Keg Purge Thought Experiment

If it is pure CO2, and if you waited a while after filling with CO2, it should displace most other gases in ambient air. You might have something else lingering around in tiny amounts, but it should work. Why would you want to do this though?

 

Yes, but for homebrewing purposes...No...have you been having problems with standard purging processes?

 

I've purged brite tanks commercially by cracking 1 psi through bottom valve for about 45 mins, thought process being filling in blanket layers from bottom on up to top and out. If you just use 1-2 psi in your corny keg over 20-30 minutes, I bet it'd be the same concept.

 

gotcha. I do a PBW soak after I used a keg. Then I let it dry until it is needed. Then I sanitize with starsan and empty it bottoms up.

 

From understanding is you purge a keg to get rid of all the oxygen so when you fill it with beer you minimize the oxygen pickup. I did that the first two times I kegged now I just fill the keg, put it on gas and purge. Never had any issues with oxygenation.

 

Oh ya I do this too. I'm just lazy about it and don't hook it up to the post. And I don't worry about displacing all the air either. I have a spare 5 lbs bottle for when my 10# bottle runs out, and I just point that into my keg and open it for a second or two. The CO2 settles to the bottom where my tube is / beer fills up from. It's not just good; it's good enough

 

Guess I'm going to go on record here and say that a cloud of CO2 in a keg is not a solid piston that will force most or all of the oxygen out of the keg as it fills with beer. The way to get rid of virtually all the O2 is to repeatedly dilute with large amounts of CO2, and release the pressure between dilutions.

 

That's true, that is the best way to purge the O2. I guess I always saw it as not necessary to use that much CO2. As long as there was a layer where the beer is being introduced I'm happy.

Of course it's not that simple, the fluid rushing in can cause air currents if the beer flows too fast, which could push co2 out and air would come in it's place. That can happen whether you purge or not though.

 

If you do the serial dilution Puge technique, it takes about 11 purges at 30 PSI to get down to 1 ppm O2. That is a boat load of CO2.

 

Exactly

 

What about filling with starsan or iodophor solution, purging with CO2, then flip the keg over and push the remaining solution out through the PRV? Might be a little messy, but should work…

Edit: disregard…just saw that is the technique you are using...

 

Carbon dioxide doesn't sink to the bottom, rather it mixes with the air (kinetic molecular theory) and ultimately is equally distributed throughout the keg. This is the same as our atmosphere . . . the heavier gases (Argon, Xeon, CO2) don't "sink" to the bottom anymore than the lighter gases (Helium, Hydrogen) "float" to the top. Rather, they are all mixed equally.

With some simple math you can compute how much air is left behind when purging a keg. Ambient (absolute) pressure, what we call "zero", is actually 14.7psi (call it 15). If you fill an empty keg with CO2 at 15psi you have mixed an equal amount of air with carbon dioxide (one atmosphere of air, one atmosphere of carbon dioxide). Given time to equalize they will mix and be equally distributed. When you vent to ambient pressure you will release half of the pressure (one atmosphere) and the result will be 1/2 air plus 1/2 CO2. If you repeat (pressurize to 15 again, then vent), the gas left behind will be 1/4 air and 3/4 CO2. A third cycle at 15psi will yield 1/8 air and 7/8 CO2. If you did the same thing at 30psi your fractions would be 1/3, 1/9 and 1/27 of air remaining with oxygen being 21% of this fraction.

Purging all the air out of an empty keg will go through some CO2. But when you fill the keg with beer you effectively "push" all the gas out (air or CO2 or whatever) until only about an inch of headspace remains. Purging this small headspace is quick and efficient.

I personally do a single purge of my kegs to force the starsan which leaves about 1/2 of the keg filled with air. Then rack in the beer and do 3 or 4 burbs. If you're really paranoid about oxygen molecules hanging around you can fill the keg completely with water and (with a normal dip tube), do a single purge . . . that'll make it completely oxygen free. Sorry to be so wordy, if I had more time I could have written less.

 

I'll take issue with this statement…heavier gases do indeed sink and is one of the main points taught during hazmat training and FF training (the reason FF's must be on air during gas spills like propane which is heavier than O2/N2 since FF's can suffocate when it pools in low lying areas). Yes there is mixing that does occur between O2 & CO2 in the keg due to turbulence (same as the effect of winds in our atmosphere), but the fact is that CO2 is heavier than O2 and will be more concentrated at the lower part of the keg compared to the upper part of the chamber where O2 should be in higher concentrations if left undisturbed after introduction.

 

Where I worked, one had to have confined spaces sniffed by security before entering. This included automotive hoist pits and such. Heavy gases can pool if there is a source, it takes time for those to dissipate through difussion.

People were killed up to 15 miles away when the CO2 cloud went down valleys from Lake Nyos in Africa.
https://en.m.wikipedia.org/wiki/Lake_Nyos

 

You're actually both right. Denser gasses (in a closed system (or mostly closed system; like a keg for instance)) will accumulate at the bottom. But the CO2 at the bottom isn't 100% CO2. You could think of it as a layer of mostly CO2, and the remaining percent was whatever was already in there. Vikeman said it best when he said it is not a piston that forces out other gases. It's better to think of it as an equilibrium with a gradient. Long story short; if you blast some CO2 in a keg and wait a couple minutes, at the bottom you will have mostly CO2 but not 100% CO2. For what we do: it's good enough I think.

 

that is my understanding as well…sorry, should have pointed this line in particular I was taking issue with "Rather, they are all mixed equally." since the CO2 will collect more heavily on the bottom and O2/N2 on top. Now I wanna get my FD's gas meter and get a reading bottom and top after 1 minute, 5 minutes, 10 minutes, etc… think I would need to do % O2 vs CO2 since my guess is the ppm would be too high for one of those multi-meters….

 

That would be really cool to do if your instrument is pretty accurate. May want to consider how are you delivering CO2 also. Just opening a bottle with no regulator and blasting it? For how long? Using a regulator with hose; what pressure, and for how long?