Fill keg headspace?

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fearwig

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I'm collecting bits and pieces so I can get in to kegging on the cheap, and in finally came across a 1/2bbl sanke I can afford. I was planning to pick up several cornies instead, but I couldn't turn down this price. Long story short, I usually make 9-12 gallon batches using a dilution method to accommodate my 7 gal kettle/tun, and I plan to fill the sanke with those kinds of volumes, 9-12 gallons.

Now, I understand there's a modest waste of CO2 involved in kegging less than the full volume, and I thought: Why not fill that dead space with full PET 2-liters and save a few bucks per keg? Duly sanitized, obviously. Now I know this isn't revolutionary or anything, but I did want to sound it off the forum and see if there are any fellow enterprising tightwads in the house.
 
"FM"

Cheers! ;)

[edit] fwiw, there are 40 cubic feet of CO2 at STP in a 5 pound CO2 tank when full.
If you filled a half barrel with 12 gallons leaving ~3.5 gallons of headspace, that's just a scoche more than half a cubic foot.
If you paid $20 to fill that 5 pound tank, you'd use roughly 25 cents worth of CO2 for each full headspace flush...

Cheers!
 
Dan has a point, of course. A spacer might not be practical anyway, certainly not a 2L (d'oh).

Day Trippr, I'm talking about carbonation and serving, though, that's definitely happening at ambient pressure, unless I'm not following you.
 
I suppose you could remove the spear and potentially fill a couple balloons or condoms (unlubricated for chrissake, you're gonna drink the beer, not screw it) with water to take up some headspace, but you probably have some risk for infection (heh) in the knots.

Not sure if this is what day_trippr was thinking or not, but consider how you are going to fill the kegs. If you have the ability to fill the keg under pressure, you may not be wasting much co2 at all. if you flush it with co2, and can fill it with beer under pressure, you'll compress the co2 that is in the flushed keg when you fill it with liquid.

That or find someone who will trade you for a smaller keg.
 
Meh, or I'll just suck it up and accept the 4oz wasted CO2 per batch, or whatever it comes to. Naturally I'm trying to "optimize" my system before I even have it. Thanks!
 
The gas used per gallon of beer for actual carbonation isn't going to change regardless of the vessel it's in - whether a fully filled corny keg, a half-fill quarter barrel, or a four-fifths filled half keg. And it doesn't matter if you carb at room temperature or barely above freezing, the volume of CO2 used for carbonation per gallon of beer will remain constant. What differs is gas "lost" to filling initial head space, and what I illustrated was just how little that would cost in this case...

Cheers!
 
Except it's under pressure, so 3.5 gal of headspace isn't equivalent to half a cubic foot STP but instead half a cubic foot at X PSI, which could be 2x or 10x that volume (I am not a physicist so 2x-10x is my guess, haha). A lot more, anyway, especially if you're using the "shake" method at high pressure, right? Because you have to get the keg's headspace up to a certain pressure and CO2 compresses way way more than beer. Unless, again, I'm missing something in what you're saying, I'm not used to keg-think.
 
Again, the amount of CO2 actually used to carbonate the beer doesn't change with temperature, or shaking/not shaking, or even pressure - if the resulting carbonation level (dissolved volumes of CO2) is the same. Not included is overcarbed or undercarbed beer.

For the head space part, you're right, I oversimplified the cost, as the head space isn't at standard pressure. So, carb at under 14.7 psi and the head space cost will be double or less - so let's say 50 cents. Carb at higher pressure and the cost will go up - yet another reason not to burst carb a keg...

Cheers!
 
The shaking method involves using higher pressure and then releasing it, which is why I mentioned that. So to achieve that PSI, again, you'd use more CO2 to get there due to headspace compression. But I get your point, I suppose most of the CO2 goes into the beer and dispensing it, not the headspace.


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The shaking method involves using higher pressure and then releasing it, [...]

Only if you like to live dangerously. The better "shake and bake" carb method is to set the pressure for the beer temperature the same as if you were going to let it sit for two or three weeks. That way you completely avoid an over-carbed outcome...

Cheers!
 
Again, the amount of CO2 actually used to carbonate the beer doesn't change with temperature, or shaking/not shaking, or even pressure - if the resulting carbonation level (dissolved volumes of CO2) is the same. Not included is overcarbed or undercarbed beer.

