How do I purge a keg?

[CMcPherson]

I'm going to be filling a corny with 4 gals of cider.
I now know that I need to do several purges to get rid of O2 in the head space.
How do I do it?
Do I fill and seal the keg, then simply repeatedly pressurize and use the PRV?

 

[FloppyKnockers]

I'm going to be filling a corny with 4 gals of cider.
I now know that I need to do several purges to get rid of O2 in the head space.
How do I do it?
Do I fill and seal the keg, then simply repeatedly pressurize and use the PRV?

You got it. It's definitely not rocket surgery. I always overdo it, but hey...better too much than not enough in this case.

 

[CMcPherson]

Good... a rocket surgeon I ain't.

 

[MagicMatt]

You got it. It's definitely not rocket surgery. I always overdo it, but hey...better too much than not enough in this case.

You may not be overdoing it as much as you think. doug293cz recently gave some great info on oxygen as it relates to purging:

You need to do more than "bleed out the o2 with a bit of co2." Commercial breweries shoot for less than 0.2 ppm or 200 ppb (parts per billion) TPO (Total Packaged Oxygen) to minimize staling from oxidation (ref: page 21 of http://www.craftbrewersconference.com/wp-content/uploads/2015_presentations/F1540_Darron_Welch.pdf.) The longer you age, the more important controlling TPO is, unless you are looking for some oxidation flavors like sherry notes (which might be ok in a barley wine, I'm not an expert on those.)

TPO is calculated as:

(Headspace Vol * Headspace O2 + Beer Vol * Beer O2) / Package Vol
Package Vol = Beer Vol + Headspace Vol​

If you fill the keg by siphoning thru the open lid, then it takes multiple pressurize and vent cycles to get the O2 down to ppm or ppb levels. The chart and table below show how much O2 is left in ppm for different numbers of purge cycles at different pressures.

View attachment 329471

View attachment 329472

You can get lower O2 levels with less CO2 usage by filling the keg completely with water or StarSan, pushing the liquid out with CO2, and then filling the keg thru the liquid out post (need to open the PRV to allow filling.)

Each brewer needs to decide for them self how OCD they want to be about minimizing TPO.

Brew on

 

[CMcPherson]

He was kind enough to do the same for me as well.
I do plan on following that regimen but just wanted to confirm that I was actually doing the mechanics right, now that I actually own some kegs.

 

[CMcPherson]

Oops... one more thing:
For each purge cycle, I should close the gas by working the in-line shut off valve right?

 

[doug293cz]

Oops... one more thing:
For each purge cycle, I should close the gas by working the in-line shut off valve right?

Yes.

Brew on

 

[303Dan]

You may not be overdoing it as much as you think. doug293cz recently gave some great info on oxygen as it relates to purging:

Yeah, these graphs were very surprising to me. Before seeing those, I was purging 6 or 7 times @ 10psi and thought I was on the anal-retentive side of purging kegs. From here on out, I plan on 13 purges @30 psi. Excessive? Perhaps, but if it could possibly make a difference, especially for my highly hopped beers, it's worth it to me. CO2 is pretty cheap, at least where I get it.

 

[MagicMatt]

Hm, I was going to say "No". I've never done such - just pull the PRV, release and let the keg repressurise, pull again, etc.

What's the rationale behind closing the shut-off?

 

[doug293cz]

Hm, I was going to say "No". I've never done such - just pull the PRV, release and let the keg repressurise, pull again, etc.

What's the rationale behind closing the shut-off?

If you leave the valve between the regulator and keg open during the vent, you will use significantly more CO2 during the process since extra CO2 flows during the venting. You could get by with fewer purge cycles by leaving the valve open, since you are pushing out more of the residual O2 during each cycle, but there is no way to calculate the number of cycles to get to a specific O2 concentration.

Brew on

 

[MagicMatt]

Ah that makes sense. Yeah I guess I never thought about the CO2 continuing to flow since the purge is so quick. I may experiment a bit to see how much extra CO2 gets used and if it's worth it (to me) to close the shut-off before each pull of the PRV.

