Quote Originally Posted by Robert
Ya but the whole should equalize in pressure over time. Pulling the gas into solution. So then it should end up someplace in between.
Butane doesn't dissolve very well in water - or in water-based solutions. The only way it could equalise is through leaking caps or diffusion through the bottle. Even water diffuses through plastic faster than butane does.
Butane is also surprisingly little explosive, and should be quite safe in the amounts used to top up a bottle. A single-use lighter contains enough butane to last several years if only used for filling bottles...

Another benefit of butane is that it liquefies easily, so relatively low pressure is needed to store it as a liquid. If your darkroom ever gets REALLY cold, you may see your butane condense to a fluid. I've only seen it outdoors...
Nitrogen needs a lot more pressure for compact storage - that's why nitrogen bottles are so much heavier. IMO the wight and pressure of nitrogen bottles make these far less safe than a bottle of butane!