Hello /sci,

I have a question about something I did and cannot explain it fully. Picture related of my question, and I'm using metrical (kilograms) units, please have patience.



So I have these two LPG gas cyilinders at home, used for cooking and water heating. Cylinder A had inside, before the transfer, 17 kilograms of LPG. I know that by subtracting the rated weight of the cylinder, listed at 19,7KG, to the whole weight I measured on a body scale, 37KG. That would be then 37-19,7=17,3KG, let's round it to 17.

Now, cylinder B was empty, I mean I literally fully opened the valve and let it vent everything to open air. 

So, I weighed the empty cyl.B and it's weight is 15,4Kg. Then I carefully arranged the two canisters like pic related, connected with an appropriate flex hose, and slowly partially opened both valves. I saw early on the freeze cooling of the valves, then when they went to normal temperature, fully opened them and let them transfer slowly for about an hour.

After the whole operation, I closed valves and detached the hose. As expected, some liquid LPG escaped slowly, vaporizing instantly. Then I weighed the "full" cylinder B: 22,2KG, so only some 7-odd kilograms of LPG went into the gas cylinder. I did't expect a fully loaded cylinder (I usually charge it with about 16 Kg), but 7KG of LPG is kinda of... too little? 

Yes of course I know that charging a gas cylinder involves a PRESSURIZED charge so they squeeze in the LPG overcoming any other antagonist pressure, but I thought that, putting the donor cylinder A upside down, in a upper position, slowly the LPG should have transfered, once the pressure stabilized, like a liquid into cylinder, almost filling it, hence why I waited an hour or so leaving them transfer. 

What happened there? Thanks.

most of it evaporated when you allowed it to warm to room temperature by opening the valves (the rubber hose is not an appropriate insulator)

>it to warm to room temperature

Everything was at room temperature anon. These are your common gas cylinders you use for, example, outdoor cooking, camping, caravans, portable heating stoves, etc. simple steel canisters that are charged, stored, sold and used at everyday's all-weather applications (well LPG doesn't work below a certain ambient temperature but we got the idea). 

The cooldown happens when you open a pressurized gas to normal pressure, like it would do a simple air compressor when you fully open the main air outlet, it would freeze temporarily due adiabatic cooling. Also there wasn't any gas leakage anywhere, I stand there (at a safety distance) but I couldn't smell even an hint of escaping LPG.

>>15413500 (OP)

Some of the liquid evaporates when it enters the second tank and takes the space the liquid could've used. You need to cool down the tank to condense some of the gas and let more liquid enter from the first tank. Heavier CO2 tanks can be simply chilled beforehand to act like a heatsink, but LPG tanks don't have enough thermal mass so you need to cool it down somehow as it's being filled.



It's easy to overfill the tank that way, so always check the weight before and after.
