Exchange bottles are only filled to 60% of capacity, not full to 80%. So instead of your 20# er 80% capacity of 4.7 gallons, you only get 3.8 gallons in an exchange bottle.
Weigh one. A typical 20# bottle tare weight is 18.4# plus 20# propane = 38.4#.
Typical "full" exchange bottles will weigh about 4# less.
It does make economic sense to exchange when your bottles are damaged or out dated. When you do that exchange, pick out the newest bottle in the rack.
Mfgr. date is stamped on the right front side of the collar in m / yr. Visual 5yr. recertification is a sticker on the front.
