I just got my 48 cubic inch steel (actually, I think it's aluminum) PMI Pure Energy nitrogen tank the other day. I noticed that it has CO2 engraved on the side of it. I'm just curious to know why that is. Do they just take a CO2 tank and slap a reg and guage on it or what? Also, I didn't notice a Hydro dating on it. Do steel tanks need to be hydroed? Isn't it every five years? Shouldn't it be on the tank somewhere?