kegging

[jeff1852]

Is it ok to keg 5 gals in a 7.5 gal keg as long as the headspace is purged with C02?

 

[day_trippr]

Sure, you can do that. Better would be a full liquid purge prior to filling...

Cheers!

 

[bpgreen]

If you put 7.5 gallons in it and pour off 2.5 gallons, you'll have 5 gallons in the 7.5 gallon keg, same as if you start with 5 gallons.

 

[day_trippr]

Umm....

 

[Qhrumphf]

Depending on how you choose to carb it, be aware that it will carbonate faster. If you set to target pressure and leave it, then all good, it'll just be ready sooner. If you do a high pressure rapid force carb, be aware that it'll carb faster than a full keg will, so be careful not to overdo it. Although the difference between 5 gals in a 7.5 gal vs 5 gals in a 5 gal will be less pronounced than 3.5 gals in a 5 gal vs 5 gals in a 5 gal.

 

[day_trippr]

Is that really true given the interface area is unchanged?
I have no idea. I work with ones and zeroes I usually look to @doug293cz for such matters...

Cheers!

 

[Qhrumphf]

Is that really true given the interface area is unchanged?
I have no idea. I work with ones and zeroes I usually look to @doug293cz for such matters...

Cheers!

Well, my usual frame of reference, 15 bbl in 30 carbs faster than 30 bbl in 60 carbs faster than 30 bbl in 30 carbs faster than 60 bbl in 60. This is with volumes of CO2 being directly measured, and carbed with a stone at set pressure and CO2 flow rate, and blown down to equilibrium pressure (based on the chart we all know) once target carb is reached.

You've got a lot more headspace at the same pressure, with a lot more mols of CO2 present to increased dissolved CO2 as it reaches equilibrium.
I don't know the math offhand. But experience dictates that extra space of CO2 means you reach target carb a lot faster. Without a stone it obviously won't happen as fast, but the physics shouldn't change.

My experience has been the same with 10 vs 20 bbl in brites, as well as with partially full cornies and SB sankeys (the latter more directly what most see at home when not using a carb stone).

 

[day_trippr]

Wait - your experience is primarily based on the use of carbonation stones.
You must realize there's a huge difference between introducing CO2 at the bottom of a vessel versus simply pressurizing a head space.

I'll wait for @doug293cz to weigh in but I'm pretty sure there's an apples vs oranges thing going on...

CHeers!

 

[Qhrumphf]

I've seen the same apply either way, stones are just a lot faster.

Whether 5 gals in a 5 gal keg vs 5 gals in a 7.75 gal keg will carb faster, I own both and have carbed both by head pressure alone and seen the same. It's just days difference instead of (partial) hours difference.

AFAIK most of your CO2 (or O2) applied by stone goes into headspace as bubbles anyway, but you do directly dissolve more in the process speeding it all. It makes a time difference (and probably alters the math and time ratios) but doesn't change my overarching experience that more headspace = faster carb.

Of course if the math says otherwise and there's something I'm not considering I'm open to correction.

 

[Qhrumphf]

Example- if I do 10 gals at home (packaged), put 5 gals into a 5 gal cornie, and 5 gals into a 7.75 gal QB sanke, and put them under the same pressure, the QB carbs faster (slim, not pony, so the surface area isn't radically different). Now I don't have a Zahm or CBox at home to give hard numbers, but I do trust my sensory perception. Or perhaps it's the slightly increased surface area making a difference. Who knows.

 

[Qhrumphf]

Ok, doing some research, seems if you set and forget it's surface area dependent (so slight increase in a slim QB and significantly higher in a pony QB vs a cornie or SB), but if you set higher and shake (or via stone) you'll see a more definite rate increase with increased headspace.

So I'm not *entirely* incorrect.

 

[doug293cz]

You rang?

The rate of CO2 absorption is affected by the surface area exposed to CO2, and the difference between the gaseous CO2 partial pressure and the equilibrium headspace pressure for the current carb level of the beer (temp dependent.) The previous statement is true for still beer carbonation by diffusion from headspace, shake rattle 'n roll carbonation, and bubbling from the bottom of a vessel. We'll look at the simple case first, and then look at bubbling.