For the head space part, you're right, I oversimplified the cost, as the head space isn't at standard pressure. So, carb at under 14.7 psi and the head space cost will be double or less - so let's say 50 cents. Carb at higher pressure and the cost will go up - yet another reason not to burst carb a keg...

Cheers!
@day_trippr I realize that this is a very old thread, but you touched on my exact question. I am getting set up to keg hard cider, and I sometimes make 1-gal experimental batches. My question was how much CO2 would I waste if I carbonate and serve that out of a 5-gal corny keg.

You point out above that the amount of CO2 needed to carb a given quantity of beer (or cider in my case) is the same regardless of keg size. Initially it takes more CO2 to fill the empty space in a partially filled keg, but if I carb at serving pressure I'm not sure that that is wasted. Once you start serving, you are going to need to fill nearly the entire keg with CO2 to force the liquid out, so I'm not sure if the total CO2 usage would be any different. Although you would use a little more initially purging the keg.

Is my thinking about this correct? I can find a used 5-gal corny keg for around $40, but used smaller kegs are difficult to find.
 
As long as you 100% liquid pre-purge the keg (using Starsan) then (partially) fill it with your gallon of homebrew through the liquid-out post, without ever opening the lid, there will be minimal CO2 waste.

Key is: the lid remains on, so no air (oxygen) can go inside. This method can use as little as 6-7 gallons of CO2 gas (left at 2-4 psi when completely full). But as much as 10 gallons of CO2 gas if left at 14 psi, which is wasteful.

I often split 5 gallon batches in 2 (e.g., using different hops), then fill 2 kegs about halfway.

Once the keg kicks (a gallon goes fast), you could simply rinse out the keg the same way as you filled it through the out post, but now, using water or better, Starsan. This is without releasing (much) pressure or removing the lid.
Rinse it 2 or 3 times that way then refill with a fresh batch of beer. Again, key is, all that time the lid remains on and CO2 remains inside the keg!
 
Some instructions on line say to sanitize the keg, then use CO2 to purge the air. Some say to take the top off the keg and use a gravity siphon, but I see the value of leaving the top on the keg.

I found an interesting discussion on this topic here: Keg Filling Easy Procedure

When you pre-purge or rinse with Starsan, do you use a gravity siphon to add the Starsan or water? I assume that you would need to somehow open the gas port so that air could escape?

Then use a little CO2 pressure to push it out? And when you put in the beer, do you use a gravity siphon to add it via the liquid out port?

Hm... I have the AIO wine pump, so if I find the right connection, I could connect the vacuum line to the gas port, and use a vacuum to draw in the water/Starsan/cider.

Thanks for all your help!
 
Is my thinking about this correct? I can find a used 5-gal corny keg for around $40, but used smaller kegs are difficult to find.
You are more likely right than wrong :)

wrt Star San purging here's my blow-by-blow procedure:

I position my bucket holding ~5.5 gallons of mix on a bench top, place a mortar mixing tub below, then set the unlidded keg in it, upright to start. The bucket has a spigot that accepts 7/16" ID vinyl tubing, and I have a short section with both ends bare that I use to fill the keg almost to the top. Then I lid the keg, flip it up-side-down and set it atop a wood fixture I made for this function (not shown in this pic that was taken before I cobbled the fixture together).


1680382410275.jpeg


I have another section of the same tubing with a ball lock gas QD that I attach to the keg, and another section with a beer QD that I also attach, and run that tube into a 2 gallon bucket set next to the keg. I open the bucket spigot and let gravity fill the keg through the gas port and vent through the beer port until the runout is completely clear of bubbles, gently rocking the keg towards the end. The Out dip tube is full length with the tip in its bottom well on the keg and I can get every last bit of air out of the keg.

To blow out the sanitizer, I put the keg on the bench and the bucket in the tub below. I use one of my 5 pound CO2 cylinders to pre-purge its gas line through its gas QD before connecting it to the keg. Then I run a vinyl hose with a beer QD at the end from the keg down to the bucket. Turn on the gas at ~2 psi, snap on the beer QD, and let it rip until it blows nearly clear, then I pop the beer QD off the keg again. I let everything sit for a few minutes to let the last of the sanitizer drip down off the sidewall down to the well then snap the beer QD back on and give the keg another clearing blast.

There will be less than a teaspoon of sanitizer left behind at the end...

Cheers!
 
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