Where did all this talk about "heat/rupture/explosion" come from? Could someone please offer a shred of physics theory to substantiate any of these "risks". Feel free to correct anything I am about to say: Pressure in a 20 lb tank is the same as in a 5 lb tank. How can the 20 lb tank "rupture" the 5 lb tank - how can more pressure be created than the 20 lb tanks starts with? And where is the "heat" coming from during a transfer? If any liquid CO2 converts to a gas, things get very COLD. The only heat that can be generated would be heating of the small volume of CO2 gas that remained in the empty 5 lb tank as that gas is compressed to around 850 PSI. I'd have to do some digging to find out how much heat that is, but the mass of that gas is about 1/500th that of the room temperature liquid that will fill the empty tank, so I have to wonder how hot things will get since the liquid will absorb that heat, and with 500X the thermal mass (no to mention the tank itself). Also, you definitely don't want a regulator or anything causing a drop in pressure from the 850 PSI liquid in the 20 lb tank. Dropping the pressure would cause that liquid CO2 to convert to a gas and then you are putting virtually nothing into that 5 lb tank (it will be 850 PSI of gas, which is about 0.2% full). Also, gas regulators don't work on liquids. Using one on an inverted liquid tank will either ruin the regulator worst case, or best case, not regulate anything. Which is why you will see that every transfer hose designed for this, is simply a high pressure hose that connects bottle to bottle (with a purge valve). And no, it does not take hours to accomplish (unless you are trying to get that last few percent just prior to equilibrium). Liquid will flow into the 5 lb tank until the pressure equalizes, going slower for the last little bit until nothing flows since tank pressures will become identical. It would be interesting to weigh the 5 lb tank as you time the fill. I would guess the tank will reach 50% full in a few minutes. Not all that different from filling propane tanks, except the LP tanks are lower pressure and they typicall use a pump to transfer the liquid rather than just gravity (although there are gravity fed transfer hoses to refill small LP bottles too).