From Brewiki: “…purge the air from the top of the keg. Repeat 3 times until you are confident all of the air is out.”
Why an I confident all the air is out after 3 times. Am i listening for something particular?
Cheers
You’re never going to get it all, but since the CO2 is being directed downward, and is heavier than air, you’ll get most of it. As far as actually checking, the only thing that comes to mind is the match test, but I’d think the force of the gas coming out of the PRV would blow it out regardless.
I just open the PRV, let the gas run for ~5 seconds, re-pressurize, open for ~5 s, then call it good.
I don’t know if my practice makes a difference or not, but I always wait a couple of minutes between purges. Here’s why: CO2 is heavier than air, so it should sink and air should rise, but I assume that it takes a couple of minutes for this to occur. My thinking is that one should get more air out if it is sitting high (by the exhaust valve).
fill the keg with sanitizer to the very tippy top brim, put the lid on and force that all out (into the next keg, a bucket or other storage device). result, sanitized, very nearly perfectly purged keg.
but yeah fill with co2, release and repeat a couple of times will get you pretty close.
I was recently schooled on this here on the forum. so as I understand it with the empty keg and/or headspace purge like this you are dilluting the air with pur c02 alot. so that each time you release the pressure there is many times less ‘air’ and many times more co2 left behind.
I do a couple purges as the incoming CO2 mixes with the existing air in the keg simply giving a higher percentage of CO2 in the mix each time. There is no effective stratification just mixing, due to the way our CO2 generally enters the keg.
After a few fills and purges you’ll be fine and will have effectively removed enough atmospheric air to be safe. After filling purge the headspace once again and you’ll be double good :-).
Mixing gases follows the partial pressure principle so “fill and purge” is the most effective way to get rid of the percentage of atmospheric air (21% oxygen) along with using a decent pressure of CO2. Each time you are mixing in 99.999 pure CO2 with what previous mix of gases remains. If you did the math you’d probably find that three fills and purges does a great job.
For discussion, say air is 1/5 O2 (close eough, as Dean said 21%).
You have 4 parts other stuff (mostly N2), and 1 part air at atmospheric pressure, approximate to 15 PSI. If you put 30 PSI on the keg, you have added 2 times the stuff, or 10 more parts. So the gas mix is 1/15 O2. Vent and repeat 2 more times.
You will have reduced to 1/(15^3), and that comes out to 0.00029 parts O2 left, which is pretty small.
I you want to use 15 PSI, you do it 3 times you are at 0.001 parts left. 4 times at 15 PSI gets you to 0.0001.
Nobody told me there would be math! 
That was just my engineering side doing approximate “quick and dirty calculations” as we used to call it.
Not to be picky (which means I’m being picky), according to your first step each dilution reduces O2 by 1/3, so after three dilutions you’d have 1/5 * 1/3 * 1/3 * 1/3 - or - 1/(5*(3^3)). Still, that’s 0.007 parts O2 at the end.
We called those “Fermi approximations”, which makes it sound really technical.
Looks like my logic was off. Thanks. Still pretty small.
I will just add
that while I do not do the math
I test the theory regularly.
Especially when there are a couple keg hopped AIPAs in the fridge.
My technique is to open fridge door, stuff head and nose into fridge
pull, expel, and take a big aromatheraputic whiff
…just to make sure everything is operating properly.
Thanks for all the replies.
I’m asking as every kegging video says you only need to do it a couple of times while they all fill and purge like 3-7 times 
I usually try and talk my way around it. With this many smart folks it can get way too detailed
Whatever method you use I would recomend purgeing the keg BEFORE putting beer in it. If you have an extremely o2 poor atmosphere inside the keg, when you rack the beer in there is very little opportunity to introducing o2. If you fill the keg and then purge just the head space you must be very careful to rack gently as splashing can introduce o2 to the finished beer.
To add to this subject, it’s also recommended to purge the entire keg (empty) with CO2 prior to racking and fill from the bottom up during the transfer. Then purge the head space with CO2 using the aforementioned methods.