On 18/02/2020 12:39, Jim GM4DHJ ... wrote:
why does raw butane set off an ionising smoke detector ?

I'd be surprised (and worried) if it got near to one. Butane is denser
than air and tends to 'settle' near the floor. If there is enough in the
room that it reaches the smoke detector (which is normally mounted high
up), there is a LOT of Butane around.

I assume this is in your caravan? You should have 'vents' in the floor,
especially where there is Butane equipment (ie possible leaks). The idea
is, any Butane which has leaked 'sinks' out of the vents. That is why
LPG and Boats can be a problem- you can't have vents! Boats should have
LPG detectors but I'm not sure how they work.