I worked for a local Hardware store for almost 20 years. I've filled thousands of propane tanks. We put 20lbs of gas into a tank the weights about 17lbs empty. The exchange tanks we see are only filled to 15lbs. But many of the exchange tanks are old expired tanks that have been repainted and put bank into service. Tanks by law are only good for 12 years. The date is stamped on the handle. The exchange tanks are painted, new valve installed and a new date sticker put on. But in the long run it's still and old tank. We've had some that are so badly rusted the bottom blows out when we start filling. My advice, buy a new tank and have it filled. It's good for 12 years.