One reference that I have gives 15 psi as the pressure when
acetylene will explosively decompose. Another states:
"Acetylene is not especially toxic but when generated from
calcium carbide it can contain toxic impurities such as traces
of phosphine and arsine. It is also highly flammable (hence its
use in welding). Its singular hazard is associated with its
intrinsic instability, especially when it is pressurized. Samples
of concentrated or pure acetylene can easily react in an
addition-type reaction to form a number of products, typically
benzene and/or vinylacetylene. This reaction is exothermic.
Consequently, acetylene can explode with extreme violence if the
pressure of the gas exceeds about 200 kPa (29 psi) as a gas
or when in liquid or solid form. It is therefore shipped and
stored dissolved in acetone or dimethylformamide(DMF), contained
in a metal cylinder with a porous filling ( Agamassan ), which
renders it safe to transport and use, given proper handling."
If the material in the tank is a liquid, then the pressure gauge is
reading the vapor pressure (which can vary highly with changes in
temperature). Opposite to this is compressed gas, where you are
reading the actual gas pressure. Vapor pressure will always be the
same as long as some liquid is in the tank. Gas pressure will
decrease as the amount of gas decreases in the tank. Propane is also
a liquid. Oxygen is a gas in the tank.
Just be careful out there.
John Atwell Rasmussen, Ph.D., AJP