the myth is that if you get an ordinary c02 cylinder refill for a drinksmaker you can quickly turn it into a small handheld scuba tank by filling it with air and ataching a mouth peice that will last you half an hour. great for a quick underwater escape!

while you can get an "escape tank" that is this size, there are two flaws in the myth:

a CO2 cylinder is made not to interchange with a breathable air cylinder, to avoid mixups.

second flaw: when I was diving, I had a 2000 CF tank. it lasted me an average of 40 minutes... now, in the fire service, we use 4500# tanks, that again, last me about 40 minutes, unless I am working hard. these bottles are much bigger than the CO2 cylinder.