CO2 Pressure question

[KC10Chief]

I have a couple of kegs in my new kegerator that I started carbonating last night. This is the first time I've kegged beer. I got the 5# bottle from my LHBS. The guy there told me that the pressure should be around 1,500 psi in the bottle. The gauge goes up to like 3,000 psi. I also realize that the pressure from the bottle isn't that important. However, when I first hooked up the bottle, it was reading a little under 700 psi. Today, after being in the kegerator all night, it's reading about 450 psi. I weighed the bottle today and it's still over 11 pounds. My scale only goes up to 11 pounds. I'd guess that it's at least 12 pounds. So, I don't think I have a leak. Everything else seems fine. Just curious what the typical pressures are. The guy at my LHBS didn't seem too sure of his 1,500 psi answer.

 

[jonp9576]

700 is fine for room temp. 450 is right for in the fridge. 1500 if it's outside in the summer time.

 

[PurpleJeepXJ]

You can not calcuate the pressure without knowing the temperature. CO2 is a liquid. All tanks should have burst disks that will blow at 3k-4.5k depending on the type of disk. if the tank was warmer when you took the first psi reading then it would be a higher psi then the tank being cooled in the kegerator... if that makes sense.

 

[Ace_Club]

+1, the pressure in the bottle is dependent on temperature.

 

[Ohio-Ed]

^^^ Yup, what he said.


Edit: OK, I've had a few but am I really "THAT" slow... this was a response to 3 posts ago... crap.

 

[Walker]

basic physics at play here. Pressure, volume, and temperature of a gas are all related.

When you cooled your tank, the pressure inside the cylinder has to go down to keep things balanced.

The high pressure gauge is totally useless on a CO2 regulator. CO2 is stored as a liquid in your tank. It takes a certain amount of pressure to keep the CO2 in liquid form, and that pressure comes from some of the CO2 evaporating.

The system keeps itself balanced... as you draw out CO2 gas, more CO2 liquid evaporates to maintain the same pressure in the tank and keep the rest of the CO2 in liquid form.

So, that gauge will read a constant value for the entire life of your tank. Only when the final bit of liquid CO2 evaporates will the pressure start to drop. And at that point, your tank is basically empty.

I don't even have a high-pressure gauge on my regulator. Well... I mean, it's THERE, but I knocked the tank over on my concrete garage floor a few years ago and the high pressure gauge was totally destroyed. The glass broke, the body smashed up, and the needle fell off.

My beer didn't seem to notice this.

 

[KC10Chief]

That's what I figured and that all makes perfect sense! Do you guys weigh your tanks often, or do you just run and change it out or fill it when it's empty?

 

[a10t2]

Well, if it's reading 450 psi, the gauge is inaccurate. That would be about 25°F.

At serving temperature the gauge will start to drop once the tank is ~90% empty. So if you keep an eye on it you'll have plenty of warning. The best thing to do is get a second tank so that you can always have a full spare waiting.

 

[bad67z]

Here is a chart that explains it and can make ones head hurt. But is shows the correlation between temperature to pressure.

 

[KC10Chief]

Well, if it's reading 450 psi, the gauge is inaccurate. That would be about 25°F.

At serving temperature the gauge will start to drop once the tank is ~90% empty. So if you keep an eye on it you'll have plenty of warning. The best thing to do is get a second tank so that you can always have a full spare waiting.

GAH! That makes sense! I put a thermometer in my kegerator yesterday and it was reading 26 degrees! Yikes! I warmed it up to about 34 today and it's reading a little over 500 psi now. Thanks for the info! That chart is great and makes sense to me!