Non contact flow sensor for heavy oil ideas please!!

how do they get the fuel to flow out of the pipe? is there a valve? you could put a sensor on the valve to detect flow :)

also, if it's a metal pipe, you could just use thermistors and monitor the temp of the pipe in two spots. when the fuel flows by them, both sensors should show a similar change of temp... triggering an alarm while avoiding false triggers.

actually... you could stick them close together and note the sequence that the thermistors change. if therm A changes, then therm B changes. then that would trigger the alarm. if both A and B change at the same rate, then that will just be the ambient temperature going up

I wonder if a speaker and mic could be used in a gas tank to find a resonant tone that would correspond to the fuel level? Then if the level changes you would know. Or if something like that would be possible from the side of the tank?

Or even if the tank is tapped, maybe it should return differently depending on how much fuel is in the way that you can 'tune' glasses of water/wine/whatever to play music? You could tap it with a piezo and probably pick up the echo with the same disk.