briefly opening a half full keg

[Maylar]

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?

 

[TexasDroughtBrewery]

I think this just depends on your comfort level. I have had to open a few kegs for various reasons in the time I have been brewing. I can't say I have ever done 8 whole purges at 25PSI. Honestly I just do one quick purge and I have never had an issue.... honestly once or twice I didn't even purge and the beer still didn't oxidize.

But I tend to think beer is a lot more forgiving than some of the members on this forum.

 

[porterpounder]

Not very much at all. It's carbonated cider that was then vented. Give it a little quick purge pull and away you go.

 

[MagicMatt]

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.

.... honestly once or twice I didn't even purge and the beer still didn't oxidize.

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.

 

[day_trippr]

Oxidation potential aside, be very careful when removing the lid to not have the O-ring end up in the bottom.

And if it does, just sanitize another O-ring and leave that one 'til the keg kicks...

Cheers!

 

[jerbrew]

@magicmatt is wrong. hes failing to consider that in order to achieve 25 PSI in a half full keg requires more CO2 (Mass or moles take your pick) then a completely full keg. If anything you'll need less purging cycles to reduce the concentration of O2 in the headspace. The 8 times at 30 PSI (there's an awesome post out there somewhere) is considering the headspace is 21% oxygen (air). And the amount of moles required to raise the pressure in the head space to 30 PSI dilutes the oxygen to whatever percent. This is assumed to be well mixed so now when the keg is purged you have considerably less residual oxygen. Do this several times and you've reduced the O2 content to almost nothing.

Opening and then closing the keg quickly will not result in the headspace to come back to 21% oxygen. And the number of moles required to increase the pressure to 30 PSI is directly proportional to the volume of the headspace (PV=nRT). So if pressure and temperature are equal V1/n1 = V2/n2. You are required to put in more CO2 to achieve the same pressure. So the 8 purges at 30PSI still holds if the headspace was 21% oxygen which it won't be since diffusion is a slow process and as long as you're not blowing a fan into it convective transfer should be minimal. There is no way to know exactly how many purges you should do but if you wanted to do a couple you'd probably be fine and if you don't mind wasting CO2 do it the full 8 times or so.

 

[MagicMatt]

@magicmatt is wrong. hes failing to consider that in order to achieve 25 PSI in a half full keg requires more CO2 (Mass or moles take your pick) then a completely full keg. If anything you'll need less purging cycles to reduce the concentration of O2 in the headspace.

Um I'll just leave this here........note this is showing percent of O2 remaining, not mass. More head space = more purges needed to get to lower levels of O2.

That will do a pretty good job of getting the O2 level down. It's not because of diffusion however. The reduction in O2 (and N2) is strictly due to dilution. The purge pressure affects how many purge cycles you need to get to a particular O2 level. The table and chart below shows how pressure and number of cycles affects the O2 levels. The values are percent of original remaining, not mass of remaining. The mass remaining depends on the headspace volume.

View attachment 323772

View attachment 323773

Brew on

Opening and then closing the keg quickly will not result in the headspace to come back to 21% oxygen. And the number of moles required to increase the pressure to 30 PSI is directly proportional to the volume of the headspace (PV=nRT). So if pressure and temperature are equal V1/n1 = V2/n2. You are required to put in more CO2 to achieve the same pressure. So the 8 purges at 30PSI still holds if the headspace was 21% oxygen which it won't be since diffusion is a slow process and as long as you're not blowing a fan into it convective transfer should be minimal.

True, it won't reach 21%, but it's not crazy to consider that 1/5th of the head space will be replaced by atmospheric air if you open the keg lid for 60 seconds. Unless you're in a draft-free room with practically no air movement. Just your breathing will be enough though. Regardless, even if 20% did get replaced with air (of which 20% is oxygen), that still leaves 5% of your headspace as oxygen. Please tell me how you think a 2.5gallon head space will be able to purge out all 5% of the oxygen in just a couple of purges?

There is no way to know exactly how many purges you should do but if you wanted to do a couple you'd probably be fine and if you don't mind wasting CO2 do it the full 8 times or so.

It's not "wasting CO2", it's "preserving beer". Beer that I took time and energy to make. I don't know about you, but I don't drink CO2, so I don't really care how much I have (or don't have) left. I do drink beer though, and oxidized beer is of no use to me. I'd rather use ("waste") the $0.25 in CO2 to preserve my $15 in beer.

I bet you're one of those people who doesn't want to "waste" the $2 on the parking meter, only to return to a $50 ticket. A little precaution goes a long way. To cite the obvious, "better safe than sorry".

