CO2 Question

[gnets99]

I just filled my first keg today!!
I have a question about the CO2 tank....I have it in the fridge with the keg and I'm not sure if this is normal....When I first opened the cylinder, the gauge on the tank side said it was in the full range, no problems. It's been going for a few hours now and the gauge has droppped significantly on the tank side (beer side is holding fine). I did do a leak check and all that before I filled the keg and everything was ok. I'm wondering if the 'loss' of pressure is just normal because of the tank being in the fridge or if I have a serious problem? Thanks for the help!!

 

[dantodd]

my tank reads about 800 warm and 600 cold. It is quite likely just due to being in the fridge. Many regulators are also effected by the temperature so you may have to adjust the pressure once the regulator comes down to temp.

 

[gnets99]

That's what I was hoping for. I checked again and after a couple of hours it seems to be holding steady right now. Guess I'll just RDWHAHB and see what happens.

 

[homebrewer_99]

I agree. It reads lower when cold.

 

[Philip1993]

Perfectly normal. Geek time:

Pressure(P) * volume(V) = gas constant (nR) * Temperature (T)
P*V = nR * T

In our case, nR & V cannot change. Hence, if T goes down, P must go down. If T goes up, P must go up.

 

[veganbrewer]

I'm new to kegging as well. I keep my 5lb CO2 tank + regulator in my Kenmore (Sanyo) fridge. I've been wondering, since the gas molecules are less active/more densely packed at a cold temperature, if somebody says "keep your keg at 12PSI" am I going to get more CO2 in my beer than somebody who has their CO2 tank outside their fridge and keeps their beer at 12PSI?

 

[malkore]

nope.
12psi is 12psi. colder liquid CO2 inside the cylinder just exerts less pressure as temperatures cool.

 

[gnef]

when your co2 is in the fridge, be careful of making pressure changes too quickly, as the diaphragm is cold as well, and usually reacts slower, so it is easy to overshoot or undershoot. so if you change the pressure, come back after an hour or so, and make sure it is where you want it to be.

 

[veganbrewer]

nope.
12psi is 12psi. colder liquid CO2 inside the cylinder just exerts less pressure as temperatures cool.

I have a hard time accepting that without some reasoning behind it. I'm not saying that the gauge reading 12psi is wrong, but I am saying that maybe a cold 12psi has more CO2 molecules than a warm 12psi, and this might affect the carbonation level of beer.

If you blow up a balloon at room temperature, the gas inside is exerting X amount of pressure (let's say 5psi). If you throw that balloon in the fridge, the balloon will shrink, and the volume of gas inside will only be exerting Y amount of pressure where X > Y (let's say 2psi). That 2psi of cold gas is a lot more dense, and when you bring it out to room temperature, it will exert 5psi again.

Wouldn't the same thing happen with beer?

 

[dantodd]

Well. when you carbonate your beer it is always cold right? If the cylinder is cold or warm makes no difference. When you look at the carbonation charts they all include atmospheres of carbonation and beer temp to give you a psi to set your regulator. So what you say is correct. It's the BEER temperature and PSI that determine the carbonation level.

 

[paanta]

If you blow up a balloon at room temperature, the gas inside is exerting X amount of pressure (let's say 5psi). If you throw that balloon in the fridge, the balloon will shrink, and the volume of gas inside will only be exerting Y amount of pressure where X > Y (let's say 2psi). That 2psi of cold gas is a lot more dense, and when you bring it out to room temperature, it will exert 5psi again.

Wouldn't the same thing happen with beer?

The balloon analogy is right. If your beer is fully carbonated and you take your keg out of the fridge, the pressure will rise like mad. The amount of gas dissolved in the beer is controlled by pressure and temperature and it's trying to reach equilibrium. Higher pressure means more gas in the beer. Lower temperature also means more gas in the beer. If the temperature starts to rise, the gas comes out of the beer and collects at the top of the keg. That makes the pressure in the keg go up. Then pressure hits a point that's high enough that no more gas can come out of the beer and you're back in equilibrium.

But as to where you store your CO2 tank, you have to break it down into two separate systems:

There's the keg, and then there's the CO2 cylinder. The two systems are linked through the regulator. The keg doesn't see the gas in the cylinder at all. It just sees a constant pressure governed by the regulator. So whether your cylinder is at 600psi or 800psi doesn't matter.