 

[CMcPherson]

If you leave the valve between the regulator and keg open...
Brew on

Thanks for explaining.
I figured that I couldn't be sure that I was getting the purges that I should be without overdoing it on the gas.
Anal retentive checkmate! :rockin:

 

[PKnotlost]

It doesn't take in account a few variables that work in our favor. Gas dose stratifi, Its provable and repeatable so i'm not going to beat that one to death.

Once you have purged a few times the the cylinder is fairly still a bit the heavy gas CO2 is going to sink, yay good for us. So just because there is going to be a few parts per million of O2 don't mean its effecting your beer.

Second thing is in the aging process requires some O2, not a lot but it dose contribute, study it out and you will find this out as well.

 

[Firewalker11]

Why purge it in the first place? Hungry yeast will gobble it up as it returns to solution and will be displaced by CO2 anyway.

 

[day_trippr]

The "CO2 blanket" is a myth...

Cheers!

 

[doug293cz]

It doesn't take in account a few variables that work in our favor. Gas dose stratifi, Its provable and repeatable so i'm not going to beat that one to death.

Once you have purged a few times the the cylinder is fairly still a bit the heavy gas CO2 is going to sink, yay good for us. So just because there is going to be a few parts per million of O2 don't mean its effecting your beer.

Second thing is in the aging process requires some O2, not a lot but it dose contribute, study it out and you will find this out as well.

I see you flunked physical chemistry. Gas does not stratify. In fact the opposite happens and gases spontaneously homogenize due to diffusion. Watch the following video. Bromine gas is about 5.5 times heavier than air and completely homogenizes in less than 30 minutes. CO2 is only about 1.5 times heavier than air so homogenizes much quicker. CO2 (mol wt 44) is about the same as NO2 (mol wt 46) so will behave similar w.r.t homogenization as the NO2 shown later in the video.

[ame]https://www.youtube.com/watch?v=_oLPBnhOCjM[/ame]

Also, you normally don't want O2 in the packaged beer. Commercial TPO (total packaged oxygen) targets are below 200 ppb (0.2 ppm.) Ref: page 21 of http://mcdantim.com/category/beer-gas-101/.

Brew on

 

[doug293cz]

Why purge it in the first place? Hungry yeast will gobble it up as it returns to solution and will be displaced by CO2 anyway.

You don't have hungry yeast gobbling up anything in a force carbed keg, as there is no remaining sugar for the yeast to eat. The O2 does not go away, and eventually some of it will chemically combine with beer components leading to stale flavors.

Brew on

 

[PKnotlost]

Good for you, doug293cz you get your education from schools and utube.

No I did not flunk chemistry, I staked my life on knowing what im talking about hundreds of time in the field. I have mixed thousands of CF of gas. How about you? Be careful of assuming you know something, I know whats taught and have seen it wrong many times, I have O2 meters, as well as CO2 meters and have been using them for years, the white coats are not always right in the field. I know gas isn't suppose to, but it dose, if your willing to take the gamble I will mix up a tank of HE and 32% O2 with no blending and you can breath it and stake your life on it. Dont worry I wouldn't let you die, but you would if someone didn't help you after you passed out. In other words I can repeat it consistently despite what anyone says or shows in a simple lab test.

I never tested the head gas in a corny so I will have to do some field work and not trust the utube. But having said all that I have never seen it stratifi after it was mixed, that I have tested and not seen myself.

Im probably wrong though cause its on the internet and has to be true.

 

[user 204491]

But seriously though, I'd just come here to say thanks to Doug as I definitely learned something from his posts in other threads and now routinely purge 13(ish) times @30PSI when I fill a keg.

The "ish" part is because it's pretty easy to lose count... fill, purge, drink, one, fill, purge, drink, two, fill, purge, drink, where was I?

