In article , "AL"
wrote:

Doesn't acetylene combust spontaneously at around 15psi?

No. It doesn't combust as normally understood (fuel plus oxygen
combining) at high pressure. What's actually happening is a breakdown of
the bonds between the atoms of the acetylene molecules into simpler
compounds with lower bond energy, releasing the energy of the previous
bonding scheme as heat in one massive dump. The proper term is
"deflagration". Any fire that results is due to the heat of the
breakdown getting combustion started on whatever acet molecules are left
as oxygen comes into the mix after the balloon breaks from the initial
pressure jump of the deflagration.

Can you get 15 psi by squeezing the baloon?

Yes. You can even get it without squeezing the balloon. First-day demo
in high school welding shop involved the teacher loading two calibrated
"color-change" balloons - They started out dark green, and as their
calibrated pressure was reached, went almost clear - in the back lot
from a remote-shutoff hose run from the acet manifold. First one maxed
out at 12 pounds, and went "pop" in standard, fairly unimpressive
balloon fashion just before it was totally clear. Second one was rated
for 20 pounds, and erupted into a suitably impressive fireball as it was
just reaching "almost transparent".

Does it have to be in the presence of oxygen?

No. As mentioned, it's not combustion as normally understood (fuel plus
oxygen combining), but a breakdown of the bonds between the atoms of the
acetylene molecules. It'll happen just the same regardless of presence
or absence of oxygen.

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