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Can a piezoelectric sensor detect constant water flow, or just the moment when the water is turned on and off?

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Actually the answer is YES. Ultrasonic flow sensors which depend on piezoelectric transducers have been used for decades in precision flow measurement of gases and liquids. The principle usually requires a pair of transducers and electronics that that (1) excites a sound pulse across the fluid flow stream, and (2) receives the pulse. Doppler or time of flight principles are then used in the electronic circuits or software signal processing to determine the fluid velocity. And if you know the cross sectional area you can get the flow rate. You can read a more detailed account of ultrasonic flow sensors here.

EDIT: There is also another flow sensing technology that utilizes piezoelectric transducers - the vortex shedding flow sensor. For this technology a hydro(aero) dynamic interrupter is placed in the center of the flow stream, and is designed so that the flow streamlines separate at the back end of the interrupter - creating vortices. By placing a piezoelectric transducer at the back side of this interupter it can 'listen' to the vortex shedding. You can then count the number of vortices you hear and divide by the time interval to get a number that is proportional to the flow rate. You can learn more about vortex shedding physics here and the way Foxboro applies it to their product here.

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  • $\begingroup$ Hello Mr. Docscience, and many thanks for your response. That's a very interesting explanation and I appreciate the link you forwarded. It's definitely something worth exploring. I was wondering, though, if I could use something sensitive to audible frequencies to "listen" to the water. Something like a microphone. This might eliminate the extra circuitry and might enable a continuous detection rather than an on/off detection. Would this make sense? $\endgroup$
    – Amygdala
    Oct 5, 2015 at 9:46
  • $\begingroup$ You want a passive flow sensor - in the sense you are not putting energy into the flow stream. There is another technology you can use - vortex shedding. See my edit momentarily. $\endgroup$
    – docscience
    Oct 5, 2015 at 15:03
  • $\begingroup$ Thank you. That's excellent advice. I was wondering also was there a way you could detect the flow just by sensing vibrations in the pipes? That is, without the sender/reflector set-up used by ultrasonic flow sensors. $\endgroup$
    – Amygdala
    Oct 7, 2015 at 9:43
  • $\begingroup$ @Amygdala unless the pipe or piping network has some feature that leads to a disturbance of the flow stream that can be characterized relative to the flow rate, then probably no. A straight length of pipe generally leads to fully developed laminar flow that's free of eddies and thus any detectable 'noise' that could be 'heard' by transducers. $\endgroup$
    – docscience
    Oct 7, 2015 at 14:32
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This is a very vague questions, but the answer is most likely "No".

Piezo sensors produce a charge proportional to how much they are squished. That charge on a known capacitance results in a voltage, but at infinite impedance. Since infinite impedance is not practical in the real world, the steady state output of a piezo sensor will decay to 0 over time. Put another way, piezo sensors only show you the high pass filter of position, not position directly.

This means that piezo sensor are no good for measuring steady state. They work on changes. Therefore, a piezo sensor can't detect "constant" anything, including constant water flow, regardless of how that is connected.

If this "constant" water flow causes small vibrations due to turbulance or whatever, then you could possibly arrange a piezo sensor to detect it. However, note that this isn't really measuring "constant flow" as you asked about.

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  • $\begingroup$ Thank you, Olin. I'm sorry that my question was vague but I'm glad that you were able to penetrate it anyway. Your answer was actually quite helpful. I am attempting to detect water flow. I felt that, if water flow generates a vibration of ~ 1,000Hz (that's a guess) then a piezoelectric sensor would pick it up all the time or "constantly". However I seem to be only able to detect when the tap is turned on or off. Your explanation explains this. But why can't it detect the flow more continually? Would I need a piezoelectric sensor which is sensitive to the frequency of movement of the water? $\endgroup$
    – Amygdala
    Oct 4, 2015 at 13:20

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