 

[treacheroustexan]

I fill my keg, seal it up, turn on gas, purge 10-15 times, set it 30 psi for 36 hrs and back down to serving pressure.

 

[MagicMatt]

It doesn't take in account a few variables that work in our favor. Gas dose stratifi, Its provable and repeatable so i'm not going to beat that one to death.

Once you have purged a few times the the cylinder is fairly still a bit the heavy gas CO2 is going to sink, yay good for us. So just because there is going to be a few parts per million of O2 don't mean its effecting your beer.

Second thing is in the aging process requires some O2, not a lot but it dose contribute, study it out and you will find this out as well.

Oh geez not another "the CO2 will sink" guy. Talk about beating something to death. This has been disproved on here numerous times, by people clearly smarter than you or I (and I'm pretty smart, so that's not easy for me to say).

And a history of science tells us otherwise as well. Take for example this paper, published over 90 years ago. Why are we still talking about this?

Gases do diffuse. Carbon dioxide is only slightly more dense than oxygen. But the difference in "weight" due to the force of gravity is so small compared to the random thermal motion of the molecules, that any sort of stratification under normal conditions is essentially impossible, especially when you consider the large amount of CO2 present relative to the oxygen present in the given circumstances.

Look I'm all for "getting it right", so I'll give you the benefit of doubt: give us a scientific example proving your theory then. If it is indeed true, you should have no problem finding a few on the internet. But my guess is you won't (because you can't, just like others before you).

 

[Firewalker11]

You don't have hungry yeast gobbling up anything in a force carbed keg, as there is no remaining sugar for the yeast to eat. The O2 does not go away, and eventually some of it will chemically combine with beer components leading to stale flavors.

Brew on

Hmmmm, did we cover that already? I just read his OP again, "I'm going to be filling a corny with 4 gals of cider.
I now know that I need to do several purges to get rid of O2 in the head space.
How do I do it?
Do I fill and seal the keg, then simply repeatedly pressurize and use the PRV?"

Sounds to me like its a fresh cider, is it not?

 

[doug293cz]

Hmmmm, did we cover that already? I just read his OP again, "I'm going to be filling a corny with 4 gals of cider.
I now know that I need to do several purges to get rid of O2 in the head space.
How do I do it?
Do I fill and seal the keg, then simply repeatedly pressurize and use the PRV?"

Sounds to me like its a fresh cider, is it not?

After rereading the OP, it sounds like OP is packaging finished cider, not fermenting cider in a keg. Why would anyone purge prior to fermentation? Maybe the OP can clarify just what stage of the process they were at.

Also, nothing gets "displaced" from a sealed keg. O2 and N2 get displaced during purge cycles, but when the purging is done, what's in the keg stays in the keg. Whatever O2 doesn't get consumed by yeast activity (if there is any), stays in the keg, either in the headspace, dissolved in the liquid, or reacted with some component of the liquid. The predominant yeast action of converting maltose (and other sugars) to alcohol does not consume O2.



Brew on

 

[CMcPherson]

Good for you, doug293cz you get your education from schools and utube.

Man... we've both been here the same length of time.
I'm brand new to all of this so I can only assume that you have more experience than I do.
These guys and gals here have been super helpful and generous with their patients with me.
Please... if you have some sort of score to settle, do it on someone else's thread okay?

 

[CMcPherson]

But seriously though, I'd just come here to say thanks to Doug as I definitely learned something from his posts in other threads and now routinely purge 13(ish) times @30PSI when I fill a keg.

The "ish" part is because it's pretty easy to lose count... fill, purge, drink, one, fill, purge, drink, two, fill, purge, drink, where was I?

Ditto!
I've grown to count on a handful of people here that are willing to take their time with me.

 

[TheMadKing]

If gasses stratified by mass, we would be breathing radon and argon at sea level, nitrogen in Montana, oxygen in Colorado, helium in Peru, and hydrogen in nepal. The atmosphere is proof.

