briefly opening a half full keg

[55x11]

What do you assume the percent remaining is if not mass?




Yes, the assumption in the charts is that the O2 concentration at the start is 21% by weight, 21% by mole or 210,000 ppm (all the same thing). Safe assumption is that in this case we can treat the opened keg the same way. Notice the scale is not linear in these graphs. After 2 purges we are down to at least 75,000ppm or 7.5% (total by weight) air. Probably close to if not lower percentage than you would get by opening the lid, maybe using the charts in this situation will end up conservative by a purge, two stretching it.

you are both somewhat correct.

I am with Magic Matt in saying that a half-empty keg is likely to have a lot more air getting in than a full keg, and therefore will require a few more purges than a full keg to get to the same purge level. Each purge will reduce oxygen by the same ratio, regardless of total volume. So if you start with say 1L of air in half-full keg, vs. 0.125L of air in another, say full keg and each purge reduces it by say a factor of 2, it will take 3 purge cycle on first keg to get to the same amount of air as second keg.

But because we are normalizing ppm by the mass of beer in the keg, there's another factor of 2 for half-empty keg (less beer to adsorb oxygen), so you would need 4 more purge cycles for half-full keg with 1L of air in headspace to get to the same ppm as in full keg with 0.125L of air in headspace.

But I don't think you need 12 purges either. In your example above, I don't see how you get that 21% oxygen in headspace is 210,000 ppm. That's not correct - you need to normalize oxygen to beer (water) in the keg. The idea is most oxygen will get dissolved into beer eventually, so when someone says you want to keep oxygen <1 ppm say, it's oxygen to liquid ratio.
You need to calculate weight of oxygen in grams (1 L of air would contain 1/22.4*0.22 moles of oxygen, or about 0.3 grams), divided by 10kg or so (half full keg, 2.5 Gallons) of beer, or about 30 ppm.

So if you want to get below 1 ppm of oxygen, and each purge cuts amount of oxygen in half you will need 2^5=32, or 5 purges at ~14 psi or so.

If you started with say ~10L of air, or 300ppm (completely filled half of the keg with air), you will need another 3-4 purges or total of say 9 purges to get down below 1ppm. So maybe 12 purges is a bit of an overkill.

Matt, Doug - check my math please.

[EDIT: some people like techbrau advocate getting to below 0.15 ppm of oxygen in packaging, which means another ~3 purges, so maybe 12 or more purges is then justified from this perspective]

 

[jerbrew]

@55x11 I think you're confusing what PPM and percent are in this frame of reference. Oxygen is 21% by volume in air (which is the same as saying by mole since it is a gas) 21% is just .21 mole fraction times 100. 210,000 PPM is just mole fraction times 10^6. It's the same thing. I'm not sure where you are introducing mass and oxygen dissolving into beer as 300 PPM in 10L of air. Are you talking about the final amount of O2 that will dissolve into the beer?

Air is 210,000 PPMv regardless of the volume its in. that being said, I see where you're going with this but I think you're neglecting that it will take more CO2 to raise the larger headpace to that pressure. Here's an example,

say you have 1 L of headpace with 0.1grams oxygen (hypothetical making it easy on me). now you have 3L containing 0.3 grams of oxygen. Yes, there is 3X the headspace and 3X the oxygen but it will take 3X the amount of CO2 to increase the pressure of that headpace. So it constitutes the same cut for each purge as a full keg. with 3X the oxygen, 3X the headspace, and the same pressure you'll requires 3X the CO2.

The only thing I've been arguing this whole time.

 

[doug293cz]

This thread is 112% too mathy for me.

It's going to get worse in a big hurry.

My previous purge dilution charts and tables are based on % or ppm of O2 in the headspace. These are easy to understand since they are independent of headspace volume or beer volume. but, what really matters for beer stability (storage life) is Total Packaged Oxigen (TPO), and to determine TPO you need to know the mass of beer, the mass of O2 in the original beer, and the mass of O2 in the headspace (which depends on the headspace volume and O2 concentration in the headspace.) So, ppm O2 in the headspace is only part of the answer.

