CO2 Transfer Question

[agentbud]

I have a 20lb CO2 tank and a transfer hose that I use to refill my 5lb CO2 bottles with. Works great. But I got to wondering. How does it know to move the CO2 from one bottle to the other. I assume that when the 20lb bottle is full, the pressure is enough to force the CO2 into the empty 5lb bottle. But what about when the 20lb tank is getting low, for example only has about 1lb of CO2 remaining. It seems that once it transferred half of that and each tank then has .5lb, it would equalize and not transfer the rest. What causes it to transfer the whole 1lb? Or does it? I have heard that getting the 20lb tank warm and the 5lb tank cold helps with the transfer but what if both tanks were the same temp, would it still transfer the entirety of the CO2?

 

[Whisky River]

No. The two tanks will equalize unless there is some force driving the liquid/gas from the 20 into the 5.

 

[Dland]

CO2 in tank is in liquid form when under pressure. As it is being released from tank, it goes to gas form, so it just goes in second tank at the gas pressure.

I'm not sure the best method of transferring in liquid form, I think a special tank with bottom feed dip tube. Good to be careful, un regulated CO2 can cause serious freeze burn when going from liquid to gas.

 

[Deadalus]

I sit both bottles down and give them a stern lecture about sharing with their younger siblings. And if they don't behave somebody is going in the fridge to cool off.

 

[agentbud]

CO2 in tank is in liquid form when under pressure. As it is being released from tank, it goes to gas form, so it just goes in second tank at the gas pressure.

I'm not sure the best method of transferring in liquid form, I think a special tank with bottom feed dip tube. Good to be careful, un regulated CO2 can cause serious freeze burn when going from liquid to gas.

the 20lb does not have a dip tube but I invert it b4 transferring so that it transfers liquid.

 

[day_trippr]

I have a dedicated 20 pound siphon tank that I use to refill my collection of 5 pounders. I usually "freeze" the 5ers for 2-3 hours before filling and with the siphon tank sitting around 60~70°F can typically push 3~3.5 pounds before both cylinders reach an equilibrium state. Right now the siphon tank is down to maybe its last 2 pounds - about when I'd usually take it in to be refilled - but this time I want to see how close to empty I can get it...

Cheers!

 

[Whisky River]

If you have a substantial amount of liquid co2 you can probably get most of into the smaller tank using the gravity method. But if the larger tank is close to empty the last bit of liquid co2 will turn to gas and then you will just have gas pressure trying to equalize between the two tanks. Maybe one of our resident math experts could figure out at what weight the last of the liquid becomes gas.

 

[Wigglebutts Brewery]

My last transfer was from inverted #20 tank to frozen empty #5 tank. #3.5 transferred easily before process bogged down so I shut everything down. That’s typical for me… I don’t think my #20 tank has been less that 50% at transfer start. I’ve got two #5 tanks and a #20 tank exchange is only $11 so this process is a real win!

Just another point of reference.

 

[IslandLizard]

This pressure equalization point makes me wonder how they actually fill tanks to 100% of their stated capacity. Or don't they...

When I had my 15# tank filled at a (semi-local) fire protection outfit, they would first transfer some CO2 (maybe a pound worth?), then blow most of it out to chill the receiving tank. Then they filled it up "completely." But I never measured (by weighing) how full that actually was.

Same for the tanks I get now with swapping (no-one seems to fill anymore, around here).

 

[agentbud]

My last transfer was from inverted #20 tank to frozen empty #5 tank. #3.5 transferred easily before process bogged down so I shut everything down. That’s typical for me… I don’t think my #20 tank has been less that 50% at transfer start. I’ve got two #5 tanks and a #20 tank exchange is only $11 so this process is a real win!

Just another point of reference.

$11 for a 20# exchange? Wow, that is a great price. A 20# exchange for me was $39. But that still a whole lot better than when I was paying $28 for each 5# exchange.

 

[agentbud]

This pressure equalization point makes me wonder how they actually fill tanks to 100% of their stated capacity. Or don't they...

When I had my 15# tank filled at a (semi-local) fire protection outfit, they would first transfer some CO2 (maybe a pound worth?), then blow most of it out to chill the receiving tank. Then they filled it up "completely." But I never measured (by weighing) how full that actually was.

Same for the tanks I get now with swapping (no-one seems to fill anymore, around here).

I just picked up a 20# exchange. The tare weight stamped on the tank said 25.0 lbs. I weighed it when I got home and it weighed just over 45 lbs so they definitely got a full 20 lbs of CO2 in it.

 

[Bramling Cross]

This pressure equalization point makes me wonder how they actually fill tanks to 100% of their stated capacity. Or don't they...

They don't.

Cheap bastard that I am, I always weigh my tanks before I take them in. I use three LHBS. The primary shop that I use really fills your tank and does a good job. They're good people and I know them well, I typically get about 4.5lb-4.75lb. If I go in on a Tuesday around opening time and I tell them I'm in no rush and I know you just opened, take your time, so I'll wait in my car, I might get 4.85lbs because they've got nothing better to do and the store is absent of idiots bragging about their swill and asking dumb questions. Getting to 4.8lbs takes a long time. I'm cool with that because they charge by the pound, so I'm getting what I pay for.

The other two shops are expensive flat-rate exchange shops, leave a cylinder, take a cylinder. That *is* a rip off. The shops themselves aren't short-changing you, it's the company that fills the tanks that rips you off. On average I get about 4lbs from the exchange shops and they're at least ten bucks more expensive for a "fill." I've also had both sell me bum cylinders that leaked before I put them inline. Both, however, honored their products and exchanged my leaky cylinders for full cylinders.

It all works out in the end, though. I simply exchange my out of date tanks at the exchange shops for certified tanks, then refill them at the shop that actually fills tanks for a couple years. Then I repeat the cycle.

 

[Whisky River]

I think how much of a fill you get depends a lot on the type of location you get your tanks filled at. If you are getting it done at a small shop or someplace that’s just filling your tank off another larger tank then you will certainly get less than a full fill. However the industrial shops use a pump to force the liquid co2 into your tank so they can fill it to the full capacity if they want. Sometimes they choose not to…maybe for a claimed safety margin in hot climates or maybe just because they are cheep idk.
This video (not mine, no affiliation) is just one example of an industrial fill. (The fill starts about 1:30)

 

[whoaru99]

I think how much of a fill you get depends a lot on the type of location you get your tanks filled at. ...
However the industrial shops use a pump to force the liquid co2 into your tank

This.

Conceptually not unlike getting a LP tank filled. Tank to tank direct is one thing, but at fill stations around here there is a pump involved to push in the scaled amount.

 

[Wigglebutts Brewery]

Our LHBS uses a pump to complete the fill