 

[bosster01]

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.

Bro. If a beer is oxidized and you can't taste a difference is it really oxidized?

 

[bosster01]

On a different note. I stupidly put my keg posts on backwards so it was super hard for me to attach my gas line.

I had to open the keg, and stick my hand in a little bit to get the posts reversed. I'm still going to drink it.

If it starts tasting bad then I'll worry.

 

[MidAtlanticBrew]

This post is way off base. Don't worry, very little O2 will get into the keg if the lid is off for a short period of time. I have no doubt you can easier minimze convection and if you do, then O2 ingress is down to diffusion which is a much slower process.

True, it won't reach 21%, but it's not crazy to consider that 1/5th of the head space will be replaced by atmospheric air if you open the keg lid for 60 seconds. Unless you're in a draft-free room with practically no air movement. Just your breathing will be enough though. Regardless, even if 20% did get replaced with air (of which 20% is oxygen), that still leaves 5% of your headspace as oxygen. Please tell me how you think a 2.5gallon head space will be able to purge out all 5% of the oxygen in just a couple of purges?
.

 

[MagicMatt]

Bro. If a beer is oxidized and you can't taste a difference is it really oxidized?

By definition, yes I would say it certainly is.

Don't worry, very little O2 will get into the keg if the lid is off for a short period of time. I have no doubt you can easier minimze convection and if you do, then O2 ingress is down to diffusion which is a much slower process.

Diffusion starts happening instantly in a sealed environment. What makes you think it won't happen more rapidly in your living room or basement? Perhaps to you "very little O2" isn't a concern, but in a place where we're trying to eliminate oxygen completely one would think that a "very little" is still too much.

Anyway, I already stated that it's not likely to warrant a large concern, but that's my opinion, just like it's your opinion. OP asked about oxidation and we shouldn't be shying away from the truth just because you don't "feel" it's important. And the truth is that oxygen will get into the beer and the only way to effectively reduce its presence is by multiple purges. It's not a requirement, and no one is telling you that you'll go to hell if you don't do it.

Will you still have drinkable beer if you open the lid and don't purge? Most likely. Will oxidation be detectable? Maybe. Is it worth the risk to you? Seems like you've already made up your mind on that one. To me, it's not.

But don't take it from me......we can go to the source if you want. @doug293cz, what say you? Care to chime in?

 

[jerbrew]

Um I'll just leave this here........note this is showing percent of O2 remaining, not mass. More head space = more purges needed to get to lower levels of O2.






True, it won't reach 21%, but it's not crazy to consider that 1/5th of the head space will be replaced by atmospheric air if you open the keg lid for 60 seconds. Unless you're in a draft-free room with practically no air movement. Just your breathing will be enough though. Regardless, even if 20% did get replaced with air (of which 20% is oxygen), that still leaves 5% of your headspace as oxygen. Please tell me how you think a 2.5gallon head space will be able to purge out all 5% of the oxygen in just a couple of purges?



It's not "wasting CO2", it's "preserving beer". Beer that I took time and energy to make. I don't know about you, but I don't drink CO2, so I don't really care how much I have (or don't have) left. I do drink beer though, and oxidized beer is of no use to me. I'd rather use ("waste") the $0.25 in CO2 to preserve my $15 in beer.

I bet you're one of those people who doesn't want to "waste" the $2 on the parking meter, only to return to a $50 ticket. A little precaution goes a long way. To cite the obvious, "better safe than sorry".

First of all, a percent can be converted into a mass not really that big of a deal just a bit of number crunching so point one is again invalid.

Second, I don't think you understand the physics governing the system. I have already gone through the reason why the amount of headspace doesn't matter. All other things equal the number of moles must increase in order to increase the pressure to the same value with a larger volume. But here you go I'll add some numbers. Units are not necessary because again its just a conversion factor. If you have 1 Liter and it requires say 50 moles to increase that pressure to 10 PSI (these numbers aren't real they're for comparison mind you) than it would take you (all other things equal) twice as many moles to increase twice as much volume to the same pressure. 1/50 = 2/100 yes?

Third, where are you getting this 1/5 from? to me it doesn't seem plausible for you to assert that it makes sense that 1/5 of the headspace would be replaced by air. Maybe it will maybe it won't but I've done a lot of these mass transfer scenarios when I was in school and it takes a long time for gasses to move into a small area like the opening of a keg. I have told you how I think a "couple" purges will knock out the O2. You have only spatt some BS numbers.

