--- In email@example.com, "Duncan in Bideford"
Or weigh it:)
All bottles should have a full and empty weight stamped on them
somewhere. I use a fish scales (spring scale) to weigh mine.
Maybe someone could invent a scale that sits under the bottle. You
tell the scale the full and empty weights and it warns you when you
are getting low and gives you a bar graph of the the amount of gas
left. Of course this would only work when stationary.
Maybe that's the route to my first million:)