# Air pressure vs explosive pressure

Alright, this might be a dumb question..

my friend and I are in a little debate. I have built two kinds of "potato cannons": one uses the force of a acetelyne gas + oxygen reaction for the energy (single explosion, flings it far). The other uses actual compressed air: I would pump air into a chamber to about 60 PSI, hit a valve, and it would decompress out with the object. He says that TECHNICALLY, both can be considered "compressed air" cannons. Any thoughts?

russ_watters
Mentor
kronchev said:
Alright, this might be a dumb question..

my friend and I are in a little debate. I have built two kinds of "potato cannons": one uses the force of a acetelyne gas + oxygen reaction for the energy (single explosion, flings it far). The other uses actual compressed air: I would pump air into a chamber to about 60 PSI, hit a valve, and it would decompress out with the object. He says that TECHNICALLY, both can be considered "compressed air" cannons. Any thoughts?
He's right. And the same goes for pretty much any gun. The "explosion" (in most guns, its not really an explosion) just creates compressed gas to push out the projectile.

Just because they both, in the end, use pressure from expanding gas to propel the potato doesn't necessarily mean they are both compressed air. For starters, the 'compressed gas' from the acetylene reacting with the oxygen isn't air, so if you're being picky enough he's wrong right there. Second, like I said, just because the gas is expanding doesn't mean it's compressed. I don't know what the reaction for acetylene with oxygen looks like, but I would expect it to increase the number of moles of gas and I know it creates a lot of heat. The energy for the product gas to expand comes from those two factors, not because it was compressed. If by "compressed air" he meant they both used expanding gas, he was right. In that case, I'd have to agree with russ. The only guns I can think of which don't use expanding gas are rail guns, rubber band guns, and coil guns.

turin
Homework Helper
ophecleide,
If you really want to get picky, then I would agree with your position. But, unless you just want to be objectionable, you can agree that you and your friend are both correct in what the two of you meant by "compressed air."

Oh, and as far as being picky, you might want to actually make sure that you are clear on the meaning of the word "air." The etymology suggests that it could refer to many common gaseous mixtures.

Last edited:
First off, I wasn't the one who had the friend who was arguing with me. Secondly, both webster's dictionary and most anybody you talk to will tell you that 'air' is the gaseous mixture which surrounds the earth and only that. Finally, I know that both definitions for "compressed" may be correct or at least acceptably accurate. That's why I think this is kind of a foolish argument in the first place. I know that dictionary definitions make for lame arguments, but here's what webster's has to say about "compressed air":

air reduced in volume by pressure and held in a container: work done by its expansion may be used to operate machines, tools, etc.

Take it for what it's worth, but this is a general dictionary, not a technical one.

Usually when someone says a device is powered by compressed air, they mean someone actually went in and explicitly compressed the air into a volume then released it, not that someone created a bunch of expanding gas and heat through a chemical reaction.