Purging sanke kegs

[MN brewer]

So instead of filling the keg with starsan and pushing out with CO2 to purge oxygen, I pushed CO2 retrograde through the dip tube with the gas port open. The CO2 is from the keezer at 36F. Like a cold front pushing out a warm front, the temperature difference between room air should cause viscosity between the air in the keg and CO2 which should limit mixing. I leave a temp probe in the gas port and watch for the temp change. After I see the temp change I let it go another 30-60 seconds then close the gas port. Seems like an easier way to purge oxygen before kegging, anyone have any thoughts on this?

 

[day_trippr]

Turbulence would totally kill that concept.
I'm sure @doug293cz would have greater insight....

Cheers!

 

[SanPancho]

Yeah, this was debated heavily aways back. I’m with you, but the only way to know for sure is with a DO meter.

 

[MN brewer]

Yes there is turbulence, just like when a cold front pushes out a warm front. But eventually it’s just the cold air that’s left. That example is with the same atmospheric gas at different temps, but retrograde filling of CO2 through the dip tube puts the heavier and colder gas inferior to the gas being purged.

 

[doug293cz]

Turbulence would totally kill that concept.
I'm sure @doug293cz would have greater insight....

Cheers!

Correct. If you could insure no turbulence and fill from the bottom, then you would get the effect of CO2 pushing O2 out of the keg. But the fill rate required to insure no turbulence brings diffusion into play, and it only takes minutes to raise the O2 content too high with diffusion. The best way to purge a keg is to use the fermentation CO2 to purge the keg. I did an analysis here that assumed complete diffusional mixing of all of the volume being purged, and it still results in single digit parts per billion O2 content.

Brew on

 

[MN brewer]

Wow P chem flashback. Looks like the diffusion constants are for two pure gases at same temps. Obviously weather happens so temperature difference is a significant barrier to diffusion with the same gas, even with significant turbulence. I don’t know enough about meteorology to model that. My thought was the temp difference from bottom up would allow for higher fill rates with limited diffusion that could be overcome by fill rate.

 

[doug293cz]

You would get some flushing action, and would eventually purge the keg, but you would need to turn over the keg volume several times to get the O2 concentration down to that of the "pure" CO2. That's a waste of a lot of CO2, that you had to pay for. Much more cost effective to use the "free" CO2 generated by a fermentation. The other benefit is that the fermentation CO2 has a lower O2 concentration than bottled CO2.

Brew on