Refilling your own tank may be illegal where you live and there are serious potential DANGERS.
Don't actually do this! It's a hypothetical to be used after the end of civilisation following the singularity, or if you are marooned on a desert island and need to do a transfer to a small tank to trek across the island to find help. That being said this is probably safer than types of transfers involving liquid syphoning where the bleeder valve has to be opened which increases the risk of fire or freezing burns. Let me stress again, messing around with HIGHLY FLAMMABLE GAS UNDER HIGH PRESSURES IS VERY DANGEROUS. 

Process Overview:
A temperature gradient is used to transfer the gas. Two tanks are joined and one is kept cooler than the other. The propane will condense in the cool tank and grandually fill it. This works because the liquid and the gas above it in a tank are in equiblibrium for any given  temperature. The equilibriums is shifted in favour of liquid by introducing the cooler tank, where gaseous propane will condense. 

1. Join the two tanks with a HIGH PRESSURE hose rated for full propane pressure.
2. Place the tank you're filling in a bucket of iced water, the source tank can sit in the sun/ bucket of luke warm water.
3. Open the cool tank valve, then open the warm tank valve.
4. I havent yet measured the rate of transfer but youre looking at half and hour plus to half fill this 2kg tank.
Keep monitoring how much has been transferred. Overfilling a tank is very dangerous.
5. Turn off the warm tank first so the residual pressure in the hose is minimised.
6. Turn off the cool tank, and remove the hose CAREFULLY. There will be some gas under pressure in it, but no liquid gas if both tanks were kept upright. 

A Few Thoughts:
Don't try turning the filler tank upside down. Trying to transfer liquids will increase the dangers hugely. There are other Instructables on liquid transfers, but I also would' t recommend unless you're a pro in this area.

This process is slow and could't be left unattended, and obviously  should have no ignition sources near. The worst case scenario I can forsee so far is a hose burst resulting it two cyclinders flowing at full tilt. I think the risk of a hose burst should be no greater than if you were attaching a normal high pressure stove to it,  in fact I imagine the pressure in the 2 cylinder system should be roughly that of propane at the temperature of the cool tank.  It probably would  not to be a good idea to heat the warm tank to more than 40 degrees Celsius, which is the suggested max temperature.
<p>In Australia LPG is basically just propane. Propane/butane LPG blends found else where should work too as my understanding is they are aneazeotropic mix that shouldn't fraction out through evaporation - otherwise the gas pressure would change dramatically as you used up the tank.</p>
<p>I've been trying to figure out how to refill a 20# BBQ grill cylinder off my 500 gallon propane tank myself and this might just be the ticket. There is an extra port on my tank and supposably on the inside it's connected to a pipe than runs to the bottom which allows the expanded gas on top to force the liquid into a smaller tank. But finding the fitting seems to be elusive to the general population because they don't want untrained people doing this.</p><p>One way to make sure the tank is not filled to over capacity (&gt;80%) would be to weigh a full tank on a bathroom scale and write the weight on the tank and stop filling when it reaches that weight. That's how they used to do it before they had automatic blow off valves.</p><p>The reason for this is it costs $10 to refill a 20# cylinder but buy renting a 500 US gallon tank (~2000 liters) for my garage and house heater, the gas company charges me $2.23 a gallon delivered. </p><p>But I just did the math and a #20 cylinder holds 4.3 US gallons so the savings would be less than $1.00 . Now if exchanging those Blue Rhino tanks which are not filled to capacity (like 3 gallons / 70%) and cost $20+ was your only option, then this would be cost effective.</p><p>However East Tennessee is like living over top of a bomb due to all the natural gas wells under my county so natural gas and propane is cheap. Sometimes half as much as petrol.</p>
This is a great method. Does it work for transferring LPG too?

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