TPO is equal to the ratio of mass of the total O2 in the keg to the mass of the beer in the keg. Commercial brewers measure TPO in ppb (parts per billion.) Dissolved Oxygen (DO) is equal to the ratio of the mass of O2 in the beer to the mass of the beer. The TPO is equal to the DO (measured in ppb) plus the ratio of O2 mass in the headspace to beer mass (also in ppb.)

To prevent premature degradation of beer flavor due to packaged O2, the Total Packaged Oxygen (TPO) should be less than about 150 ppb (0.15 ppm) (ref: page 21 of http://www.craftbrewersconference.com/wp-content/uploads/2015_presentations/F1540_Darron_Welch.pdf.)

Let's do a little math. If we assume the beer starts with 100 ppb of DO, then the headspace contribution to TPO should be less than 50 ppb, based on the mass of the beer, not the volume of headspace. If we have a beer with an FG of 1.009, then the total O2 allowed is:

1.009 kg/L * 150e-9 = 1.5135e-07 kg/L = 1.5135e-04 g/L = 0.15135 mg/L​

2.5 gal of beer is 9.46 L, so the total allowed O2 in the keg would be:

1.5135e-04 g/L * 9.46 L = 14.323e-04 g = 1.4323 mg​

And the total allowed O2 in the headspace would be:

1.4323 mg * 50 ppb / 150 ppb = 0.477 mg​

Air at 0&#730;C (32&#730;F) has a density of 1.293 g/L. A "five" gal keg has a total volume of about 5.3 gal, so with 2.5 gal of beer, the headspace volume is about 2.8 gal, or 10.6 L. Air is 21% by weight O2. So, if we get 20% air in the headspace after removing the lid, making an addition, and replacing the lid, the mass of O2 in the headspace will be:

1.293 g/L * 10.6 L * 0.21 * 0.20 = 0.576 g => 576 mg​

This is way more than the 0.477 mg target for O2 in the headspace. Thus we must dilute the O2 in the headspace by:

576 mg / 0.477 mg = 1208 times reduction in O2​

The dilution equation for pressurized to atmospheric purging is:

Dilution = ((P[atm] + P[purge])/ P[atm]) ^ N, where N is the number of purge cycles​

We can solve for the required N by doing some algebra on the above equation to get:

N = ln(Dilution) / ln((P[atm] + P[purge])/ P[atm])​

If we purge at 30 psi, then for the dilution target of 1208, the equation becomes:

N = ln (1208) / ln((14.7 psi + 30 psi) / 14.7 psi) = ln(1208) / ln(3.041) = 6.38​

We can't do a fractional number of purges, so we need to round N up to 7. Thus, you need seven purges at 30 psi to get the headspace TPO contribution down to less than 50 ppb for the assumptions used.

Brew on

 

[MidAtlanticBrew]

Love the math and can hang with it, but referencing the handbook of common sense provides the OP the assurance she's requested. The headspace has low O2 to begin with. After relieving pressure, and removing the lid in an enviromnent without gross convective currents, I'd expect little exchange in the minute it takes to perform the procedure. The O2 will be far less then 21%, a couple of farts of CO2 should do it

 

[TexasDroughtBrewery]

This thread escalated...

 

[Terek]

Completely disagree with the above posters.

First, the "8 purges at 25psi thing" is for a completely full keg. If your keg is half empty, you'll need many more purges than that, likely 20+.

Now is it that serious? Probably not. But I would definitely do it at least 10 times. If you plan on this beer being around a while before it kicks (longer than a couple of weeks), I'd highly recommend 20 or more times. Losing a keg, even a half keg, to oxidation is no fun.



