A few technical questions regarding Co2 pressure and temperature

[thehopbandit]

I don't actually have any Co2 equipment in front of me, but I am wondering more about the science/specifics behind the process. I realize I'm probably digging way too deep into the subject, but it is interesting for me and I'd like to get the scientific understanding behind it. Please excuse my ignorance if I describe something that doesn't make sense or if I am way off base.

I understand that when you move the Co2 tank from room temp to a colder temp say, inside the fridge, the pressure reading on the tank will decrease due to the colder temperature making the Co2 more dense. The Co2 in the tank is the same.

1. How does temperature affect the Co2 regulator controlling how much gas goes to the keg? I'm not talking about the gauge on the Co2 tank that tells you how much gas is left, but more the regulator that is controlling the PSI on the keg. Does the mechanical operation of the regulator keep output PSI constant regardless of temperature? For instance, if you had a keg at 12 PSI and dropped the temperature of your equipment, would that regulator continue to output 12 PSI or would the output PSI drop without adjustment from the user?

I understand that pressure is pressure. So, I am not sure if I am scientifically describing this correctly, but wouldn't you actually be transferring more Co2 molecules at 12 PSI at 32F than at 12 PSI at 50F, ignoring the solubility of the liquid and considering just the amount of transfer through the line.

An analogy would be a balloon filled with gas. When you drop the temp, the balloon shrinks and the pressure inside the balloon goes down. However, the amount of gas in that balloon stays the same, even though the volume and PSI decreased. Say the PSI on that ballon was 5 PSI. You decrease the temp, it shrinks in volume and the pressure goes down.

2. If you wanted to keep the pressure constant at 5 PSI, wouldn't you technically need more gas in the balloon to keep that PSI at that lower temperature?

3. In this regard, does keeping output PSI constant as temperature changes actually affect the physical amount of Co2 being delivered to the beer? As with the example above, if you are delivering 15 PSI to the beer at 32F and 15PSI to the beer at 50F, wouldn't there be more Co2 molecules transferred to the keg at the lower temp, even though pressure was constant?

I also understand that there are two main things that affect how much Co2 gets into the kegged beer. The first being the PSI of the Co2 and the lower the temp (to an extent) of the beer, the greater the solubility of a gas into the liquid.

4. How does the temperature of the liquid (beer/keg) work in conjunction with the temperature of the gas to determine how many volumes of Co2 you end up with in the beer. I understand the Co2 volume charts on the net, but it seems to me they only take into the account the solubility and temp of the beer, not the temperature of the gas, if my above thinking is somewhat correct.

Thank you for reading, and let me know if I can clarify anything.

 

[Yooper]

1. Yes, the regulator manages the pressure. If it's set at 12 psi, the temperature doesn't matter. The regulator won't change.

2. I don't understand this question.

3. You need a higher pressure at a higher temperature to have the same carbonation. If you want, say, 2.4 volumes of co2 in the beer, you'd use 19 psi at 54 degrees, but only 9 psi at 35 degrees.

4. Gas temperature doesn't matter. Gas is gas.

 

[thehopbandit]

1. Yes, the regulator manages the pressure. If it's set at 12 psi, the temperature doesn't matter. The regulator won't change.

2. I don't understand this question.

3. You need a higher pressure at a higher temperature to have the same carbonation. If you want, say, 2.4 volumes of co2 in the beer, you'd use 19 psi at 54 degrees, but only 9 psi at 35 degrees.

4. Gas temperature doesn't matter. Gas is gas.

Thanks for the reply. I apologize if my questions were not scientifically sound. I am approaching this from my limited knowledge on the subject.

As for question 2, my example was with a balloon. If you have a gas in a balloon, it has a certain volume and exerts a certain pressure. If you decrease the temperature of that balloon, the balloon shrinks in size and the pressure goes down. Say the original pressure was 5 psi inside. After the temperature drop, the psi inside the balloon is now 3 psi even though there is the same amount of Co2 inside. If you wanted the psi in that balloon to stay at 5 psi, wouldn't you need to pump more Co2 inside to get that pressure back up to 5 psi at that temperature, thereby increasing the actual amount (number of molecules, etc. etc.) of Co2 inside the balloon?

As for question 3, is that strictly due to the solubility level of the liquid at a certain temperature or are there any other factors there?

Thanks!

 

[mdgagne]

FWIW, I think what you're describing with your balloon example is the ideal gas law, pv=nrt. It applies here.

 

[zachattack]

1) The diaphragm inside the regulator may be affected very slightly by temperature, but like you said the gauge isn't. So you may have to tweak it a little bit, but it's not significant. As long as you don't make a habit of changing the temperature, you won't even notice.

2) Yes you would. The ideal gas law: PV = nRT. If the volume, pressure and gas constant R are all constant, which they are in this case, and the temperature drops, "n", the number of moles of gas, must increase.

3) Yes it does. Keeping the keg's headspace pressure the same as the temperature of the beer changes will certainly affect the amount of carbonation. The carbonation of your beer is determined by Henry's Law which states that the dissolved CO2 in the beer (carbonation) is directly related to the pressure of CO2 above the beer. The pressure is what affects the carbonation, not the total number of CO2 molecules above the beer. If that were the case, the carbonation would change as you consumed beer and the headspace volume increased. The "directly related" means that you multiply by a solubility factor (aka Henry's constant) which is different for every gas/liquid combo. For CO2 going into water (beer) the constant is highly temperature dependent.

4) The pressure of CO2 and the beer's temperature are the only two variables you need to figure out the volumes of CO2. Again, look at Henry's Law. The only way the CO2's temperature can affect things if it it causes a pressure change, but since we're maintaining a set pressure this can't happen.

 

[thehopbandit]

Zachattack, thanks for the detailed reply. That helps clear things up. You answered all my questions brilliantly!

 

[Demonantis]

1. The outlet pressure might change with the inlet pressure changing. It depends on the regulator. Wikipedia has a decent article about it.

2. A balloon is like a weighted air piston. The volume changes to address the temperature change to balance PV=nRT. The pressure is fixed and is determined by the weight of the piston F=mg, P=F/A. In a balloon the pressure is determined by atmospheric pressure. In a keg the volume is fixed and the pressure changes.

zachattack covered the other questions perfectly.

 

[JRems]

Most regulators are built to keep the output pressure the same regardless of input fluctuations. Unless the input drops below 500psi the output should remain unaffected. If the input pressure gets below 500 it's usually an inverse reaction, the output pressure actually goes up.

 

[day_trippr]

^ Given how a primary regulator works, with a membrane fighting against a simple spring with adjustable tension to try to seat the pin valve, I don't see how that is possible. Wouldn't be much "regulation" going on, that's for sure...

Cheers!

 

[zachattack]

No that's actually true, though I've never stopped to think about it much. But it's pretty common. As your tank runs out, the regulated pressure starts to go up a little bit unless you actively tweak it. This is why for more critical applications in a lab you'll buy a more expensive dual stage regulator.

 

[zachattack]

http://www.alspecialtygases.com/Prd_single-stage_versus_two-stage_regulators.aspx

From the sidebar of that site:

Supply and Pressure Effect

The change in delivery pressure as the inlet pressure changes. For most regulators, a decrease in inlet pressure causes the delivery pressure to increase.