Gas Bottle Pressure Under Different Temps

[lonte]

I keep my CO2 gas bottle in the fridge with my kegs and so it permanently sits at around 4*C. I have a dual guage reg and the 'tank' guage seems to always sit at a particular reading (and I've just realised the two guages might be calibrated differently, so I won't give an absolute for the tank guage value).

Now yesterday a condensor fan motor bearing seized and when I got home I found the fridge sitting ~19*C. Now the interesting thing I also noticed (once panic subsided) was that the tank guage was at least 2x and closer to 3x the 'normal' reading. I understand there is a relationship between pressure and temperature, so this sits fine with me - what I want to know is how temperature might affect the reading on the outlet side of the reg?

Are the guages calibrated at a particular temperature? If I set my cold (4*C) CO2 tank up to apply (as per the guage) 12psi to a 4*C beer (about the right amount to get 2.5 volumes) then is the guage really reading (and supplying) 12psi or not? From last night's experience it would appear possible that if I set it up as above, then moved the tank outside the fridge, it might slowly change to show as somewhere between 24 and 36psi?

Any comments appreciated. Lonte.

 

[Tony M]

My contents gauge (tank gauge) reads around 1000 psi at ambient and perhaps 10% less when I used to keep it in the fridge. This 2 to 3X you are speaking of sounds very strange.
I do not have any measureable difference in line pressure readings in or out of the fridge. (I just put my bottle in the fridge for 1/2 hr to check this)

 

[afromaiko]

From what I understand, the pressure in the CO2 bottle will change according to the temperature of its environment. So, if you have the CO2 bottle in the fridge, the pressure inside the bottle will be reduced. If you were to take it out in the middle of the desert, for example, it would greatly increase the pressure. The same applies to the pressure inside your kegs, if you were to gas them while cold then let them warm up to room temperature you'd find that the pressure inside them has risen.

I'm considering putting my CO2 bottle inside the fridge too, and the thing I'm wondering about is - do you get less usage out of a cold CO2 bottle? It's a big concern for me because I'm using Sodastream cylinders.

My thinking is that really it shouldn't make a difference whether the CO2 bottle is inside or out, as long as the keg itself is kept cold. While the high side gauge will read lower when cold, once the CO2 enters the cold keg I can't see why the temperature of the supply bottle would have any effect. So, theoretically it should be the same usage amount of gas. Feel free to correct me anyone.

 

[kevnlis]

There is still just as much gas in the cylinder at 30C as there is at 1C reguardless of the pressure reading you will gas just as many kegs.

 

[Adamt]

The gauges measure the pressure in the headspace of the bottle, which is related to the total amount left in your bottle. This relationship changes with temperature.

 

[bigholty]

Useless fact #32: At fridge temperatures, a CO2 cylinder should have a distinct liquid and gas phase. Above CO2's critical pressure and temperature of about 7.4Mpa and 30 degrees it becomes a supercritical fluid, not really a liquid or a gas, with properties of both phases. Supercritical CO2 is used as a solvent to extract certain compounds in some processes.

 

[Thirsty Boy]

Supercritical CO2 is used as a solvent to extract certain compounds in some processes.

For instance - Alpha acids from hops. Or at least thats when the plant hasn't recently blown up

 

[lonte]

The gauges measure the pressure in the headspace of the bottle, which is related to the total amount left in your bottle. This relationship changes with temperature.

Thanks everyone, Ross set me straight this afternoon. Yes the bottle pressure will change in relation to temperature but (and I'm paraphrasing Ross here) the whole idea of a regulator is to deliver the gas at at regulated pressure regardless of ambient temps.

Blonde moment on my behalf!

 

[Paul H]

Thanks everyone, Ross set me straight this afternoon. Yes the bottle pressure will change in relation to temperature but (and I'm paraphrasing Ross here) the whole idea of a regulator is to deliver the gas at at regulated pressure regardless of ambient temps.

Blonde moment on my behalf!

Yeah life's too short to worry about calibrated gas gauges