CO2 Usage

I'll start out by saying I'm completely new to kegging (have bottled my fair share), but I've read a ton on this site and learned a lot (thanks!).

My situation:
I did a ~15-gallon AG batch that got split into four 5-gallon carboys. My share was three, one went to my buddy. So each carboy had about 3.5 - 4 gallons of liquid gold in them. I fermented and dry-hopped (3 weeks total), then cold crashed for a day at ~35F. I kegged two of the carboys into their own 5-gallon corny, so it wasn't completely full of liquid, probably about 3.5 gallons got into each corny, so quite a bit of head space (which I purged with CO2).

Target CO2 volume was 2.3 (referenced http://www.kegerators.com/carbonation-table.php). I have a 5 pound CO2 tank. So to carb, I put 30psi on, rotated the corny upside down until I heard the CO2 stop, then rotated again, and repeated for about 4-5 minutes. I let the corny sit at ~35F for roughly 36 hours with gas on at 30psi. I periodically switched the gas between the two kegs. Then I brought the pressure on the corny down and let it sit on gas for a day at 10psi at ~40F. I then drew some liquid gold from my picnic tap (at 10psi) that is on a 6ft line and got foam. So I dialed it back to 8psi and got a good pour, but, it is a bit over-carbed. And, my initially full 5# CO2 tank read 1000 when I started, but now reads 500.

My questions:
1 - Is that much CO2 usage typical (I sprayed to check for leaks and found none)? Half of my 5# tank for two 5-gallon kegs seems like a lot to me. Maybe because I had so much headspace in the kegs?

2 - I'm thinking that carb volume is also (along with temp, pressure, and time) a function of the ratio of headspace to liquid. If so, is there a calculation or table that takes that into account?

3 - What could have I done differently?

Any insight and advice appreciated.

I brew and keg 10 to 20 gallons a month. For force carbing, serving, and general CO2 usage, I get 2.5 to 3.5 months worth of usage from a 5lb CO2 bottle.

Don't pay a lot of attention to the high pressure gauge. They are pretty much worthless, except to tell you when you are almost out of gas.

Hi

The CO2 in your bottle is in a liquid state when the bottle is filled. Through > 90% of the cycle, all that's happening is the liquid is boiling off. The pressure reading will go up and down with the temperature of the bottle (it boils at a higer pressure at higher temperature). Once you get right to the end of the bottle contents, the pressure will start to drop.

Bob

yea either your co2 tank was almost empty before this batch, or you have a leak and it emptied during that time.

Thanks for the replies. I was about ready to go buy a 20 pound tank :cross: , and I don't need SWMBO asking any more questions about new equipment showing up in the garage. She is still trying to figure out why I need two refrigerators (1 for bottles, 1 for fermentation and cold crashing) & one 14cf chest freezer (for kegs).

Anyone have any info on question #2?

Silentnoiz said:

Thanks for the replies. I was about ready to go buy a 20 pound tank :cross: , and I don't need SWMBO asking any more questions about new equipment showing up in the garage. She is still trying to figure out why I need two refrigerators (1 for bottles, 1 for fermentation and cold crashing) & one 14cf chest freezer (for kegs).

Anyone have any info on question #2?

Click to expand...

Hi

Headspace has pretty much nothing to do with carb volume (other than it's filled with CO2). Surface area will affect the carb rate, but headspace volume - not so much.

Bob

The PSI reading 1000/500 only indicates the temperature of the CO2 (as long as there is still some CO2 in the liquid phase). To know how much CO2 you have used you will have to know what the tank weighed when you started and what it weighs now.

carlisle_bob said:

Hi

Headspace has pretty much nothing to do with carb volume (other than it's filled with CO2). Surface area will affect the carb rate, but headspace volume - not so much.

Bob

Click to expand...

Ok, that makes sense, thanks.

shelly_belly said:

The PSI reading 1000/500 only indicates the temperature of the CO2 (as long as there is still some CO2 in the liquid phase). To know how much CO2 you have used you will have to know what the tank weighed when you started and what it weighs now.

Click to expand...

Got it. Now SWMBO is going to wonder where the scale went.

Hi

One *very* handy gizmo: A digital luggage scale. It straps on the top of a keg or a CO2 bottle and gives you a quick weigh of the item. Small, cheap, and plenty accurate enough for the job. "How much is in the keg" is just as real a question as "how much is in the CO2 tank".

One example:

http://www.amazon.com/dp/B004EZG9X4/?tag=skimlinks_replacement-20

There are many others at a variety of prices.

Bob