This is a false statement. I can guarantee that your beer oxidized on some level if you opened it and didn't purge afterwards. What you really should have said is that you didn't taste it, not that it didn't happen. We all have different tolerances and taste thresholds. It may be that you don't detect (or maybe even prefer) oxidized beer, or that it wasn't enough to detect. But oxygen made it to your beer, you can bet on that.

this is the guy that star sans everything 10 times before using, including his pen, before writing down notes, lol. Some people get WAY too technical than needed. 1 or 2 purges is fine, co2 is so dense, and will fall below any oxygen, and 95% will come out on the first purge. Plus the fact that when you open the keg, the co2 blanket in there doesnt go anywhere. That blanket isnt going to fly out of there and be replaced with oxygen. You may get a bit of o2 in there with what you are doing, but not much at all. 20 or more purges does only one thing, waist co2

 

[Terek]

i think everyone on this thread is forgetting one major fact. Co2 is more dense than o2. When you take the lid off the keg, the co2 blanket does not go anywhere. Its not going to run away and be replaced by o2. You may get a tiny bit of o2 in there, but 95% of the co2 blanket is going to stay put. 1 or 2 purges will get rid of that tiny amount of o2 no problem.

 

[jddevinn]

this is the guy that star sans everything 10 times before using, including his pen, before writing down notes, lol. Some people get WAY too technical than needed. 1 or 2 purges is fine, co2 is so dense, and will fall below any oxygen, and 95% will come out on the first purge. Plus the fact that when you open the keg, the co2 blanket in there doesnt go anywhere. That blanket isnt going to fly out of there and be replaced with oxygen. You may get a bit of o2 in there with what you are doing, but not much at all. 20 or more purges does only one thing, waist co2

i think everyone on this thread is forgetting one major fact. Co2 is more dense than o2. When you take the lid off the keg, the co2 blanket does not go anywhere. Its not going to run away and be replaced by o2. You may get a tiny bit of o2 in there, but 95% of the co2 blanket is going to stay put. 1 or 2 purges will get rid of that tiny amount of o2 no problem.

No one is forgetting that CO2 has a higher molecular weight than "air"; however gasses mix by diffusion and do not stratify. There is no "blanket" and the CO2 IS actually going to "just fly out of there and be replaced with oxygen".

 

[TexasDroughtBrewery]

I have a solution. Just open it up, do what you need to do, purge it once. if it Oxidizes you can send it to me. I'll take one for the team and drink it.

 

[doug293cz]

this is the guy that star sans everything 10 times before using, including his pen, before writing down notes, lol. Some people get WAY too technical than needed. 1 or 2 purges is fine, co2 is so dense, and will fall below any oxygen, and 95% will come out on the first purge. Plus the fact that when you open the keg, the co2 blanket in there doesnt go anywhere. That blanket isnt going to fly out of there and be replaced with oxygen. You may get a bit of o2 in there with what you are doing, but not much at all. 20 or more purges does only one thing, waist co2

i think everyone on this thread is forgetting one major fact. Co2 is more dense than o2. When you take the lid off the keg, the co2 blanket does not go anywhere. Its not going to run away and be replaced by o2. You may get a tiny bit of o2 in there, but 95% of the co2 blanket is going to stay put. 1 or 2 purges will get rid of that tiny amount of o2 no problem.

Sorry, but you are just plain wrong. As @jddevinn noted, gases homogenize spontaneously, rather than stratifying. This is well known physics. The video below demonstrates how homogenization via diffusion works. Note that Br2 (used in the first demo) is 3.63 times heavier than CO2, and it homogenizes with air in about 30 minutes, when no convection is present. A later demo uses NO2, which is similar in weight to CO2 (46 vs 44). As shown the NO2 homogenizes much faster than the Br2. The presence of any convection will speed up the homogenization by orders of magnitude. The only time you can have a CO2 blanket is when you are generating CO2 faster than diffusion and/or convection dissipates the blanket.

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

Brew on

 

[jddevinn]

The only time you can have a CO2 blanket is when you are generating CO2 faster than diffusion and/or convection dissipates the blanket.


Brew on

Which is why the simple answer to this question is to generate a bunch of CO2 :fro: Stream a bunch through the gas inlet while adding the addition with the minimum opening for diffusion possible. Then purge a good amount for good measure. CO2 is pretty damn cheap.

 

[PapaO]

The only time you can have a CO2 blanket is when you are generating CO2 faster than diffusion and/or convection dissipates the blanket.

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

Brew on

You are generating CO2 its coming out of solution because there is on head pressure.

 

[doug293cz]

You are generating CO2 its coming out of solution because there is on head pressure.

It's unlikely that CO2 is coming out of solution fast enough to overcome the convection and diffusion causing mixing at the lid opening.

