flow switches are available (try a commercial plumbing outlet)
otherwise the can be made... here are two ideas.
stick two stainles steel wire electrodes along opposite sides (of the inside) of the tubing and put a magnetic field through the gap. (not across) measure the voltage with a high impedance voltmeter.
may not work well if mounted too close to the pump or if the pump develops an electrical fault. running the water through a metre or so of metal pipe (copper/stainless/galv) pipe before the sensor may ba all that's needed,
This one may collect rust particles if they are present in the water and the flow is too slow.
some boats use a setup like this for their speedometers.
you can use copper wire electrodes to test-drive this one, but stainless lasts much longer in service.
another option is this, (cross section)
if garbled view in a fixed-pitch font (like courier) or paste into notepad
| | | | | |