Most people don't realize that while CO2 is dissolving into the beer from the headspace, CO2 is simultaneously escaping from the beer into the headspace. CO2 is going in both directions at the same time, just different molecules going in different directions. That's how diffusion works. You get carbonation because the flow into the beer is faster than the flow out of the beer when the headspace pressure is higher than the equilibrium pressure for the current carbonation level of the beer. When the headspace pressure equals the equilibrium pressure for the current carb level, then the flow rates into and out of the beer are the same, so the difference between the flows is zero, and carbonation no longer increases or decreases.

The flow rate of CO2 into the beer is determined by the following equation:

Dissolution Rate = C1 * A * PH

C1 = a temperature dependent constant
A = surface area exposed to CO2
PH = gaseous CO2 partial pressure
​

The flow rate of CO2 from the beer into the headspace is determined by the following equation:

Exhalation Rate = C2 * A * Peq

C2 = a temperature dependent constant
A = surface area exposed to CO2
Peq = equilibrium pressure for the current carb level of the beer
​

The net flow rate of CO2 into, or out of, the beer is then given by:

Net Dissolution Rate = A * (C1 * PH - C2 * Peq)​

If the net dissolution rate is positive, then carbonation is increasing. If negative, then carbonation is decreasing. We also know that when PH = Peq, the net dissolution rate is zero, which implies A * P * (C1 - C2) = 0, so C1 must equal C2. Thus we can simplify the net dissolution rate equation to:

Net Dissolution Rate = A * C * (PH - Peq)​

The units for dissolution rate are mass/time (eg, g/sec, kg/hour, etc.)

The carbonation rate is then given by:

Carbonation Rate = Net Dissolution Rate / V = A * C * (PH - Peq) / V

V = volume of beer​

The units for carbonation rate are mass/(volume·time). The most useful units (at the homebrew level) are probably g/(liter·hour). One volume of carbonation is equal to 1.98 g/L, so you could also use units of volumes/hour (divide g/(liter·hour) by 1.98.) If you have half the beer in a cylinder, then the carbonation rate is doubled.

Note that as carbonation increases, so does Peq, and that causes the carbonation rate to decrease. The more carbonated you get, the slower carbonation proceeds.

When using a carb stone you create lots of bubbles with lots of surface area - orders of magnitude more area than the area on the surface of still beer. A typical corny has a diameter of about 9" and a liquid surface area of about 64 in^2, or 413 cm^2. The volume of a sphere is 4*pi*r^3/3, and the surface area is 4*pi*r^2. So, 10cm^3 (0.01 L, 0.02 g) of CO2 @ 1 atm would be 5 cm^3 @ 14.7 psig. If we had 250um diameter bubbles (about 0.01"), 5 cm^3 would have a total surface area of about 1200 cm^2. If we reduced the bubble size to 100um, then the total surface area would be about 3000 cm^2.

Another factor that speeds carbonation with bubbles is that as the bubbles dissolve and get smaller, the pressure inside the bubble increases, speeding up the rate at which CO2 dissolves from the bubble into the beer. It should be possible with a carb stone to find a bubble size and CO2 flow rate that would result in bubbles completely dissolving into the beer before they reach the free surface of the beer. This would minimize excess pressure build up in the headspace, which would then have to dissolve back into the beer (or be vented.)

As you can see from the above, beer volume differences affect the carb rate (for the same vessel geometry), but headspace variation does not.

Headspace volume might have a secondary effect when doing shake rattle 'n roll carbonation in that you have higher surface area with larger headspace volume when you put a cylinder on its side. If you shake the keg, then more headspace could allow you to create more surface area when shaking. As described above, increasing the surface area exposed to CO2 increases the carbonation rate.

Brew on

 

[Qhrumphf]

Based on that, shaking is gonna do much like an (ineffective) stone and dramatically increase the surface contact, especially if you're vigorously shaking it.

So to the OP, whilen my reasoning why might have been incorrect, my original point still stands, unless you're putting 5 gals in a larger keg with identical inner diameter and therefore surface area of gas/beer interface (not aware of one), the larger keg will carb faster.

Thanks doug. Always good for the math I don't care to or are too drunk to look up. Cheers!

 

[Vale71]

Another factor that speeds carbonation with bubbles is that as the bubbles dissolve and get smaller, the pressure inside the bubble increases, speeding up the rate at which CO2 dissolves from the bubble into the beer.