Brew on

 

[Terek]

No one is forgetting that CO2 has a higher molecular weight than "air"; however gasses mix by diffusion and do not stratify. There is no "blanket" and the CO2 IS actually going to "just fly out of there and be replaced with oxygen".

so do you purge your fermenter every time you open it to check gravity? Because you are basically saying that if you dont, all your beer is now oxidized. Every time you open your fermenter.

It takes time for diffusion to happen, the OP says that he opened it briefly. As in seconds. Not for a day

 

[Terek]

Sorry, but you are just plain wrong. As @jddevinn noted, gases homogenize spontaneously, rather than stratifying. This is well known physics. The video below demonstrates how homogenization via diffusion works. Note that Br2 (used in the first demo) is 3.63 times heavier than CO2, and it homogenizes with air in about 30 minutes, when no convection is present. A later demo uses NO2, which is similar in weight to CO2 (46 vs 44). As shown the NO2 homogenizes much faster than the Br2. The presence of any convection will speed up the homogenization by orders of magnitude. The only time you can have a CO2 blanket is when you are generating CO2 faster than diffusion and/or convection dissipates the blanket.

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

Brew on

i will say the same as i did to the other guy.


so do you purge your fermenter every time you open it to check gravity? Because you are basically saying that if you dont, all your beer is now oxidized. Every time you open your fermenter.

It takes time for diffusion to happen, the OP says that he opened it briefly. As in seconds. Not for a day

 

[MagicMatt]

so do you purge your fermenter every time you open it to check gravity? Because you are basically saying that if you dont, all your beer is now oxidized. Every time you open your fermenter.

I can't speak for these guys, but yes I absolutely do purge the fermenter with CO2 if I need to open it. However, for the past year or so I've just been waiting until I keg it to take the sample. I leave the beer in the carboy for several weeks and am in no rush to keg usually, so checking gravity isn't important. In the off chance that the beer isn't finished (which has never happened), it would finish in the keg, no big deal.

P.S. even if you don't purge the fermenter, "all [of] your beer" is not oxidized. The risk of oxidation is definitely increased, but only a very small portion is surface area and will allow for diffusion, which limits the rate of which that can happen. Personally I want to eliminate all oxygen at every step possible, but the threshold for oxidation is generally higher than what you describe. Opening the fermnter just once and only for a minute, especially if it's a carboy (i.e. narrow opening), will likely not have a noticeable impact. It really depends on your methodology, but personally if I'm taking a reading from the carboy and it's done, I'm transferring to a keg which will then purge any exposed O2. If the beer isn't done when checked, then there should be enough off-gassing to eventually push most of the exposed oxygen out via the airlock (natural purging).

 

[jddevinn]

so do you purge your fermenter every time you open it to check gravity? Because you are basically saying that if you dont, all your beer is now oxidized. Every time you open your fermenter.

It takes time for diffusion to happen, the OP says that he opened it briefly. As in seconds. Not for a day

I ferment in conicals and use a sampling valve while the conical is under positive CO2 pressure. From the time I pitch and oxidize I do not see the beer again (and it does not see air) until it is in my cup to drink.

Diffusion is slowed through the narrow opening of the carboy, bucket will have no transfer. If you check the gravity and it is done you are transferring anyway, if not it is actively releasing CO2. When I did use carboys I used a syringe with a ss tube that I put through the airlock bung to minimize air ingress.

How many times are you taking gravity?? For established recipes I'm normally "one and done" on measurements.

 

[doug293cz]

i will say the same as i did to the other guy.


so do you purge your fermenter every time you open it to check gravity? Because you are basically saying that if you dont, all your beer is now oxidized. Every time you open your fermenter.