Lastly, I only say waste because it might mean you have to refill your tank a little sooner. Sorry for using a word you must think offensive. I don't mind much about the cost of C02 either. I purge my kegs 12 times after I've transferred from my carboy (which is a closed transfer mind you) I purge my carboys with co2 if I need to take a sample. CO2 is not the issue.

 

[MagicMatt]

I don't know where I got the more head space thing from those graphs. And you're the only one talking about moles here. All of my comments and the links I posted referenced percentages. You say that converting from percent to moles is just simple number crunching, but you would definitely have to know the head space volume to do the conversion (which is why I used percent, as it's more universally applicable).

My "numbers" were just SWAGs as I wasn't trying to dig into any textbooks for gas-gas diffusion constants when this isn't in the Brew Science forum and I wasn't trying to over complicate this more than it needed to be. But fine, just to prove my point of how much the air mixes into a keg I dug one up, and this is the equation to calculate the penetration distance of diffusing gases:

D = √(Ct)​

where D is the distance diffused (cm), C is the diffusion constant, and t is time (s). The function assumes 1 atmosphere of pressure at room temp in a controlled environment.

For O2 diffusing into CO2, C = 0.16 cm²/s

Plugging it in:

[t=1] → [D=0.4 cm]
[t=60] → [D=3.08 cm]
[t=120] → [D=4.38 cm]

Now extrapolate that into a real world situation. The A/C is on, the dog and kids are running around, you're breathing, the cat gets nosy, whatever....you get the picture. Just removing (and replacing) the lid to the keg itself will push enough air into the keg to warrant a few extra purges. And likely you removed the lid because you want to put something in the keg, which will also pull O2 with it.

Is it tons of oxygen? No, I think we all agree on that. But to say it's not worthy of attention isn't correct. If the OP didn't care about oxidation, he wouldn't be here asking about how to reduce it in this instance. Perhaps you needn't purge any more than you would when kegging the beer initially, but I would say certainly not less either. I still hold that one or two purges will not void the oxygen you let in to less than the acceptable industry threshold.

And as I stated earlier...if the keg is going to be emptied within a week or two, it's highly unlikely the effects of oxidation, if any, would present themselves by the time it's gone.

Otherwise....we go through all this trouble to keep the dreaded O2 out of our beer throughout fermentation, transfers, etc, only to neglect it now, after all of that?

 

[benco]

It's actually pretty simple, just put your gas back on at serving pressure and purge a few times.

 

[jerbrew]

Never said it wasn't worthy of attention. I still only wanted to hinder you from giving out incorrect information. Again sorry if I use something I'm familiar with as in moles. Would you prefer mass? It's no biggy. Moles of gas = PV/RT

grams O2 = moles of gas * percent/100 * 32. These are the simple things. Moles is just a better unit to quantify amounts of things ubiquitously.

4.38cm in 2 minutes. Thanks that's a whole hell of a lot of distance consider the size of a keg. I don't think that will get air to completely displace 1/5 of the headspace.

This is the second time you've gotten all bent out of shape when I say a "few" purges. The information is out there and OP has obviously seen it and is thinking about it. Few is subjective but the hard numbers are out there next time I'll be an arse and stick a BS number like 20+ without knowing what I'm talking about.

 

[jddevinn]

Magic Matt the doug293cz graphs that you posted above are in part per million O2, this is a percent unit, volume (ie keg fullness) doesn't matter. ** https://www.homebrewtalk.com/showpost.php?p=7461310&postcount=24. Moles are generally the correct quantity measurement when dealing with gas mixtures.

Easiest way to add to the keg is through one of the posts. Even better pull the gas in line, put your addition in the line, vent the keg of pressure and reattach the line so that the CO2 pushes your addition in.

 

[Maylar]

Easiest way to add to the keg is through one of the posts. Even better pull the gas in line, put your addition in the line, vent the keg of pressure and reattach the line so that the CO2 pushes your addition in.

I thought about that, but can't come up with a transfer mechanism that wouldn't add some air. I'm only looking at adding about 1/4 cup of liquid to the keg but that won't fill the gas line.

 

[jddevinn]

I also can't think of a way to do NO oxygen, maybe put it in a large syringe and insert through the PRV while allowing the CO2 to continue to come in the in port might be the most O2 limiting you can get.

If you REALLY want to drive yourself crazy.... how much DO is in the liquid malic acid that you are adding :fro:

 

[IslandLizard]

I also can't think of a way to do NO oxygen, maybe put it in a large syringe and insert through the PRV while allowing the CO2 to continue to come in the in port might be the most O2 limiting you can get.

If you REALLY want to drive yourself crazy.... how much DO is in the liquid malic acid that you are adding :fro:

^ That's what I would do.

