Detecting water flow in a pipe

Thread Starter

cmartinez

Joined Jan 17, 2007
6,593
I need to detect the flow of water inside a pipe, as sensitively as possible. I initially considered either a small turbine or a paddle wheel, with magnets attached so that a hall sensor could detect their rotation. But I'd like the device to be long lived, and a rotating shaft might wear its bushings prematurely. I only need to detect the presence of flow. Precision measurement is not necessary.

The pipe I'm using is 1-1/4" Schedule #40 PVC. A reasonable lower limit for flow detection would be 0.1 lt/min. That would yield a flow linear speed of approximately 1 mm/sec.

If I were to use a turbine, then a small jewel acting as a bearing would be needed at the downstream end of its shaft, and that's a little too complicated for what I want. And if I were to use a paddle wheel, the device would be limited in that the pipe would have to be installed in such way that the wheel's shaft would have to be horizontal, otherwise the wheel would lean its weight on one of its sides and grind against the bushing.

I own a 3D printer, so fabricating a complex part in ABS is not a problem for me.

A sort of astable vibrating mechanism... something like a wiggling snake comes to mind. But I haven't seen anything like it out there ... any thoughts?
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,593
Since you have a 3D printer, you could look into Venturi meter, no moving parts, long lasting :)
Thanks for the suggestion, but a venturi would only create a pressure differential that I could not be able to measure under the constraints I mentioned.
 
An ultrasonic system may work, even at that low flow rate. I would put a transmitter between two receivers and measure the differential frequency shift in order to get better resolution. The down stream receiver would see an increase in frequency and the up stream would see a decrease. You could also do a phase comparison to detect the shift. If the transducers are place equal distance apart then at no flow you would have no phase shift and increasing phase shift as flow increases.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,593
An ultrasonic system may work, even at that low flow rate. I would put a transmitter between two receivers and measure the differential frequency shift in order to get better resolution. The down stream receiver would see an increase in frequency and the up stream would see a decrease.
Thanks for trying to help. But I must insist, I need to either rotate or shake a small magnet so a hall effect sensor will pick its presence. That's a design constraint and it can't be changed.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,593
In the previous discussion the constraint was that there was no access to the internals of the water pipe.
Without such constraints I would investigate measuring Doppler shift of a laser beam.
Well, in this case there is access to the internals of the pipe. And my question is, other than a paddle or a turbine, what else is there that I could use to detect its motion?
 

AnalogKid

Joined Aug 1, 2013
8,140
What is the fluid?
Any suspended solids?
Any minimum aperture between the edges of the sensor (vane, blade, whatever) and the wall of the pipe?
Any maximum back pressure spec?

ak
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,593
What is the fluid?
Water, as I stated in post #1

Any suspended solids?
Nope... it's domestic water.

Any minimum aperture between the edges of the sensor (vane, blade, whatever) and the wall of the pipe?
No minimum aperture required.

Any maximum back pressure spec?
Only that present in a domestic water pipe.

I don't know if it would work at such a low flow rate, but how about use a Karman vortex street to waggle a thread-suspended magnet?
That looks very, very interesting, Alec. I'll take a look at it. Many thanks! :)
 
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