It takes time for diffusion to happen, the OP says that he opened it briefly. As in seconds. Not for a day

I don't open fermenters to take gravity readings. I used to remove the air-lock from the bucket lid and pull a sample using a straight piece of rigid tube. I used a refractometer to check for fermentation endpoint, so only needed a few drops for a sample. Currently I use a Tilt wireless hydrometer to monitor fermentation, so there is no need to even remove the airlock to sample. I have also switched from bucket fermenters to PET wide-mouth jar fermenters to reduce air ingress (primarily due to the iffy seal around bucket lids.) With the new fermenters, I can do a pressurized transfer from the fermenter to a pre-purged keg (liquid method) to avoid air exposure during the transfer. The weak point I still have is air suck-back thru the airlock during cold crashing. I plan to eliminate this exposure by getting a 6.5 gal corny keg, and transferring to it prior to completion of fermentation, and then finishing with a pressurized (via spunding valve) fermentation and cold crash in the keg. At this point, my wort will never be exposed to the atmosphere after initial oxygenation in the fermenter.

As the video demonstrates, the time frame for complete interdiffusion is minutes, not days, and you don't need complete interdiffusion to have too much O2 in the headspace. As @jddevinn pointed out, the amount of air that enters a vessel when opened is dependent on the area of the opening. A carboy is much better than a bucket, and a keg is somewhere in between those two. OP asked about opening up a keg, and the removal and replacement of a keg lid adds a significant amount of convection to the equation. Convection mixes gases orders of magnitude faster than diffusion alone. Thus a keg will definitely contain enough O2 after opening to adversely affect the beer over time. Exactly how much O2 is impossible to determine a priori. If you don't have the ability to measure ppb levels of O2 in the keg, then the only safe option is to purge for the estimated worst case, and that is the analysis I presented in a previous post.

The fact that you seem to not like the science does not invalidate the science. If you expose your beer to O2 post fermentation, you will get some amount of oxidation. The more O2, the more oxidation. Temperature plays a big role in the rate of oxidation, so colder beer takes longer to reach the same level of oxidation as warmer beer.

If you are happy with how your beer tastes over its entire lifetime, then you don't need to make any changes to your procedures. Different people have different taste thresholds for oxidation effects. If yours is high, then you don't need to be as careful.

When people ask about best practices, or underlying science, I will attempt to provide the best information available (as I understand it), assuming I have some expertise in the area of interest. What anyone does with the information is up to them. I only try to educate, not proscribe behavior.

Brew on

 

[MagicMatt]

Doug, to solve the cold crash issue I initially used solid bungs, which would allow negative pressure to build in the carboy, but opening them when it came time to transfer let in a whoosh of air. I wasn't too concerned as I still did open air transfers at the time (auto-siphon into the keg with lid off) and would be purging the keg just a few minutes later.

Now, I use a gas ball lock post attached to a short length of tubing inserted into a bung. This bung goes on the carboy prior to cold crashing and allows the negative pressure to build (no air entering), and when it's time to transfer I hook up my CO2 to the post and apply 1-2psi until the pressure stabilizes.

 

[normonster]

Doug, to solve the cold crash issue I initially used solid bungs, which would allow negative pressure to build in the carboy, but opening them when it came time to transfer let in a whoosh of air. I wasn't too concerned as I still did open air transfers at the time (auto-siphon into the keg with lid off) and would be purging the keg just a few minutes later.

Now, I use a gas ball lock post attached to a short length of tubing inserted into a bung. This bung goes on the carboy prior to cold crashing and allows the negative pressure to build (no air entering), and when it's time to transfer I hook up my CO2 to the post and apply 1-2psi until the pressure stabilizes.

Dude, you and @doug293cz just changed my life.

Straight brilliant!! Makes me wonder what type of oxidation threshold I have though, because I have essentially let a **** ton of air into my fermenters and kegs but I have never had a beer come out that I thought matched up with an oxidation flaw. Maybe it's all that Copenhagen!!!!!

 

[jcav]

Ok so I keg and do the best I can to do oxygen free transfers from my conical into the kegs, under Co2 pressure. I also purge the sealed keg 10 or 12 times at 25-30 psi when I am done transferring just to make sure. But what about all those who transfer into a bottling bucket to prime with corn sugar, and then bottle. Doesn't this oxidize the beer as it sits in the open bottling bucket. Even if you covered it, it wasn't purged first. So is everyone then that bottles and uses a bottling bucket essentially oxidizing their beer? Your thoughts....

