[flbrewer]

How much total volume do 5 gallon corny kegs typically have?

Is there any issue kegging say 3 gallons in a 5 gallon keg?

[Stevie]

No issue, just make sure you purge all the O2 out. The volume is just about 5 gallons below the gas dip tube.

[Philbrew]

No issue, just make sure you purge all the O2 out. The volume is just about 5 gallons below the gas dip tube.

What is a good way to purge the O2 out?  Would reversing the ball locks and pumping CO2 slowly in the out (thru the dip tube) be a good way?

[jtoots]

No issue, just make sure you purge all the O2 out. The volume is just about 5 gallons below the gas dip tube.

What is a good way to purge the O2 out?  Would reversing the ball locks and pumping CO2 slowly in the out (thru the dip tube) be a good way?

Not necessary... just hook up the gas line to the gas inlet and tug on the relief valve.  The CO2 will sink to the liquid level and work its way up.

[HoosierBrew]

Let CO2 pressure into the keg and vent the PRV a few times. This will purge the O2 out.

[Philbrew]

Aah.  Sure, simple.  Thanks

[ajk]

Gases don't stratify the way oil and water do. They always mix, regardless of their relative weights. So the best way to purge a keg is to fill it to the top with sanitizer, then push the sanitizer out with pure CO₂.

[jtoots]

Gases don't stratify the way oil and water do. They always mix, regardless of their relative weights. So the best way to purge a keg is to fill it to the top with sanitizer, then push the sanitizer out with pure CO₂.

I'd have to respectfully disagree here...  Gasses certainly stratify effectively enough for homebrewer's purposes.  I come from the HVAC design world and one proven type of air delivery called displacement ventilation depends on it (think "hot air rises").

https://www.youtube.com/watch?v=4TpMd5lZSQA

I think this suggestion is a bit over the top complicated (although I can't argue against this being the BEST way to purge, I just think it's excessive).  The OP is looking to purge oxygen out of a keg once it has beer in it, so the sanitizer strategy is a little off topic too, unless you're suggesting the sanitizer, purge, fill with beer, purge again. 

IMHO this is a KISS scenario.

[dkfick]

Your talking temperature stratification.. not stratification of the different gasses in the air.  Pushing the sanitizer out of the keg would be your ideal way to get the most o2 out of the keg... though I think for practicality sake the put co2 in the 'in' of the keg and purging gets the vast majority of the o2 out via displacement.

[HoosierBrew]

I push sanitizer out of the keg to purge an empty keg, but venting the prv a few times on a keg I've added gelatin, more hops, coffee beans, etc, to never gives me oxidized beer. So in practice I feel this purges 02 just fine at the homebrew level.

[Joe Sr.]

The gasses will mix, but once you purge a couple of times the o2 is negligible.

In a thread from past years, which I shall not look for, someone posted an analysis of the theoretical mix of o2/co2 when purging.  The o2 percentage drops precipitously as you fill and purge with co2.  It's never 100% gone, but effectively so.

No doubt that filling with star san, pumping out, and then filling with beer in a closed loop is the best way to get all of the 02 out.  It's just not really necessary for most of us.

[jtoots]

Your talking temperature stratification.. not stratification of the different gasses in the air. 

Good catch!!