You analysis is off.
First, there is no way (at least not easily) of knowing how much Co2 (in weight) 10 gallons at 30 psi is. You would need to factor in temperature, altitude, etc.
Sorry, but I don't think my analysis is off.
If you double the pressure in a fixed volume with a fixed temperature, which is exactly the case here, you're going to double the amount of gas in that volume. Go from 15psia to 30 psia in a 10 gallon space, and you now have twice the mass of gas. I used 15 psi as a nice number because it's close to the amount of one atmosphere at sea level. Of course altitude is going to change things a bit, but my argument is still very valid.
If you apply CO2 to a vessel containing 15 psia air and stop when you hit 30 psia you will now have a mixture of 50% air and 50% CO2. Vent it back to 15 psia, and you'll still have 50% air and 50% CO2. Pressurize again to 30 psia with CO2 and you'll now have 25% air and 75% CO2. Vent and purge again and you have 12.5% air. Etc.
I never stated any amount in weight, but I don't think that's a difficult matter at all if I wanted to calculate it. Why don't you think that's easy? Yes, you would need to know the absolute pressure and the temperature, but these are fairly standard assumptions to make. Let's assume you're near sea level, and at 70 degrees F. That means the atmospheric pressure is 14.7 psia, and one gallon of CO2 at this pressure and temperature is 0.157 moles = 6.9 grams = .015 lbs.
In that case, pressurizing the 10 gallons of headspace to 14.7 psig then venting it would use .15 lbs of CO2. Purge it 3 times and you'll use about half a pound of CO2, like you estimated (nice one by the way!). But like I said above, I wouldn't want 1.25 gallons of air still in my keg. If a full corny keg has 1/2 gallon of headspace (just a guess) you'd be down to 1/16 of a gallon (4 fl oz) of air after 3 purges. To get to that same level in the Sanke you'd need to purge about 10 times, which would use 1.5 lbs of CO2. That's a whole lot of CO2, especially considering you haven't carbonated yet.
At $2 per lb. is it really that much?
It's not the cost as much as it being a PITA. Even if you only use half a pound extra each time you fill a keg, that's basically doubling your CO2 usage. CO2 may be relatively cheap, but it's annoying to remove the cylinder, bring it into a shop, pick it up again when it's full, etc. At least in my area my best bet is a fire extinguisher place that does fills with a next-day turnaround time. Obviously having an extra cylinder would make things easier and negate my argument somewhat.
Second, if you fill slowly from the bottom up, you don't need to waste a bunch of Co2.
I agree that this would be a much better method. Though a Sankey isn't set up for such a thing, so you'd have to drop a gas tube to the bottom with the spear removed and then very slowly purge that way. Is that what you were picturing?
Personally I would convert it to a boil kettle and keep your eyes open for Corny's.
Agree again. CO2 issue aside, 1/2 bbls Sankes are big and won't fit in a 7 cu foot keezer. I don't really see any advantage.
