How much gas am I wasting by kegging a small batch in a 5 gal keg?

I just did a split 5 gal batch with 2 different yeasts, so I will be kegging around 2.5 gal into a 5 gal keg. I know I could buy a smaller keg, but that will be at least $50 plus shipping. How much gas am I wasting by kegging in a bigger vessel?

Is wasted gas the only disadvantage?

Or course I could just bottle it.

How much gas are you wasting when a 5-gallon batch in a keg gets down to 2.5 gallons left?

I suppose if you force carbonate that batch, you might use a bit more than if it were a full batch, but it's going to be negligible in the grand scheme of things. If you "set and forget" at serving pressure, you'll "waste" none.

Besides--isn't the expenditure of CO2 in the pursuit of beer a loving sacrifice anyway?

You're wasting about 2.5 gallons of gas.

mongoose33 said:

How much gas are you wasting when a 5-gallon batch in a keg gets down to 2.5 gallons left?

I suppose if you force carbonate that batch, you might use a bit more than if it were a full batch, but it's going to be negligible in the grand scheme of things. If you "set and forget" at serving pressure, you'll "waste" none.

Besides--isn't the expenditure of CO2 in the pursuit of beer a loving sacrifice anyway?

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I do force carb. I'm not convinced it doesn't waste gas, but you're right, it's worth it. Bottling sucks.

PGEduardo said:

You're wasting about 2.5 gallons of gas.

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PGEduardo said:

You're wasting about 2.5 gallons of gas.

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^^ what he said It's not worth worrying about IMO.

It takes about 4oz of CO2 to serve a 5g keg. Half of that gas (2oz) is left in the keg when it empties, the other half (2oz) is dispensed with the beer (<-- ratio is probably way off, but I'm on a roll now).

In the case of the OP, he's still going to leave 2oz in the 5g keg when empty, but only dispense 1oz of gas since it's only 2.5g of beer. So, in total, you're going to use 3oz.

If you had a 2.5g keg, it would take 2oz to serve a full keg. Since you used 3oz in your 5g keg, you've wasted 1oz of gas.

I pay $10 for 5# CO2 fill. That's 12¢ per ounce. Probably cheaper than the caps if you decided to bottle

Depends on how much you have to purge.
Outside of purging, you would use the same amount as kegging and dispensing a 5 gal batch assuming you didn't force carb.
Force carbing you actually use less than you would by force carbing a full 5 gal batch.

mongoose33 said:

How much gas are you wasting when a 5-gallon batch in a keg gets down to 2.5 gallons left?

I suppose if you force carbonate that batch, you might use a bit more than if it were a full batch, but it's going to be negligible in the grand scheme of things. If you "set and forget" at serving pressure, you'll "waste" none.

Besides--isn't the expenditure of CO2 in the pursuit of beer a loving sacrifice anyway?

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mtnagel said:

I do force carb. I'm not convinced it doesn't waste gas, but you're right, it's worth it. Bottling sucks.

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My point was that when a keg is mostly empty, that empty space is taken up by CO2. No matter how much beer you start with, you'll end up with a keg that is filled with CO2 at the end.

I suppose the way to think about it is comparing two 2.5-gallon batches versus one 5-gallon batch. To serve those two 2.5-gallon batches you will end up using more CO2 than if you'd done one batch of 5 gallons.

In the 2.5-gallon instance, you'd end up with two kegs full of CO2 versus only one keg full of CO2 with the 5-gallon batch. So on that basis, PGEduardo is correct, though I would not call it a "waste."

"Waste" is pouring good beer down the drain.

I am sure there is some technical boring site that will give you equations to see the amount of gas displacement in liquid for an enclosed space etcetc.. maybe even http://pubs.acs.org/doi/abs/10.1021/ac60326a005 but in the end I think we can all agree you are talking pennies wasted in the end, hardly even worth the effort of looking it up. I judge my time is worth more per hour than the loss of the co2 in your keg after a year.

You won't waste any CO2. If you started with a full keg it would get down to that point eventually anyway!!!