Adding small amounts of something doesn't need to be invasive. You could use the out port too, even remove the dip tube. Just have a slow flow of CO2 flushing the system while you work.

When adding dry hop bags to a (mostly) full, 100% pre-purged keg, I stream CO2 through the in-port at a decent flow, remove the lid, slowly drop the squeezed bag and replace with the new lid with a nut welded on the bottom to which the bag is tied. I then purge 5-10 times.

 

[MagicMatt]

Magic Matt the doug293cz graphs that you posted above are in part per million O2, this is a percent unit.... Moles are generally the correct quantity measurement when dealing with gas mixtures.

Read Doug's comment directly above the graphs again. The graphs I posted show percentages, not ppm. Sure, you could translate this into ppm if you know the diffusion rate and how much O2 has made it into the keg. But the graph SHOWS percentages. The numbers are not in ppm units, they are in percents. You can apply that to whatever unit of measurement you want (mols, ppm, etc). The point is that the chart themselves are not indicating units. The chart tells you what percent (of whatever unit you are measuring with) escapes with each purge.

You say the chart shows ppm, but it doesn't. Even the ppm version of the chart is worthless in this case as there isn't 100% atmospheric air inside the keg. Anyone who took high school chemistry understands what a mole is, no need to explain it to me. My point still stands that in order to know the mols, you have to know the amount of O2 in the keg. And unless you have an organoplatinum target in your keg that you can point an O2 sensor at, I think guesstimating is going to be as good as it gets.

Again sorry if I use something I'm familiar with as in moles. Would you prefer mass? It's no biggy. Moles of gas = PV/RT

grams O2 = moles of gas * percent/100 * 32. These are the simple things. Moles is just a better unit to quantify amounts of things ubiquitously.

Mols are not ubiquitous though, because you need extra information (you even just pointed that out - you need to know the quantity of oxygen). With percentages, you don't need such information. Percentages are way more ubiquitous than mols.

And next time you pop the lid on a keg, watch the CO2 vapor swirling around and out of your keg. You may think the air around you is calm and still but it's not. If you open a container of any type, air will get in. In a completely still environment, air will penetrate over an inch into the keg. Add to that the natural air movement around you, and it's more like 2-3 inches. Putting the lid back in the hole pushes this air further down and sealing the keg locks it inside. It's not rocket science.


I'm tried of spinning in circles around you guys and picking nits. We're all talking about the same thing and y'all insist on using different units and calling it out each time as if that changes anything about the fact that there is oxygen in the keg. If you want to keep debating moles vs ppm vs percents and how any of that matters in preventing air from magically getting into your keg when you open it, it's a free country, go for it and have a blast.

 

[Upthewazzu]

This thread is 112% too mathy for me.

 

[jddevinn]

Um I'll just leave this here........note this is showing percent of O2 remaining, not mass. More head space = more purges needed to get to lower levels of O2.

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

Read Doug's comment directly above the graphs again. The graphs I posted show percentages, not ppm. Sure, you could translate this into ppm if you know the diffusion rate and how much O2 has made it into the keg. But the graph SHOWS percentages. The numbers are not in ppm units, they are in percents. You can apply that to whatever unit of measurement you want (mols, ppm, etc). The point is that the chart themselves are not indicating units. The chart tells you what percent (of whatever unit you are measuring with) escapes with each purge.

You say the chart shows ppm, but it doesn't. Even the ppm version of the chart is worthless in this case as there isn't 100% atmospheric air inside the keg. Anyone who took high school chemistry understands what a mole is, no need to explain it to me. My point still stands that in order to know the mols, you have to know the amount of O2 in the keg. And unless you have an organoplatinum target in your keg that you can point an O2 sensor at, I think guesstimating is going to be as good as it gets.



Mols are not ubiquitous though, because you need extra information (you even just pointed that out - you need to know the quantity of oxygen). With percentages, you don't need such information. Percentages are way more ubiquitous than mols.

And next time you pop the lid on a keg, watch the CO2 vapor swirling around and out of your keg. You may think the air around you is calm and still but it's not. If you open a container of any type, air will get in. In a completely still environment, air will penetrate over an inch into the keg. Add to that the natural air movement around you, and it's more like 2-3 inches. Putting the lid back in the hole pushes this air further down and sealing the keg locks it inside. It's not rocket science.


I'm tried of spinning in circles around you guys and picking nits. We're all talking about the same thing and y'all insist on using different units and calling it out each time as if that changes anything about the fact that there is oxygen in the keg. If you want to keep debating moles vs ppm vs percents and how any of that matters in preventing air from magically getting into your keg when you open it, it's a free country, go for it and have a blast.

