it is a gas,its a v.o.c an organic compound when compresses into a tank the chemical structure slightly changes to a liquid but will always remain a gas it's just a chemical reaction. so to the point,most people run bigger tanks like 20's or the 80 pounders,every place i have had fills from uses scales and gauges from there fill center,they check the gauge as its on a weigh scale during filling,from personal experience a full 20 pound tank weighs in around 35-40 pounds full, but honeslty it only costs about 12 bucks to fill a 20 pounder,the bitch part about using co2 bottles is trying to be incognito about the several tanks your bringing in and out of your house all the time"well thats my paranoia"