Sodastream refilling woes - what anm I doing wrong?

[RoomWithABrew]

It's not weight or mass it's pressure that matters.
You are right once the pressure is equal in both cylinders flow will stop. That is why you get a better fill with a fresh donor cylinder than a nearly empty one.
When your main CO2 cylinder is filled they use a compressor between their large CO2 cylinder and your smaller one. Otherwise you'd have the same scenario as large cylinder to sodastream.
Dip tube cylinders 're normally used as fire extinguishers so must be vertical.
CO2 gas pressure at the top of the cylinder exceeds atmospheric pressure and forces liquid CO2 up dip tube and out nozzle where it vaporises in warm atmosphere and lower pressure.
Think of our cylinders having a very short dip tube so need to be inverted so the tube can be filled.
Can make your ead hurt as it's nearly rocket science!

 

[peebee]

Weight/mass doesn't matter? Gravity is the only force available; mass must matter moving a liquid. Liquids are not famous for their compressibility. Pressure is the same throughout, the system must be sealed, atmospheric pressure plays no part: If it did, you'd have a fire extinguisher.

I was a "rocket scientist" ... well sort of ... okay barely then. Benwell Fireworks (gawd, they were bad!).

 

[RoomWithABrew]

Think it's pressure gradient that moves the co2 liquid from big cylinder to sodastream or my beer from keg to tap which is 4 foot above it.
You're not suggesting a pressure difference wouldn't work in zero gravity are you?
Also if gravity was the only force how would the heavy liquid CO2 go up the fire extinguisher dip tube and come out the top?
Similarly a small mass of very high pressure gas in the sodastream bottle if it could be done safely would move into the other cylinder even if full of heavier liquid until equilibrium occurs.
Liquid rocket fuel a little more complex than solid fuel rockets!!
But all this aside a sealed connection between the two cylinders should transfer fine if the valve behaves.

 

[peebee]

Pressure gradient plays its part. I think (i.e. no references, no proof, and a significantly messed up head!) this is behind cooling the receiving cylinder? Cooling reduces the pressure needed for the liquid CO2 not to boil off. The equilibrium between liquid and gas changes and liquid from the warmer cylinder moves to the cooler to equal the pressures. It won't take long for temperature and pressure differences to be the same again, and flow stops. Unless something takes over, like gravity.

A fire extinguisher is not a closed system, we can ignore that. A liquid fuel rocket isn't a closed system either, but they don't rely on pressure to shift the fuel and have stomping great big pumps (I never needed anything like that for my firework rockets!).

Pressure in a liquid gas cylinder, whatever its size, is entirely down to temperature. I think it's something like 850PSI for CO2 at 20°C.


Something that is missing from my attempt to explain this (and I mean explain it to myself); with liquid gas travelling to the lower cylinder through the connecting "pipe" (or whatever) how does the gas travel back to keep the pressure equal? Same pipe, (which would involve a lot of "glugging" if it were water)?

 

[RoomWithABrew]

Don't think of it as two separate spaces but one. Gas and liquid phases will be in balance if you were able to measure headspace ( gas element) and liquid pressure in either cylinder once equilibrium occurs which doesn't take long.

 

[peebee]

... if you were able to measure headspace ( gas element) ...

What's this? You're changing tactics. You altering the definition I'm using of pressure? That'll do it, ... nope I can't keep up, I surrender

Now, do something useful, like answer that question I asked about keeping the pressures the same in two containers when the only connector is also used for transferring the liquid one-to-other? In this I think, is the answer for people having trouble filling their Sodastream cylinders?

 

[RoomWithABrew]

@peebee
The valve closes on the sodastream once the pressure on the inside of the pin is greater than the donor cylinder gas coming in, there isn't a spring that needs to be " fought " against by the incoming CO2 but if the inside of the sodastream pin is a relatively large surface area then could the pressure which is force over area slam it shut prematurely.

Perhaps position of the sodastream cylinder would influence this? ie donor cylinder inverted but is it better to have the sodastream cylinder angled down, horizontal or slightly up. My gut says sodastream angled up, then detach when full vertical and vent a bit of gas then retry and see if more goes in. You'd need to weigh between these attempts.
We need a physicist.

 

[peebee]

Stop saying "gas"! You are confusing people. "Liquid" or "Liquid CO2" reminds people what they are dealing with. There's no pressure (nearly incompressible), but it has weight, but that can be confusing do perhaps doesn't need emphasising? Anyway, I said I'd surrendered that argument, so moving on to that question I posed ...

In a sealed pressure system and a substance at equilibrium with its gas and liquid state, with two tanks one higher than the other and connected by a "pipe", does it actually need gas to flow back up to replace liquid moving down (the same pipe)? Would the gas in the lower tank not just condense into liquid to maintain the equilibrium? And visa-versa. And changing state does things to temperature ... now me 'ead's really hurting! But this is just "A" level Physics stuff, "O" level even? ... Then why doesn't the lower tank simply fill 100% in practice? (Aaaarrhh!).

We need a physicist.

I regularly see a psyc... ahhhh!, "physicist", I get it now (Greek words and their stupid spelling ... )

 

[peebee]

Don't think of it as two separate spaces but one. Gas and liquid phases will be in balance if you were able to measure headspace ( gas element) and liquid pressure in either cylinder once equilibrium occurs which doesn't take long.

Another "Ahhh". Isn't this probably relevant to my last post? (Agreeing with it?).

You know, your writing is as bad as mine! It fails to convey its meaning very well. Don't take up teaching kids at infant school! I know this from experience 'cos I'm only three you know: I have been that for the past eleven years.

 

[RoomWithABrew]

@peebee
I think we agree that equilibrium should occur quite easily whilst a very simple connection occurs.
It may well be something to do with the dynamics of that sodastream valve which doesn't screw open or shut as the main cylinder valve does.

 

[Steven Monkman]

I guess you've read my battle to get these to fill?

https://www.thehomebrewforum.co.uk/threads/refill-sodastream-bottles.83180/post-839298
(From that post #20 onwards - don't skip through, there are errors covered in later posts).

But you do seem to be doing all that?

But one thing you do talk of that might be relevant: You mention "gas" a lot and "hear the gas start flowing then it stops after a few seconds": You should not be interested with transferring "gas", you are transferring "liquid". Make sure the liquid can flow (no air lock or expecting liquid to flow uphill).



And always remember: The operation is full of potential danger! High pressure gas obviously, and escaping gas causing very cold temperatures (skin instantly sticks to materials at such temperatures).

 

[Steven Monkman]

Hi I have had similar issues, but read on liberty the the new soda streams have a NRV fitted so with the dover upside down just crack the valve a tiny amount and leave for 5.or 10 min. Its not.oerfect and takes a bit of playing around but gives better results.
Ps, I have not tried freezing the bottle, but does make sense.

 

[Steven Monkman]

Apologies for the spell checker and fat fingers!!!