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How I refill little propane bottles.
A story of savings and convenience
John Fuhring
Engineer Emeritus and Backyard Mechanic

    I use a lot of propane from small bottles.  I have a tent heater, a gas camping stove, a small patio heater and various torches I use in my micro blacksmithing hobby and they all require those small (one pound) propane bottles.  

     Small propane bottles are very well made and it is such a shame and such a waste throwing them away after they are empty and goodness knows I can empty one pretty quickly when I'm working.  Then there is the inconvenience of having to go to the store and buy more when I'm down to the last few onces of propane and I still have work to do.

     I have solved my propane problem by buying a high quality five gallon tank -- with a gage that tells the amount left.   Filling that large bottle at a station is quick, easy and cheap and then you have months and months of propane without having to worrying about running out.  Because I am so cheap, I have accumulated a whole collection of small propane bottles and now I'm glad I have them because they are now all refilled.

     Soon after buying a good five gallon tank, I bought a brass fitting that allows the transfer of liquid propane from large bottles to small bottles.  While experimenting with the bottles, I learned two important things.  One is obvious and that is that you have to have the big bottle upside down for the liquid propane to flow into the smaller bottle.  The other one is also very important.  In order to transfer the maximum amount of liquid, the small bottle must be cold and quite a bit colder than the big bottle.

     To get the small empty bottles really cold, I place them in the freezer section for a few hours or over night.  Being so cold, the pressure in the small bottle is very low and when you go to connect it to the supply of warmer, higher pressure liquid from the large bottle, the propane flows right in and completely fills up the small bottle.  You can actually get substantially more propane in the cold small bottle than what is in a new bottle of propane so you have to be careful.  When using my big propane torch, I have to let it burn for a time while vertical or else liquid gets into the torch and causes unstable operation.

     Through experimenting I have discovered that gravity feed is not enough to fill a small bottle, there also must be a pressure differential so that the liquid propane from the large tank is "sucked" into the small tank.  The Gas Laws tell us that propane bottles will be at a relative vacuum compared to the warmer supply bottle, so this makes sense, but I didn't realize how much it really mattered until trying to fill a warm bottle and getting very poor results.

     With the big bottle right side up and the adapter valve closed, screw on the small bottle so it sticks out the side in a horizontal position.  Carefully and without putting any stress on the bottle sticking out, turn the large bottle upside down and then open the valve (yes, it is awkward).  With the valve open, you will hear the liquid flowing into the small bottle and after several seconds, the sound will decrease and stop.  When you can no longer hear liquid flowing, close the valve while the big bottle is still upside down (yes, it is awkward).  After the valve is closed, stand the big bottle right side up, but be very careful not to bump or put any stress on the small bottle sticking out the side.  Screw off the small bottle and you will get a big "PUFF" of gas that will come out when the two bottles are released from each other and you are all done.  Remember, if your small bottle was really cold, it will probably have more liquid propane in it than a new bottle.

     There is one final warning: please folks, for goodness' sake DO NOT perform this task inside a house or enclosed garage or workshop or you may not live to regret it.  Do this ONLY out in the open and far, far away from any sparks or open flames that might set off the propane that always leaks from this operation.  Never, ever bring a large propane bottle inside your house and it is probably wise to have no more small bottles in the house or workshop than the one(s) you are actually using at the time.