowmeter to work for you.

When you enter an application with your flowmeter, knowing the challenges ahead of time is very helpful. First, make sure air is not a problem and if you think it might be, then go to a place where it may not be a problem. Keep the pipe surface clean of debris that could prevent the transducer from fully contacting the pipe and use coupling gel or coupling pads liberally. If you suspect air is in the pipe, then place the flowmeter on the side of the pipe (instead of the top) to avoid the air bubbles and pockets that collect at the top of the pipe.

Now that we know how the flowmeter works, we can also use it as a tool to detect empty or partially full pipes. SoundWater flowmeters have an “ultrasonic signal indicator” on the measurement screen, telling you the strength of the received ultrasound. If there is no ultrasound signal strength, then it is very likely that the pipe is empty, or partially full (or in some cases massively corroded). This guides you in placing your SoundWater flowmeter to ensure the ultrasound is completing its journey from one transducer to the other. This is a simple, yet effective tool to make sure you’re able to focus on getting the job done quickly and effectively.

I hope this has armed you with the knowledge to more effectively use your SoundWater flowmeter!

Next time, we will talk about another ultrasound phenomenon “reflection.”

Best Regards,
Jeff Peery, CEO
SoundWater Technologies