Make sure to purge well (fill with sanitizer and push out) or you will oxidize the beer from all the O2 in the headspace.

Thanks everyone. I guess the only market for the small kegs are people who don't have the space for the 5 gal kegs.

mtnagel said:

Thanks everyone. I guess the only market for the small kegs are people who don't have the space for the 5 gal kegs.

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I have a 3g. More portable.

jddevinn said:

You won't waste any CO2. If you started with a full keg it would get down to that point eventually anyway!!!

Make sure to purge well (fill with sanitizer and push out) or you will oxidize the beer from all the O2 in the headspace.

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agreed. you will actually use slightly less CO2 than if you served a full 5G keg. But more than if you kegged into 3G keg.

5 Gallons of CO2 at 1 atm is very close to 22.4L which is 1 mole of gas (for CO2 it means 12 (C)+16 (O)+16 (O)=44 grams [EDIT: thanks to doug293cz for catching an error]. Since 22.4L is actually 5.9G, the actual number for 5G is 38g at 1 atm). If you serve at low temperature the pressure will be slightly higher than atmospheric but not by much - let's say 7-8psi in addition to 1atm which is 14 psi. So say roughly it is ~60 grams of CO2 - this will be in the keg when it's almost empty, no matter what you put in it. (For 2.5G keg it would be half that or 30g). If you purge with CO2 first (fill with starsan and then push it out using CO2 - and then fill beer backwards using out line - good practice - you will "push out" those 60 grams. But that's a low price to pay for unoxidized beer (and much less CO2 than repeated purging of air headspace with CO2).

Then there is additional CO2 that will go into solution. If you carb, say 3 Gallons to 2.5 volumes, it means that you will use 3G*2.5=7.5 Gallons of CO2 that will be dissolved in the beer. Roughly what will be in the keg when it's empty - so say another 60g. (actual number is 56g).

If you used 5 Gallons, it would be 5*2.5=12.5 Gallons of CO2 at 1 atm or 93g of CO2 - slightly more than what will be left in the keg when it is empty.

At ~150g of CO2 per keg, a 5 lb CO2 tank should last> 30 keg fills. First problem is that it's rarely filled with 5 lb - more like 3.5 lb if you fill it at room temperature (can fill more if it is cold).
The second problem is that if you overcarb and purge some CO2 out and then re-carb again, you will use more and more CO2. If you purge multiple times, again this uses more CO2.

Most important, third problem is that most of the dramatic losses of CO2 is due to various leaks in your system - which could eat the entire 5lb of CO2 overnight if you are not too careful (happened to me once). So make sure your kegs seal properly and do not leak.

2.5 gallons is 0.33 cubic feet. CO2 has a specific volume of approx 8.5 cubic feet per pound. You are wasting 0.039 pounds.

55x11 said:

agreed. you will actually use slightly less CO2 than if you served a full 5G keg. But more than if you kegged into 3G keg.

5 Gallons of CO2 at 1 atm is very close to 22.4L which is 1 mole of gas (for CO2 it means 12 (C)+8 (O)+8 (O)=28 grams (since 22.4L is actually 5.9G, the actual number for 5G is 24g at 1 atm). If you serve at low temperature the pressure will be slightly higher than atmospheric but not by much - let's say 7-8psi in addition to 1atm which is 14 psi. So roughly it is ~40 grams of CO2 - this will be in the keg when it's almost empty, no matter what you put in it. (For 2.5G keg it would be half that or 20g). If you purge with CO2 first (fill with starsan and then push it out using CO2 - and then fill beer backwards using out line - good practice - you will "push out" those 40 grams. But that's a low price to pay for unoxidized beer (and much less CO2 than repeated purging of air headspace with CO2).

Then there is additional CO2 that will go into solution. If you carb, say 3 Gallons to 2.5 volumes, it means that you will use 3G*2.5=7.5 Gallons of CO2 that will be dissolved in the beer. Roughly what will be in the keg when it's empty - so say another 40g. (actual number is 36g).

If you used 5 Gallons, it would be 5*2.5=12.5 Gallons of CO2 at 1 atm or 60g of CO2 - slightly more than what will be left in the keg when it is empty.

