$10 solution? I guess you don't build many things. How much is your time
worth? How much effort do you think it will take you to source the
apprpriate parts, even if you could? If you couldn't, do you have the tools
to make them? Material cost? Then you have the performance, accuracy,
liniarity, data display and verification issues to deal with. I have both
the knowledge, the test equipment and the machine shop to do this and I know
that for me to make one would cost me several kilobucks in time and
materials. Perhaps you want to share what your smoking.
Steve

"Sjouke Burry" wrote in message
...

If you want to build one, two solutions.
1.
Put a small propeller in a piece of glass or
transparent tube, a LED lichtsource on one side,
a phototransistor on the other side.
Clean up the phototransistor signal, and add a counter.
Then calibrate your counter by withdrawing a
known amount of water.
2.
Put a small propeller with magnet attached in the tube.
On the outside place a reed relay close by.
use switch to produce pulses, for the rest see solution 1.

Both are 10 dollar solutions.