Question about kegerator kit

dontman said:

that is an interesting article. Too bad it doesn't attempt to address force carbing. Because either force carbing takes a lot more gas than they are estimating ( which is pretty obvious that this is the case.) or I am leaking a lot of gas.
I am getting 3-4 cornies carbed and dispensed with a 5 lb tank.

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Bobby_M said:

Right, so if it takes a 1/2 pound to dispense 1/4 barrel, (1 volume, fully displaced) and you want to carb to 2 volumes, it should take 1.5 pounds to carb and dispense. I know that's not true at all so Micromatic's estimate seems way off.

Here's something I posted on my website and I'd love for anyone to tell me where I went wrong, if I did.

One of the questions I see a lot is "How many kegs can you carbonate and dispense with a single fill on a xx pound CO2 tank"? There are a lot of variables of course based on how many volumes you carb each keg to (volumes) and how much you waste into the atmosphere. The typical average in my opinion is 2 volumes meaning there will be 2 times as much gaseous CO2 dissolved as there is beer. Example, 5 gallons of beer take on 10 gallons of CO2. It will take another 5 gallons of CO2 to push all that beer out of the keg. The real issue is figuring out how much liquid CO2 it takes to create a certain number of gallons of gaseous CO2.

According to http://www.ca.airliquide.com/en/business/products/carbon_diox/index.asp , 1 pound of CO2 (it's sold by the pound) is equal to 8.741 cubic feat of gaseous CO2 at sea level / 70F. If my logic serves me, this volume would actually be halved at typical carbonation pressure of say 14 psi (2ATM) for 4.37 cuft. I'm going to ignore the temperature.

5 gallons is .668 cubic feet. 4.37cuft (1lb co2) divided by .668 = 6.54. That means 1 pound of liquid CO2 is 6.54 volumes in a 5 gallon keg. For example, that would be enough to carb two kegs to 2.25 volumes and dispense both to empty. So if 2.25 volumes was an average carb level, and there was no wasted Co2 on purging/venting:

CO2 Tank Size Total Volumes 5 gal @ 14psi Total Kegs, Carb 2.5 vol + Dispense
2.5 lb 16.35 5.3
5 lb 32.7 10.6
10 lb 65.4 21.2
20 lb 130.8 42.4

I admit, I don't understand the ideal gas law in the slightest so I can't really know how the temperature of the beer is playing on the volume requirements. However, Micromatic, an authority on keg dispensing claims that it takes about 1lb of CO2 to "DISPENSE" a 1/2 barrel. This would mean that it takes 1/3rd or .33 pounds per volume on 5 gallons. Remembering that carbing to 2 volumes and dispensing actually takes 3 volumes, that's 1 pound per 5 gallon keg. On the same page, the chart shows a 20lb tank capable of dispensing at least 62 five gallon kegs but you have to remember that dispensing only takes ONE volume so carbing will require 200% more gas. So, the number I'm comfortable with right now is "about 1 pound per 5 gallon keg".

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Arkador said:

1 volume = the amount of CO2 to dispence a keg.
2.25 = a typical carb level when force carbing
If you are just dispensing, you will use 1 volume.
If you force carb to 2.25 volumes, then dispence, you are using 3.25 volumes of CO2.

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dontman said:

Additional thought on the 20 oz container. The extra money to upgrade to 5 lb is well worth it, especially when you take into account the cost to refill - ~ $10 for 8 oz, $13 for 5 lb. OTOH I have the 5 lb and every once in a while I wish I had another tank and regulator setup for force carbing, purging tanks and lines, and for a portable setup for parties. If you don't plan to brew and keg your own then I would recommend you just pony up for the 5 lb tank.

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