On Thu, 18 Apr 2019 17:39:24 +0100
newshound wrote:

That seems a pretty neat idea to me. My analogous problem is a 100
litre electric sprayer that sits in the back of my all terrain
vehicle, recognising when the spray stops for any reason. I have
tried a little wireless TV camera with a monitor on the dashboard but
it wasn't really effective.

Fit a flow meter and monitor it with a microprocessor that can flash
LEDs at you.