For non-invasive measurement of flow in pipes, TRANSIT TIME and DOPPLER are two widely used technologies. They are both ultrasonic and both measure flow by using sensors clamped onto the outside of a pipe. The biggest difference between them is that they actually work best in opposite situations. Understanding the differences between these technologies will help you making the right choices regarding a successful and efficient installation.
How it works
Transit time flowmeters measure – as the name suggests – the time required for an ultrasonic signal (sound generated above the human hearing range – above 20 kHz) transmitted from one sensor, to cross a pipe diagonally and be received by a second sensor. Upstream and downstream time measurements are compared. Without flow, the transit time would be the same in both directions. With flow, the signal travels faster in the flow direction and slower against it. The measured time difference is directly dependent on the speed of the medium flowing through the pipe. This time difference is the relevant value to measure and monitor the flow of liquids and gases.
Clear liquids and gases
Due to the fact that the ultrasonic signal must cross the pipe to a receiving sensor, the fluid inside the pipe may contain only a minimum concentration of air bubbles or solids (<30%). Otherwise the high-frequency sound will be attenuated and therefore too weak to reach the other side of the pipe. If that is the case we advise you to opt for a doppler flowmeter.
In order to perform a bi-directional measurement, our transit time flowmeters are always supplied with two sensors. The two sensors can function both as transmitter as well as receiver of the ultrasonic signal.
The current generation of transit time flowmeters achieve results with 0.5 – 1 % accuracy of full scale.
Transit time flowmeters are suitable for pipes and tubes made of a wide range of materials (also double-walled). Their diameters can vary from 3 mm up to 6500 mm. Applications include potable water, cooling water, water/glycol solutions, hydraulic oil, fuel oils and chemicals.
How it works
The doppler flow measurement technology is based on the principle that sound waves will be returned to the transmitter at an altered frequency if reflectors in the liquid are in motion. We all hear examples of the ‘Doppler effect’ every day; The tone change of a passing train, a flying plane, or the exhaust from a race car. We hear this changing tone because we are stationary and the transmitter of the sound is in motion.
The operation of a doppler flowmeter is based on a linear relationship between this frequency shift and the velocity of the fluid. This is measured as accurately as possible by the instrument to calculate the flow rate.
Doppler flowmeters are used solely for measuring fluid flows and depend on the reflection of sound. Only liquids with solids and / or gas bubbles are therefore eligible for this technology. These are ‘difficult’ liquids that may damage regular flowmeters; Slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. At a concentration of <30% (refectors), consider choosing a transit time flowmeter.
One or two sensors
Unlike transit time flowmeters that always have two sensors, doppler flowmeters exist in different versions; Either with one or with two sensors. In the single sensor models, both transmitter and receiver are combined in the same housing. In the models with two sensors, one sensor transmits the ultrasonic signal and the other acts as the receiver.
Even though the doppler flow measurement technology assumes that the reflective particles move at the same speed as the liquid, in practice this is never the case. Particles tend to lower while gas bubbles want to rise. As a result, a doppler flow measurement provides results with an accuracy > 2% of full scale.
Doppler flowmeters are suitable for pipes and tubes of a wide range of materials. Their diameters can vary from 30 mm up to 4000 mm.
Applications include dry-running detection of pumps, overflow applications and sand extraction.
Doppler flowmeters werken het beste op vervuilde en luchtbellen bevattende vloeistoffen zoals rioolwater en afvalwater. Looptijdverschil flowmeters werken met heldere vloeistoffen zoals water, olie en chemicaliën. Neem contact met ons op voor specifiek advies en informatie over welke methode het meest geschikt is voor uw toepassing. Doppler flowmeters perform best in polluted or aereated liquids such as sewage and wastewater. Transit time flowmeters work best with clear fluids like water, oil and chemicals.
Contact us for specific advice and information on selecting and applying these technologies succesfully in your situation.
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