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I've got an inground swimming pool, and was inspired by an ill-formed question that was recently posted. I would like to non-invasively measure the volume of water my pump is circulating through the pool over a given period of time. By non-invasively, I mean I would rather not have to hack into the plumbing and embed a sensor into the stream, but rather I'd like to measure it inferentially somehow (e.g. perhaps the water current actually has an electrical current that can be measured through the PVC pipe with a hall effect sensor an an instrumentation-amp — not to bias the answers or anything).

For the sake of bounding the problem with some requirements, I'd be happy with an accuracy of ±1 gallon/min and a component cost of less than $100. To be clear though, these are artificial requirements, since I just want to build something like this for myself.

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A quick google search turned up a website with a variety of flow measurement techniques and even full products for sale (albeit, quite expensive).

Some of these are invasive while others are not.

The most promising non-invasive ones I've noticed:

Using ultrasound waves.. Takes advantage of the Doppler effect to measure fluid velocity, which can be used to calculate volume flow rate.

Using magnetic induction.. Takes advantage of Faraday's law generate a voltage from moving ions through an electric field.

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  • \$\begingroup\$ For Doppler flow, you need some sort of sound-reflective substance in the water. People often use cornstarch or something similar. Magnetic induction would do the job, but signals are small and it can get tricky. \$\endgroup\$ Aug 28 '12 at 22:59
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    \$\begingroup\$ There are ultrasonic flow meters that are entirely non-invasive and don't require anything to be added to the water. \$\endgroup\$ Dec 29 '13 at 22:07
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One very simple method if you know the pump specs (gal/min) and assuming pump runs at a constant speed, would be to time the on period of the pump and calculate total from there.
For example (just for completeness) if the pump pumps at 0.5 gal/min and it runs for 5 mins you have a total of 2.5 gal.
Just hack into the pump and tap into the on/off circuit then send to micro.

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    \$\begingroup\$ If it runs at 0.5 gal/min then you can assume 0 water flow and still be within spec. runs \$\endgroup\$ Aug 28 '12 at 21:58
  • \$\begingroup\$ It was just an example, I have no idea what the real pump specs are. The OP says the stated requirements are artificial. \$\endgroup\$
    – Oli Glaser
    Aug 28 '12 at 23:02
  • \$\begingroup\$ I don't want to assume anything about the pump... \$\endgroup\$
    – vicatcu
    Aug 31 '12 at 3:27
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Proton spin / Proton Precession / Nuclear Magnetic Resonance
Swimming pool flow meter ! :-) :

This would actually work ! :-)
Surprisingly cheap to build once you have it sorted out.

Coil around pipe or even against (plastic) pipe.
High current pulse to produce brief strong magnetic field.
Protons in water have "spin" induced with a characteristic frequency related to the field strength, which decays over some seconds. Spin induces RF field.

RF detector downstream slightly looks for time to peak of RF signal which depends on flow rate.

Wikipedia Magnetometer

DIY Proton Magnetometer - with circuit diagrams for transmitter. They use this for field measurement so pick up signal from the TX coil.

Practical guidelines for building a magnetometer by hobbyists.

PPMs - superb report

Related - may be usefu

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Does threading instrumentation down one of the plumbing ducts count as non-invasive?

1- You could thread a non-instrumented propeller down the plumbing, with a magnet on one of the arms and a waterproof-cased hall effect device near the outer radius. The frequency of the hall effect output would be linear with flow. Optical interrupt would work the same way.

2- measure inflow and outflow pressure, and flow should scale with the pressure difference as the hydraulic analog of ohm's law.

3- thread a thermistor down the plumbing in self-heated mode, and it will cool in a way related to flow. If memory serves, if you get the heating current correct this will be fairly linear.

4- Some form of thermodilution-- inject a bolus of hot water somewhere and measure temp a bit downstream, and the area under the temperature change curve will be linear with flow.

5- Thread a pitot tube down the plumbing -- the type with ports both aligned with and perpendicular to the flow.

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Magnetic inductive flow meter.
Fluid needs to be somewhat conductive.
Pool water should be OK.

Alternating magnetic field at right angles to flow.
Voltage is induced across water normal to flow and field.
Non invasive measurement of voltage could be challenging.

[Wikipedia](Manetic flow meter](http://en.wikipedia.org/wiki/Magnetic_flow_meter)

Commercial product

Simple description

Useful

Video with annoying sound

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You could just use any flow sensor and put it on the jet instead of in line with the PVC pipe.

This is not invasive in the sense that it requires no modification to the 'plumbing'. If it was mounted against the jet output it would be minimally invasive mechanically.

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  • \$\begingroup\$ He clearly says he doesn't want to use an invasive sensor. \$\endgroup\$
    – Finbarr
    May 30 '20 at 20:44
  • \$\begingroup\$ But it wouldn't be invasive if it's just held over the jet with some duct tape temporarily. \$\endgroup\$
    – Randomaker
    May 31 '20 at 18:05
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    \$\begingroup\$ 1. I've just looked through your questions and answers and upvoted two of them. 2. I note the frustration you are having with the site. Hang in there !. It gets easier after some experience AND it's very worthwhile being part of it. It may not seem that way at first :-(. 3. This question is just under 8 years old. It has some good answers already. Your answer has some merit (you know that) but the chances of it getting upvotes is smallish, and the chances of it getting downvotes and of being closed is reasonably high. ... \$\endgroup\$
    – Russell McMahon
    Jun 1 '20 at 11:36
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    \$\begingroup\$ ... If you answer old questions it's a good idea to choose ones where you can make a substantial positive contribution. Single sentence (or very short) answers are liable to be treated poorly. You lose rep on downvotes . As you don't have a lot of "rep" when first getting going it's probably wiser to initially avoid answers that do you harm rep wise - having a certain amount of rep allows you to do more things so its actually of some use. \$\endgroup\$
    – Russell McMahon
    Jun 1 '20 at 11:36

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