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.

 

[jerbrew]

Read Doug's comment directly above the graphs again. The graphs I posted show percentages, not ppm. Sure, you could translate this into ppm if you know the diffusion rate and how much O2 has made it into the keg. But the graph SHOWS percentages. The numbers are not in ppm units, they are in percents. You can apply that to whatever unit of measurement you want (mols, ppm, etc). The point is that the chart themselves are not indicating units. The chart tells you what percent (of whatever unit you are measuring with) escapes with each purge.

You say the chart shows ppm, but it doesn't. Even the ppm version of the chart is worthless in this case as there isn't 100% atmospheric air inside the keg. Anyone who took high school chemistry understands what a mole is, no need to explain it to me. My point still stands that in order to know the mols, you have to know the amount of O2 in the keg. And unless you have an organoplatinum target in your keg that you can point an O2 sensor at, I think guesstimating is going to be as good as it gets.



Mols are not ubiquitous though, because you need extra information (you even just pointed that out - you need to know the quantity of oxygen). With percentages, you don't need such information. Percentages are way more ubiquitous than mols.

And next time you pop the lid on a keg, watch the CO2 vapor swirling around and out of your keg. You may think the air around you is calm and still but it's not. If you open a container of any type, air will get in. In a completely still environment, air will penetrate over an inch into the keg. Add to that the natural air movement around you, and it's more like 2-3 inches. Putting the lid back in the hole pushes this air further down and sealing the keg locks it inside. It's not rocket science.


I'm tried of spinning in circles around you guys and picking nits. We're all talking about the same thing and y'all insist on using different units and calling it out each time as if that changes anything about the fact that there is oxygen in the keg. If you want to keep debating moles vs ppm vs percents and how any of that matters in preventing air from magically getting into your keg when you open it, it's a free country, go for it and have a blast.

Dude, the point of all this is that you were wrong to say that a half full keg would need 20+ purges as opposed to the 12 in those graphs needed to get into sub PPM levels. Using that high school chem you talked about you can see that the volume doesn't matter as long as temperature and pressure are the same. percentage oxygen doesn't matter either because we know that the upper boundary condition is 21% and everything will always be that or lower. So the N2 required to purged the keg increases granted (oh snap the number of moles) but as long as you fill it to the same pressure the volume doesn't matter.

Get over the units dude. Don't argue about not knowing the percentages when it doesn't matter. That's the point. Oxygen is the great killer of beer but as long as you follow the steps to purge the head space as mentioned many times before. 12 purges at 30 PSI as per that table will get you into sub PPM independent of the volume.

Finished. I'm done trying to convince you of something that's true but that you're failing to grasp.

 

[Maylar]

^ That's what I would do.

Adding small amounts of something doesn't need to be invasive. You could use the out port too, even remove the dip tube. Just have a slow flow of CO2 flushing the system while you work.

When adding dry hop bags to a (mostly) full, 100% pre-purged keg, I stream CO2 through the in-port at a decent flow, remove the lid, slowly drop the squeezed bag and replace with the new lid with a nut welded on the bottom to which the bag is tied. I then purge 5-10 times.

You guys have given me some great ideas. I could bubble CO2 in through the out port while the lid is off. That serve both to keep positive pressure flowing and also help mix the acid solution into the cider. Or I could remove the gas in port and pour through that, which would reduce the area that air could enter.

... and there are no moles in my cider

 

[chudsonvt]

The other two options rather than worrying about purging are to either bottle what is left or just hurry up and drink the stuff!!! haha ;-)

 

[Limbrizik]

This is why I come here. A heated argument on oxygen diffusion. :rockin:

 

[jddevinn]

This is why I come here. A heated argument on oxygen diffusion. :rockin:

Big Oxygen has me in their pocket. Or maybe its small Oxygen.

 

[TAK]

Hold on. I've gotta go grab some popcorn. BRB

 

[behindthetimes]

I'm not good at math, but to answer your actual question I'd say a couple purges would be enough in your situation, because I don't believe cider is prone to oxidation like beer, unless we're talking about a very long period of time. Lots of Good Will Hunting on this thread.

 

[Maylar]

I'm not good at math, but to answer your actual question I'd say a couple purges would be enough in your situation, because I don't believe cider is prone to oxidation like beer, unless we're talking about a very long period of time. Lots of Good Will Hunting on this thread.

Not true. Cider is very prone to oxidation. We cider makers are just as anal about it as you brewers. Better safe than sorry.