How ultrasonic flow meters work?
There are several types of ultrasonic flow meter technologies available on the market. This information was designed to help users understand some basic differences between the technologies so that it will be easier to select the correct type for their application.
What is an ultrasonic flow meter?
An ultrasonic flow meter uses sound waves above the audible hearing level. Humans can detect sounds in a frequency range from about 20 Hz to 20 kHz. Certain animals pick up higher frequencies and have their own natural sensing techniques in higher frequencies for short distances e.g. Bats produce echolocation by emitting high frequency sound pulses through their mouth or nose and listening to the echo, while elephants can communicate with each other at subsonic levels to signal their location over long distances.
The animation above shows a device that is both a wave emitter and receiver. This is how high frequency transducers are able to measure distance. If the sonic velocity of the media (in this case air) is fixed and known, then the time taken for the wave to transmit and bounce back to the source will be directly proportional to the level or distance.
Even though the sound waves are above the hearing level, we can understand how the meters work by observing how sound waves act at the audible level. By visualizing this phenomena, it is easier to comprehend how ultrasonics are used in industry using transit-time technology.
Lets take for example how transit-time ultrasonic technology is used in flow measurement. In this case, the distance is now fixed and transducers are placed upstream and downstream of the flow. The variable is now the flow rate of the fluid.
Most transit-time flow meters use a simple technique to measure flow. If this technique is combined with cross-correlation and advanced signal processing, it is possible to measure liquids with some amount of solids and aeration. EESIFLO design and manufacture transit-time cross-correlation flow meters which vastly improve accuracy, repeatability as well as user experience. Sometimes it is helpful to visualize the speed at which these things happen. The following animation produced for a University in Houston TX gives us an idea of just how many signals are constantly being sent to and fro.
Accuracy and repeatability will depend on how well the initial and subsequent signals are interpreted and the need for cross-correlation techniques and advanced signal processing. The processor capability becomes an issue that needs careful consideration.
The clamp on ultrasonic flow meters have some unique advantages over traditional flowmeters such as the Coriolis, turbine, vortex, magnetic and positive displacement types. Of course, each technology has it it’s own plus and minuses but unlike fixed flow meters (where the pipe must be cut and the process halted to install a meter) the clamp on ultrasonic flow meter can be tested in the field without having to shut down anything. In fact, since there are so many manufacturers of clamp on ultrasonic flow meters, it is possible to test them all on site to see which technology works best. This takes the pressure off the buyer who is able to “kick the tyres” before deciding to make a larger capital investment in the clamp ons.
What is the difference between Transit-time and Doppler flow meters?
Doppler flow meters come in handy in situations that are often impossible for transit time flow meters to work well. This is because we are measuring the doppler shift. A continuous wave is sent through the pipe into the liquid which will reflect off particles or undissolved gas or air bubbles. The return signal will be a different frequency (doppler shift) which is proportional to the flow velocity of the liquid. This technology assumes that the particles or air bubbles are moving at a speed close to the flow velocity, hence the need for a minimum velocity in order to obtain the required accuracy.