Gas does not conduct current. Use a float switch or potentiometer or one
of those graded optical sensors. If the tank is transparent you can use
miniature reflective optocouplers glued to the wall. I have used this and
it works also with model fuel, but only with a level tank. Ignition is a
problem if you have enough Joules to spend on a spark or glow, no matter
what the voltage. Keep it well under 0.0001 Joules and you won't be able
to ignite it probably. This is easy with low voltage. F.ex. 10uF at 5V
decoupling that could discharge through a short in the tank will be
0.00025 Joules, i.e. too much imho. The danger of explosion will be
greatest when the tank is empty. Use an approved tank with an approved air
valve built in (this is important) and screw it on tightly every time.

Peter

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