John

 

[day_trippr]

Sans purging the bucket there's substantial surface area exposed to oxygen for the duration of the bottling phase, so certainly there's some degree of take-up. Plus some degree of agitation while mixing in the primer will add to that by exposing a greater volume of beer to the surface.

Perhaps the saving factor is the yeast may consume some of that O2 during the carbonation process...

Cheers!

 

[jcav]

That's what I thought....makes sense about the yeast too, forgot about that!

John

 

[doug293cz]

Doug, to solve the cold crash issue I initially used solid bungs, which would allow negative pressure to build in the carboy, but opening them when it came time to transfer let in a whoosh of air. I wasn't too concerned as I still did open air transfers at the time (auto-siphon into the keg with lid off) and would be purging the keg just a few minutes later.

Now, I use a gas ball lock post attached to a short length of tubing inserted into a bung. This bung goes on the carboy prior to cold crashing and allows the negative pressure to build (no air entering), and when it's time to transfer I hook up my CO2 to the post and apply 1-2psi until the pressure stabilizes.

I don't think the solid bung/stopper would work well on a bucket just because the lid seals are so bad. It could work on the PET vessels I have now, but I don't know how much negative pressure they can take before collapsing. I don't think I'm curious enough to test one to failure, cause they're not cheap.

Brew on

 

[MagicMatt]

I don't think the solid bung/stopper would work well on a bucket just because the lid seals are so bad. It could work on the PET vessels I have now, but I don't know how much negative pressure they can take before collapsing. I don't think I'm curious enough to test one to failure, cause they're not cheap.

Brew on

Not sure which brand of PET carboy you're using, but those things can handle a great deal of negative pressure from what I've seen:

I use glass and haven't had any issues (yet, though it did concern me for a while), and I would think glass would break before a PET vessel would. Soda bottles are PET and can withstand >60-70 psi. Not sure how negative pressure correlates though.

I wonder if there's essentially a backwards spunding valve, that would only draw gas from the CO2 tank when pressure drops below a certain threshold.

 

[doug293cz]

Not sure which brand of PET carboy you're using, but those things can handle a great deal of negative pressure from what I've seen:

I use glass and haven't had any issues (yet, though it did concern me for a while), and I would think glass would break before a PET vessel would. Soda bottles are PET and can withstand >60-70 psi. Not sure how negative pressure correlates though.

I wonder if there's essentially a backwards spunding valve, that would only draw gas from the CO2 tank when pressure drops below a certain threshold.

I'm not worried about a catastrophic implosion, but rather partial collapse inwards with creasing of the vessel walls. Soda bottles crease pretty readily (I know 'cause I use them instead of growlers.) Repeated creasing could weaken the walls, and the creases are not pleasing aesthetically.

I really like the reverse spunding valve idea. Such a device is definitely feasible. Now, can we figure out how to implement one in a cost effective way?

Brew on

 

[TexasDroughtBrewery]

I have a small keg of cider that's cold and carbed where I want it. I want to add some malic acid to improve the taste. Turn off the gas, bleed pressure, pop the lid, add the acid (dissolved in cider), close the lid...

How thorough do I need to be in purging the keg afterward? Will one or two purges at serving pressure be enough, or do I need to do the whole 8 purges at 25 psi thing again?

Ive got to know after 6 pages of debate what you ended up going with and if your cider is fine?

 

[Maylar]

Ive got to know after 6 pages of debate what you ended up going with and if your cider is fine?

Heh - Haven't done it yet. I'm stuck on a different aspect of this problem, which is figuring out exactly how to make the acid adjustment that I want first.

What I've decided to do is to push a couple psi of CO2 through the liquid post for the few seconds it'll take me to pour a 1/4 cup into the keg. I figure that'll hold back any O2 trying to sneak its way in. Probably purge a couple times anyway just because.

I'll report back when it's a done deal.

 

[jddevinn]

Daytripper had a good method of positive CO2 pressure in carboys https://www.homebrewtalk.com/showpost.php?p=7419568&postcount=28

and there is a whole thread discussing it here https://www.homebrewtalk.com/showthread.php?t=573127

 

[MagicMatt]

Digging through that thread now, but what's the purpose of the balloons?

I've seen people use balloons to keep the pressure up when cold crashing, but not like this.