Why do pressurized cans warn against puncturing them "even if empty"?
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@robryk Because empty doesnt always mean empty... A tank can have gas stop coming out and then when you turn it off pressure will build up again as it warms. This is particularly true if they are a liquid at pressure.
So it can read as empty and gas might not come out but when you go to puncture it it may have re pressurized by then.
The thing that caused me to wonder now was a shaving cream can, where I could test the pressure inside by deforming it with my fingers first.
I also saw such a warning on camping gas bottles (some mixture of propane and butane), which did have a way of ensuring they're empty provided: there's a plastic shim you can push into the valve to permanently open it (so that you can vent the bottle after you used up the gas). In such cases I don't really see a reason not to amend it to say "don't puncture unless the shim has been inserted for a few minutes".
@robryk butane is particularly troublesome because its a liquid at pressure.
Sure, but if there's an open vent in the tank, and there is noticeable sound coming from the vent, then I'd assume that it's at atmospheric pressure and thus can be punctured without much risk (assuming nonsparking tooling and/or long duration venting upside down so that it's unlikely to flash). I wonder whether the warning is trying to warn me of some remainign risk that I don't see or whether it's (overly) simplified.
@robryk yea if you hold the vent open you can safely puncture it. But for liability reasons they wont say all that.