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Ultrasonic Flowmeters: Portable Flow Technology

Updated on February 17, 2012

The Principles Behind Ultrasonic Flowmeters

Ultrasonic Flowmeters Give Companies An Advantage

photo by 36760597@N08 on Flickr
photo by 36760597@N08 on Flickr

Accurate Measurements With Ultrasonic Flow Meters

Ultrasonic flowmeters are an excellent example of applied physics in action. As a must-have tool for any accurate flow measurement task, ultrasonic flowmeters make use of the Doppler effect (also known as the Doppler shift) in order to calculate the volumetric or mass flow rates of liquid flows and gas flows. The ultrasonic flowmeter is a non-intrusive tool which means that the user is able to take these measurements from the outside of the pipes in question. A high quality meter is able to measure flow rates in metal, plastic and even concrete lined pipes to a very accurate tolerance.

The Doppler effect is a principle of physics that was first realized in Prague in 1842 by Christian Doppler. The Austrian physicist realized that there is a change in sound wave frequency when an observer is moving relative to the source of the sound. It is the same principle that most have probably heard about in explaining why an ambulance or police car siren may sound like it is at a higher pitch during the approach than when it has passed by on its way to the Regional Medical Center or the state prison, whichever might be the case. Ultrasonic flowmeters can sometimes be known as transit time flow meters because the transit time is what is actually being measured. Ultrasonic pulses are sent into the pipe and it is there that the sound beam reflects off of the substance in question. For the measurement of natural gas or other gases, the obtained speed is compared to a known speed for the gas. Deviations are used to extrapolate the actual speed and displayed on the screen of the ultrasonic flowmeter. In the case of liquids, the sound beam will need to bounce off of entrained air bubbles or particles in the liquid flow and then the Doppler shift is used to calculate the flow measurement. The latest in ultrasonic flowmeters are capable of measuring most gases and any type of liquid flow, whether it be ultra-pure water or an extremely dirty or toxic liquid. The non-invasive methodology means that the pipe does not have to be opened which might disrupt the process or slow operations.

The new ultrasonic flow meters are inventions that were probably nowhere in Christian's mind when he began writing about the Doppler effect in his treatise on the colored light of the universe's binary stars and "some other stars of the heavens." Yet digital ultrasonic flowmeters have become a very important tool in all types of industries. They see a lot of use as pollution prevention equipment in areas of water, wastewater, hazardous waste and air contamination where emissions control and flow monitoring are key approaches to business. The newer transit-time ultrasonic flowmeters have much improved correlation algorithms to make even more accurate measurements in varying conditions. A buyer should look for a model that has a good clamping apparatus included in the kit and will find that there are different meters depending on the size of pipe in which the flow measurement will be taken. Look for ultrasonic flowmeter models that incorporate the range of pipe diameter that the tool will be used on. For example, one portable ultrasonic flowmeter kit by GE Sensing has the capabilities to measure a pipe with a diameter of anywhere from a half an inch (1/2") to 24 inches (24"). Other models cover only a portion of that range (say 2" to 12") so it is important to know what measurements will be needed. The price of an ultrasonic flowmeter with a wider range may be slightly more in the short-term, but it will pay off as the total cost of ownership will be lower as multiple meters will not be needed.

Another area where these doppler meters show some differences is in the flow rate that they are capable of measuring. You might see a meter that can measure say .3 to 30 feet per second. So be certain to get a flowmeter that has the specific tolerances to your company's needs. Some flowmeters are sort of meant to more or less clamp onto the pipe and remain fairly stationary after that. They act as a sort of sensor or ad hoc meter. The really cool type of ultrasonic flowmeters are the portable units that can fit into a toolbox and be taken on the job anywhere. Clamp-on meters can come in two different types. There is a single sensor model and a dual sensor model. In a single sensor model, the sensor will house both the transmit and receive crystals. While in the dual sensor model these crystals will be separated. The dual sensor models are more common and tend to be more accurate.

Add a fleet of ultrasonic flowmeters to your company's arsenal today and you will have a technical advantage over the competition.

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