Gas only appears to stratify when it has a constant emission source that emits gas at a rate that exceeds its diffusion speed (like CO2 pockets in caves). Gas DOES however form a gradient of density by elevation, but all gasses mix equally throughout that gradient. So the amount of radon may be higher at lower levels, but the amount of oxygen is equally higher, so the relative concentrations stay the same because of gas diffusion.

Thanks for the great thread OP! And thanks again for all you do Doug!

 

[CMcPherson]

Oh geez not another "the CO2 will sink" guy. Talk about beating something to death. This has been disproved on here numerous times, by people clearly smarter than you or I (and I'm pretty smart, so that's not easy for me to say).

I am (was?) a "the CO2 will sink" guy because that's what I've been reading in some of the beginning cider books.
The kid at the local brew supply agreed that filling my carboy first with CO2 would protect my finished cider from O2 when racking.
So... how best to keep O2 off my cider when racking?

After rereading the OP, it sounds like OP is packaging finished cider, not fermenting cider in a keg. Why would anyone purge prior to fermentation? Maybe the OP can clarify just what stage of the process they were at.

Finished cider. Sorry for the confusion.

 

[MagicMatt]

I am (was?) a "the CO2 will sink" guy because that's what I've been reading in some of the beginning cider books.
The kid at the local brew supply agreed that filling my carboy first with CO2 would protect my finished cider from O2 when racking.
So... how best to keep O2 off my cider when racking?

Ha, tbh I was once 'that guy' too. We all have to start somewhere, right?

As for oxygen-free transferring, you can first purge your keg (many of us fill it completely with sanitizer and use CO2 to push it out, effectively leaving very minimal residual O2 in the keg). Then you can connect your tubing from your racking cane to a quick-disconnect, and have the beer flow into the keg via the 'out' post (diptube). You'll have to either pop up the PRV and keep it open so that the air in the keg can escape as the beer fills it, or you can connect a gas QD to the gas post and let the air out that way.

Here's a pic to illustrate:

Purging the keg afterwards is still recommended.

If you really don't want the beer to touch O2, you can also get one of the carboy caps with 2 ports and use one for your racking cane, and the other to push CO2 into the carboy (and thus use the CO2 to push the beer out via the racking cane). I don't go this far, as I feel that the few minutes that the beer may have in contact with any O2 drawn into the carboy will be purged anyway. It's such a short period of time and I don't find the beer has a chance to absorb any O2 (though I'm sure it happens on a very small scale).

 

[CMcPherson]

As for oxygen-free transferring, you can first purge your keg (many of us fill it completely with sanitizer and use CO2 to push it out, effectively leaving very minimal residual O2 in the keg).

Thanks for the visual.
What about between two carboys instead of into a keg?

 

[MagicMatt]

Same basic principal. I do purge my carboys with CO2 before transferring to secondary (which I rarely do anymore), but it's not nearly as effective as using a keg. Basically I would just drop my gas line all the way to the bottom of the carboy and open the valve. Usually 5-10psi. I'd let it run for a minute or so, then remove the hose and cap the carboy until I was ready to fill it. Generally I would still have a couple of gravity points to drop when I did this, so I knew there would be some CO2 production by the secondary fermentation, however minimal this actually was. I'm not sure how effective this really is, but it made me feel better.

But there should be no reason you can't treat it the same as the keg. Fill the carboy to the brim with sanitizer, put a two-port cap on it and push the sanitizer out with CO2 via a racking cane. That'll be as close as you'll get to minimizing the O2 I believe.

It's all a matter of how important is it to you if you want to do all of that or not. We all have our thresholds. Many people have homebrewed beer for a long time without doing any of this and I'm sure most of those beers came out fine. As Doug pointed out, the longer you plan to age the beer, the more effect oxygen tends to have. If you're aging a barleywine or something for a year, then yeah it's probably worth it. If it's an IPA that will be kicked within a couple of weeks, personally I wouldn't worry too much.