I really can't get my head around this statement. Why should pressure increase just because the bubble gets smaller? In an equilibrium state pressure inside the bubble will always equal hydrostatic pressure outside the bubble. If this weren't the case then the bubble would just contract or expand until both pressures equalize. The former (contraction) is exactly why the bubble gets smaller as CO2 is absorbed. As CO2 diffuses into the beer pressure in the bubble decreases and the bubble contracts keeping the internal pressure equalized with the external.
What does happen though is that the surface to volume ratio of the bubble increases exponentially and so does the rate at which the bubble is further absorbed, hopefully leading to complete absorption before the bubble can reach the surface of the liquid.

 

[doug293cz]

I really can't get my head around this statement. Why should pressure increase just because the bubble gets smaller? In an equilibrium state pressure inside the bubble will always equal hydrostatic pressure outside the bubble. If this weren't the case then the bubble would just contract or expand until both pressures equalize. The former (contraction) is exactly why the bubble gets smaller as CO2 is absorbed. As CO2 diffuses into the beer pressure in the bubble decreases and the bubble contracts keeping the internal pressure equalized with the external.
What does happen though is that the surface to volume ratio of the bubble increases exponentially and so does the rate at which the bubble is further absorbed, hopefully leading to complete absorption before the bubble can reach the surface of the liquid.

The pressure inside a bubble is higher because of surface tension. Surface tension provides a force trying to collapse the bubble to eliminate the surface which has higher energy than the bulk. The surface tension force must be balanced by a pressure increase inside the bubble or the bubble would collapse. Read more about it here: https://en.wikipedia.org/wiki/Laplace_pressure. The Laplace pressure is the difference in the hydrostatic pressure outside the bubble and the pressure inside the bubble.

Also, the surface area to volume ratio increases linearly with decrease in diameter, not exponentially. The surface area to volume ratio is given by:

4 * pi * r^2 / (4 * pi * r^3 / 3) = 3 / r
​

Brew on

 

[day_trippr]

^that's^ my dog!

 

[balrog]

Rats. My confirmation bias perhaps, but I thought I had found a decent burst carb sheet, giving reasonably good results, but it definitely shows higher carb achieved with more headspace(4.5 gal in 5.25 Vttl versus 4.8 gal in same 5.25 Vttl).

Or maybe it's showing faster. That might be it. I don't have it here to check.

 

[doug293cz]

Rats. My confirmation bias perhaps, but I thought I had found a decent burst carb sheet, giving reasonably good results, but it definitely shows higher carb achieved with more headspace(4.5 gal in 5.25 Vttl versus 4.8 gal in same 5.25 Vttl).

Or maybe it's showing faster. That might be it. I don't have it here to check.

Yes, a smaller volume of beer will carb faster than a larger volume of beer in the same style keg. It has nothing to do with headspace, it’s that less beer requires less CO2 to reach the same carb level. So, at a fixed rate of carbonation, a smaller volume will get more carbed in the same amount of time.

 

[balrog]

Right, of course. Lower vol of beer carbs faster.
But changing headspace, in the calculator I found, from 5.25 to 5.5 while leaving beer volume alone at 4.75, did change the overall vols CO2 by .03 so if headspace has no bearing then the math/physics of that calculator are in question.

 

[doug293cz]

Right, of course. Lower vol of beer carbs faster.
But changing headspace, in the calculator I found, from 5.25 to 5.5 while leaving beer volume alone at 4.75, did change the overall vols CO2 by .03 so if headspace has no bearing then the math/physics of that calculator are in question.

Depends on just what the calculator is assuming. If, for example, you pressurized at 30 psi for X hrs, and then sealed off the keg, there would be more CO2 in the headspace at higher than equilibrium pressure, and that excess CO2 would get absorbed into the beer until the headspace pressure is in equilibrium. If you just pressurized at 30 psi for X hrs and then bled the pressure to equilibrium pressure, then no extra carb would result from the extra headspace.

I should look at the calculator to see just what it is doing.

Edit: Ok, I looked at the description of what he says the calculator is doing, and it is the first case above. He assumes that when you turn down the regulator, you have a check valve that is 100% leak free, and no CO2 leaks back thru the regulator (which is now set for a lower pressure.)

Brew on

 

[balrog]

Right, the name makes sense (no purge), in that regard, and in use, or in my particular use, I hook up 30psi for the 18-23-or-whatever hours and then disconnect.

(It's always fun to find any pesky leaking lid pressure relief valves a few days later)