The gas entering the keg through the regulator reaches the temperature of the beer very quickly, and it isn't flowing into the cylinder very fast to begin with. The rate it flows into the keg is governed by how fast the CO2 molecules are making their way into the beer, which is slow slow slow. A molecule of CO2 dissolves in the beer, which drops the pressure in the space at the top of the keg, which causes the regulator to let in another molecule. SLOW. It takes at least a couple of days to let in that 10ish gallons of gas. Plus, the gas comes into the cylinder _incredibly_ cold even if you're storing it outside the fridge. Ever sprayed canned air on yourself? You can get frostbite almost instantly because the temp of the gas drops when something expands from high pressure to low pressure.

BTW, first post! Hi!

 

[veganbrewer]

I like that explanation.

The important part, I think, is the part where you mention that when gas comes out of the cylinder, it becomes super cold anyways. Is this maybe because the pressure decreases enough in the cylinder for more liquid CO2 to convert to gas CO2, and this phase change steals energy from the gas CO2, like in evaporative cooling?

So, 12PSI really is 12PSI, because the temperature of the gas when it goes into the regulator is something super low whether the cylinder + regulator is stored cold or not.

BTW, hi!

 

[Philip1993]

I have a hard time accepting that without some reasoning behind it. I'm not saying that the gauge reading 12psi is wrong, but I am saying that maybe a cold 12psi has more CO2 molecules than a warm 12psi, and this might affect the carbonation level of beer.

You're absolutely right in your logic. Remember the equation I posted earlier? P*V = n*R*T. If P&V do not change and T goes up, then n (the number of molecules) must go down. Hence, a gas delivered at a higher temp has LESS molecules per unit volume. As an analogy, think of a room packed with people. If they are just standing there, you can pack many in. If they are running around, you can fit a lot less in.

Beer carbonation levels are determined by the beers ability to absorb and hold CO2. And that is determined by beer temp & pressure. So, gas temp has little effect on carbonation levels.

One final note: A warmer gas at the same pressure should take longer to carbonate the beer to equilibrium, but whether that extra time is significant would require testing. And since we condition for much longer than carbonation times, it's probably not worth the effort.

 

[anottomous]

I talked to the guy that I buy my CO2 from when I was designing my system. He said you should not put the regulator in the refrigerator as it can damage the diaphragm.

 

[TheJadedDog]

Here is a good link which shows the relationship between temperature, pressure, and volume...

http://www.northernbrewer.com/instructions/co2.htm

The chart is an excellent way to determine what the pressure should be at any given temp to achieve the desired volume.

 

[user 4295]

Here is my generic response whenever I see this topic come up. There is a direct pressure/temperature relationship with CO2 in a liquid/vapor state, as there is with the refrigerants I use every day. The pressure will stay the same, at a given temperature, until all of the liquid in the tank has been vaporized. Once only vapor exists the pressure will drop rapidly.

One thing to note when keeping a CO2 tank and regulator in the fridge, is that the elastomer used in the regulator diaphragm will respond more slowly to a change in output pressure when it is cold. For example, if you set the regulator to 12 psi when cold, when you come back in an hour it may have creeped up to 14-15 psi. It's not a problem as long as you are aware of it. I set mine a few psi below the desired pressure and it will stabilize at the pressure I want.

Here is a pressure/temperature chart for CO2 that shows the relationship:

 

[user 4295]

I talked to the guy that I buy my CO2 from when I was designing my system. He said you should not put the regulator in the refrigerator as it can damage the diaphragm.

That is pure BS! I have had my CO2 regulator in my fridge for 2 years without ill effect. Also, I have regulators of all sorts in my service truck, IE; acetylene, oxygen, nitrogen, some of which are decades old, that have been exposed to temperatures ranging from +100˚F in summer to below 0˚F in winter. Nowhere in the literature that came with the regulators does it state to bring the regulators in from your service truck when it gets cold outside. As I stated in my previous post, the only effect that cold temperatures have on regulators is that the elastomer that the diaphragm is made of will react more slowly to a change in output pressure.