At ~100g of CO2 per keg, a 5 lb CO2 tank should last> 20 keg fills. First problem is that it's rarely filled with 5 lb - more like 3.5 lb if you fill it at room temperature (can fill more if it is cold).
The second problem is that if you overcarb and purge some CO2 out and then re-carb again, you will use more and more CO2. If you purge multiple times, again this uses more CO2.

Most important, third problem is that most of the dramatic losses of CO2 is due to various leaks in your system - which could eat the entire 5lb of CO2 overnight if you are not too careful (happened to me once). So make sure your kegs seal properly and do not leak.

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Good analysis, except the molecular weight of CO2 is 44, not 28 (atomic weight of oxygen is 16, not 8.)

Brew on

Oops you are right! Was doing it from memory (used atomic number instead maybe) and it's been over 20 years since I took chemistry. I will fix the numbers in original post.
it appears a full keg (5G) will have about 90g of CO2 dissolved in beer and another 60g or so that will be used to push it out (assuming low temperature ~5C and 2.5 volumes/8psi or so carbonation). So total of 150g per keg. If you use CO2 to purge the keg before backfilling it with beer, that's another 60g of CO2 or so.

Let's see... *starts mathing...* ... .... ... 29 min later.. Nooooo it is next to no co2 and you wasted enough of my time to brew a batch!

ballandkeg said:

CO2 has a specific volume of approx 8.5 cubic feet per pound. You are wasting 0.039 pounds.

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That's at STP. This is at 12psi. 0.039 x (14.7+12)/14.7 = 0.070.

passedpawn said:

That's at STP. This is at 12psi. 0.039 x (14.7+12)/14.7 = 0.070.

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Assuming he doesn't purge the head space, let's say, 3 times with 30PSI to seal the lid? This would waste more gas than if the keg were full. By how much, I don't know.

I have a 2.5 gallon keg and love it. Fits great on my compressor hump and for experimental half batches.

beersk said:

Assuming he doesn't purge the head space, let's say, 3 times with 30PSI to seal the lid? This would waste more gas than if the keg were full. By how much, I don't know.

I have a 2.5 gallon keg and love it. Fits great on my compressor hump and for experimental half batches.

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You should never "purge the headspace", in my opinion (as in fill keg with air around and then put CO2 in, release, repeat xN times).
One should fill a keg full with starsan (need to do it anyways) and then push starsan out with CO2. As you see in my post above, the cost here is 60g of CO2 if pushed at 7psi - could be reduced to as low as 40g if pushed at super-low pressure ~ 1ps but it takes forever (most of which will be emptied to atmosphere as you put the beer in).

Now, let's say he filled keg when it was open to air, then sealed and purged 3 times at 30 psi. 30 psi is about 3 atmospheres (30+14=44psi vs. 14 psi), and say he fills half of the keg with CO2, so he would use about 40*3/2=60g - same 60g, *per purge*. So total of 180g.

But the amount of air will go down from 2.5 Gallons, to 2.5G/3 after purge 1 (max pressure is 3 atm. - 1 atm. of air plus 2 atm. of CO2), to 2.5G/9 after purge 2 and down to 2.5G/27, or about 0.1 Gallons of air after purge 3. That's still about half a liter (or half a quart) of air in your keg, and over long term that would be a problem with oxidation - at least for me that's a troubling sign.

On the other hand, 0.1G of air to 2.5G of beer, or 1:25 ratio is probably about what you get in the bottle so maybe that's ok.

Still, pushing starsan with CO2 uses a lot less CO2 and leaves a negligibly small amount of oxygen.

55x11 said:

You should never "purge the headspace", in my opinion (as in fill keg with air around and then put CO2 in, release, repeat xN times).
One should fill a keg full with starsan (need to do it anyways) and then push starsan out with CO2. As you see in my post above, the cost here is 60g of CO2 if pushed at 7psi - could be reduced to as low as 40g if pushed at super-low pressure ~ 1ps but it takes forever (most of which will be emptied to atmosphere as you put the beer in).

