Doppler and transit time are two very popular types of flow meters for non-invasive measurement of flow in full pipes.
These technologies are often confused because they are both ultrasonic, and both measure flow by using sensors clamped onto the outside of a pipe.
They actually work best in opposite applications and successful installation depends on understanding the differences and making the right choice.
The transit time flow meter utilizes two transducers that function as both ultrasonic transmitters and receivers. The transducers are clamped on the outside of a closed pipe at a specific distance from each other.
The transducers can be mounted in the V-method in which case the ultra sound transverses the pipe twice, or W-method in which case the ultra sound transverses the pipe four times, or in the Z-method in which case the transducers are mounted on opposite sides of the pipe and the ultra sound transverses the pipe only once.
The selection of the mounting method depends on pipe and liquid characteristics. When the flow meter works, two transducers transmit and receive the ultrasonic signal which travels firstly downstream and then travels upstream. Because ultrasound travels faster downstream than upstream, there will be a difference in time of flight (Δt).
When the flow is still, the time difference(Δt) is zero. Therefore, as long as we know the time of flight both downstream and upstream, we can work out the time difference, and then the flow velocity (V) and flow volume (Q).
The flow meter operates by transmitting an ultrasonic sound from its transmitting transducer, the sound will be reflected by useful sonic reflectors suspended within the liquid and recorded by the receiving transducer.
If the sonic reflectors are moving within the sound transmission path, sound waves will be reflected at a frequency shifted (Doppler frequency) from the transmitted frequency.
The shift in frequency will be directly related to the speed of the moving particle or bubble. This shift in frequency is interpreted by the instrument and converted to various user-defined measuring units.
There must be some particles large enough to cause longitudinal reflection – particles larger than 100 microns.
When installing the transducers, the installation location must have enough straight pipe length upstream and downstream. Commonly, the upstream needs 10D, and the downstream needs 5D straight pipe length, where D is pipe diameter.
The Doppler ultrasonic flow meter is designed to measure the volumetric flow of liquid within the closed conduit, the pipeline must be full of liquids, and there must be a certain amount of air bubbles or suspended solids in the liquid.
Transducers are clamp-on or hot-tapped insertion types, user don’t need to shut down the pipe flow when installing transducers.
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