Can I half fill a keg (corny)

[A Clueless Bloke]

So basically.

Most of my corny kegs are of 19L capacity.

However. My next brew will be a 9L stout. That’ll be carbed and served in nitro.

Presuming the basic clean the keg, Syphon the beer into the keg, purge the keg. Leave it a few months and the beer will be fine?

It’s the headspace I’m concerned about.

Or buy another 10L keg?

 

[Larse]

Well I'll ask the obvious question first. Why can't you brew an 18L stout?

In any case, the headspace shouldn't be an issue if you purge it with CO2. After all, when you're drinking a keg, at some point it's going to be half full and then quarter full etc.

But also I'd put in a shout for a 10L keg. I bought one early on and it's been a very useful bit of kit. I often use it to take the remainder from a half full 19L to free the bigger one up without losing any beer. Or the one time I made a 27L brew and filled a keg and a half.

 

[Rwilts]

If you keep it flushed of 'air' and under CO2 i dont se why not..

 

[Agentgonzo]

I did some calculations a while ago (for bottling rather than kegging, but you can just scale it up and the ratios work out the same). Essentially, yes, the headspace is significant. Bottle conditioning will have the yeast consume the oxygen, but with kegging you don't have this luxury

I don't keg, but from a scientific point of view, here's a good explanation.

If you fill the keg with starsan etc, then use CO2 to push the beer out, you'll have (close enough to) no oxygen in there, so when you fill the keg (assuming you don't open the lid etc) the headspace will be (close enough to) 100% CO2 and you'll be fine.

However, If you fill first, then try to flush the headspace with CO2, what essentially happens is that you mix/dilute the air in the headspace with CO2 with each "purge/flush". For a small headspace, you can get away with it, but if you've got 10L of headspace, the following will happen.
* Put in 10L of CO2 and purge. You're now at 50% dilution.
* Put in another 10L if CO2 and purge. You're now at 25% dilution.
*Put in another 10L of CO2 and purge. Your now at 12.5% dilution.
* Another 10L of CO2. 6.25% dilution.

So after 40L of CO2 purging You're at the equivalent to 625ml of air (6.25% of 10L) in a "full keg". Which I think is still a lot...

So yes, you can do it with purging the headspace, but it's an inefficient way to do it

 

[Larse]

Just on that purging. Is it not the case that CO2 is heavier than air? So if you introduce CO2 into the headspace and leave for a few minutes, the air will be displaced to the top and will purge first?

 

[Agentgonzo]

Just on that purging. Is it not the case that CO2 is heavier than air? So if you introduce CO2 into the headspace and leave for a few minutes, the air will be displaced to the top and will purge first?

CO2 is denser than air. Yes.
Will the co2 "sink"? No. That's not how gases behave.

A good analogy is to think about milk and water. They are close but different in density (like CO2 and air are) and mix according to the same laws of physics (fluids - both liquids and gasses are fluids).

So get a glass half full of water (representing the air). Now pour the same quantity of milk into the glass (representing the co2). Milk is denser than water. Will the milk "sink to the bottom"?. Sadly, no.

 

[MashBag]

Yes. (prime it)

 

[stripeyjoe]

Just on that purging. Is it not the case that CO2 is heavier than air? So if you introduce CO2 into the headspace and leave for a few minutes, the air will be displaced to the top and will purge first?

If that was the same we would all be dead as air is made up of 0.04% CO2, it doesn't 'sink' to ground level.

 

[MickDundee]

If that was the same we would all be dead as air is made up of 0.04% CO2, it doesn't 'sink' to ground level.

We’d only be dead if we were lying on the ground

 

[JockyBrewer]

Yes you can half fill a keg, but you need to liquid purge it first:

Fill it with water (or preferably sanitiser), then push all the water out with CO2.

Your keg is now 99.9% fully of CO2.

 

[JockyBrewer]

Just on that purging. Is it not the case that CO2 is heavier than air? So if you introduce CO2 into the headspace and leave for a few minutes, the air will be displaced to the top and will purge first?

There’s too much entropy for atmospheric gases to stratify like that.