Now, let's say he filled keg when it was open to air, then sealed and purged 3 times at 30 psi. 30 psi is about 3 atmospheres (30+14=44psi vs. 14 psi), and say he fills half of the keg with CO2, so he would use about 40*3/2=60g - same 60g, *per purge*. So total of 180g.

But the amount of air will go down from 2.5 Gallons, to 2.5G/3 after purge 1 (max pressure is 3 atm. - 1 atm. of air plus 2 atm. of CO2), to 2.5G/9 after purge 2 and down to 2.5G/27, or about 0.1 Gallons of air after purge 3. That's still about half a liter (or half a quart) of air in your keg, and over long term that would be a problem with oxidation - at least for me that's a troubling sign.

On the other hand, 0.1G of air to 2.5G of beer, or 1:25 ratio is probably about what you get in the bottle so maybe that's ok.

Still, pushing starsan with CO2 uses a lot less CO2 and leaves a negligibly small amount of oxygen.

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Not as negligible as you think. If you fill the keg so the StarSan overflows out the PRV, you still have about 3 fl oz of headspace in the keg due to the way the lid is designed (I measured.) If you then push the StarSan with CO2, and do a closed transfer of 5 gal to the keg, the headspace will still contain about 900 ppm O2 (if my memory is correct. Posted the calcs in another thread.) Still need to do some purges to get rid of the residual O2.

Brew on

doug293cz said:

Not as negligible as you think. If you fill the keg so the StarSan overflows out the PRV, you still have about 3 fl oz of headspace in the keg due to the way the lid is designed (I measured.) If you then push the StarSan with CO2, and do a closed transfer of 5 gal to the keg, the headspace will still contain about 900 ppm O2 (if my memory is correct. Posted the calcs in another thread.) Still need to do some purges to get rid of the residual O2.

Brew on

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hmm... maybe you are right, but it seems high.
3 fl oz is about 0.025 Gallons by volume (0.5%)
after emptying starsan and backfilling with same amount of beer (effectively 1 full "purge") you will have 0.023/5*.023=0.0001058 Gallons of air in that headspace.
Since air is roughly 1g of oxygen per Gallon, this means you will have 0.0001 grams of oxygen dissolved in 18 kg of beer, or 0.005 ppm.

What I am doing wrong (if anything) in this calculation?

If you want to properly purge it of all oxygen you absolutely will waste more gas. You can't just pressurize it like you do a 5 gallon batch and purge once and be done. In order to get all of the oxygen out I think you need to purge it at least 8 times with that volume, there's a chart floating around here somewhere on that.

Here is that chart. See post #14

https://www.homebrewtalk.com/showthread.php?t=538199&page=2

arnobg said:

If you want to properly purge it of all oxygen you absolutely will waste more gas. You can't just pressurize it like you do a 5 gallon batch and purge once and be done. In order to get all of the oxygen out I think you need to purge it at least 8 times with that volume, there's a chart floating around here somewhere on that.

Here is that chart. See post #14

https://www.homebrewtalk.com/showthread.php?t=538199&page=2

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it's simple enough math - it depends on pressure. If purging using 14 psi (1 atm.) each purge will reduce amount of air by a factor of 2. If purging using 28 psi, it will be a factor of 3, etc.

You won't ever get rid of "all oxygen" (I guess technically you could once you get to single molecule limit). But it can be down to manageable values.
So 8 purges of 14 psi would reduce volume of original air by a factor of 2^8=128. So if you started with say 2.5G of air in the keg, you will still end with 2.5/128=0.02 G of air. Much more actually than if you fill keg with starsan to the top and push it out using CO2.

55x11 said:

hmm... maybe you are right, but it seems high.
3 fl oz is about 0.025 Gallons by volume (0.5%)
after emptying starsan and backfilling with same amount of beer (effectively 1 full "purge") you will have 0.023/5*.023=0.0001058 Gallons of air in that headspace.
Since air is roughly 1g of oxygen per Gallon, this means you will have 0.0001 grams of oxygen dissolved in 18 kg of beer, or 0.05 ppm.

