It doesn't matter. A gas is a gas. Heated, they expand. Cooled, they contract. Alternately, compress them they get hot, or evacuate them and they cool. Boyle's Law is pretty simple. Steam just provides a dramatic example because it contracts so very much when it is cooled that it actually creates a vacuum that will crush a can."nomojo65"I think we might be confusing water vapor (stm.) vs. breathing air, Remember atmospheric psi is 14.7 psi @ sea level or 29.9in. Hg, and you can boil wtr. @ about 75 Deg.F. in a perfect vacuum, If I remember my chem. & Phys. right? This coment is referring to original post. I sure hope the breathing air from my scba tank does not contain any H2O