What I am doing wrong (if anything) in this calculation?

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I need to revisit the calculations. The residual O2 charts are based on ppm by volume, since they are for gases. To look at the potential effects on DO, the volume numbers should be converted to mass. More later.

Brew on

Was thinking about kegging, so I stumbled upon this thread, read about half of it before getting a headache and curling up in the fetal position in the corner of my bedroom. Seems too complicated.

David_Trucks said:

Was thinking about kegging, so I stumbled upon this thread, read about half of it before getting a headache and curling up in the fetal position in the corner of my bedroom. Seems too complicated.

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You drive a car without understanding all the fine details of combustion engine. You use computers and phones without even knowing how transistor works. Never mind a modern silicon based CPU.

What's the problem again?!

What is good enough in a home-brew sense that we are not packaging commercial beer that might be mistreated (stored warm for weeks). I only have (2) 5# c02 tanks and doing 11 purges at 30PSI as the chart/link suggests seems overkill.

https://www.homebrewtalk.com/showthread.php?p=7530398#post7530398

My process is 7 purges at 30 psi after filling which translates to
0.6 gal * 87 ppm / 5.3 gal = 9.8 ppm (is this too high?)

Should I be treating my distilled water starsan solution for kegs with campden tablets to remove DO in the water?

The thread that makes me want to quit is https://www.homebrewtalk.com/showthread.php?t=580532 all 16 pages of it. TLDR is we have to brew in the vacuum of space now because oxidation in the mash ruins malt flavor after a few minutes. I do agree oxidation is the biggest issue in home brewing but how far down the rabbit hole do we go?

khillian said:

What is good enough in a home-brew sense that we are not packaging commercial beer that might be mistreated (stored warm for weeks). I only have (2) 5# c02 tanks and doing 11 purges at 30PSI as the chart/link suggests seems overkill.

https://www.homebrewtalk.com/showthread.php?p=7530398#post7530398

My process is 7 purges at 30 psi after filling which translates to
0.6 gal * 87 ppm / 5.3 gal = 9.8 ppm (is this too high?)

Should I be treating my distilled water starsan solution for kegs with campden tablets to remove DO in the water?

The thread that makes me want to quit is https://www.homebrewtalk.com/showthread.php?t=580532 all 16 pages of it. TLDR is we have to brew in the vacuum of space now because oxidation in the mash ruins malt flavor after a few minutes. I do agree oxidation is the biggest issue in home brewing but how far down the rabbit hole do we go?

Click to expand...

Are you happy with how your beer tastes over time in the keg? If so, then there is no need to change your process. If you think your beer is degrading as time goes by, then reducing packaging O2 is something to think about.

Brew on

Dude Gas is Cheap! Dont stress enjoy your hobbie and do as you will. Chur

khillian said:

What is good enough in a home-brew sense that we are not packaging commercial beer that might be mistreated (stored warm for weeks). I only have (2) 5# c02 tanks and doing 11 purges at 30PSI as the chart/link suggests seems overkill.

https://www.homebrewtalk.com/showthread.php?p=7530398#post7530398

My process is 7 purges at 30 psi after filling which translates to
0.6 gal * 87 ppm / 5.3 gal = 9.8 ppm (is this too high?)

Should I be treating my distilled water starsan solution for kegs with campden tablets to remove DO in the water?

The thread that makes me want to quit is https://www.homebrewtalk.com/showthread.php?t=580532 all 16 pages of it. TLDR is we have to brew in the vacuum of space now because oxidation in the mash ruins malt flavor after a few minutes. I do agree oxidation is the biggest issue in home brewing but how far down the rabbit hole do we go?

Click to expand...

Fill the keg with StarSan solution and then push it out with CO2 instead of multiple purges. You'll have less O2 concentration and use less CO2.

55x11 said:

You drive a car without understanding all the fine details of combustion engine. You use computers and phones without even knowing how transistor works. Never mind a modern silicon based CPU.

What's the problem again?!

Click to expand...

I drive the car, not take apart the engine, lube it, replace some parts and put it back together. But, point made. I'm sure I'll get on the keg wagon.