Blended CO2 Levels not making sense

[cactusgarrett]

I'm not getting any hits in the Bottling forum, but I wrestled with possibly submitting here due to the technical nature. Hopefully someone can enlighten my on this concept:

I'll be blending two barrel aged sours soon, and I'm confused about some details when using the excel sheet referenced by Mike here: Priming Barrel-Aged and Blended Sour Beers | The Mad Fermentationist - Homebrewing Blog

I plan on bottling 5gal of of the base beer (a funky saison), as well as a 50/50 (2.5gal/2.5gal) blend of that and another sour. The calculator states for the 5gal of the base saison alone has: 0.43 Residual CO2, and 0.43 Predict CO2 w/o Sugar -> needing 7.75 oz table sugar to hit 3.2 vols

But when doing the 50/50 blend, the sheet states the combined components have 0.43 Residual CO2 and 1.71 Predict CO2 w/o Sugar -> needing 4.18 oz table sugar to hit 3.2 vols

It seems from a practical aspect, blending two similar beers with the same residual CO2 should yield the same "Predict CO2 w/o sugar" as only one. Why does the "Predicted CO2 w/o sugar" more than quadruple when blending? I'd like to wrap my head around this and be able to trust this sheet, as it's the matter of a difference of 3.6 oz sugar and the potential to vastly undercarbonate a great beer.

 

[Vale71]

It appears that the calculator is largely based on guesses, however educated.
First guess is how much residual CO2 from fermentation will be left in the beer after barrell aging.
Second guess is how much of the residual extract will be fermented by the wild bacteria during bottle conditioning. This seems to take FG into account and then guesses how much of that will be fermented out.
My guess is your second beer has a higher FG at bottling than the first one? Keep in mind that the whole thing is based on guesses so you're just as likely to end up with 3.2 vols as with 5.0 or 2.0...

 

[cactusgarrett]

My guess is your second beer has a higher FG at bottling than the first one?

It does: 1.010 versus 1.005, but I don't think this tool is specifically for sour beer, so I don't think the assumption of bugs driving down gravity beyond the components' FG is necessarily a good one.
It seems like being that far off is way outside the realm of guessing, though. I guess it boils down to trying to determine what the rationale or calculation behind this factor is.

 

[Vale71]

...I don't think this tool is specifically for sour beer...

Yes, it clearly is. A regular beer fermented with a regular yeast, either lager or ale, would not increase in carbonation after bottling without any priming sugar. A prediction of increased carbonation after bottling without priming only makes sense if there are overattenuating yeasts such as diastaticus or brettanomyces present.

 

[cactusgarrett]

A prediction of increased carbonation after bottling without priming only makes sense

I get that, and this is the stuff I'm trying to suss out with this from people familiar with this. Is this sheet assuming all components will drop to down to the FG of the lowest component?

Practically speaking, why would the component with the higher FG be expected to drop further when it was fermented in the same manner? The rationale I'm using is that this batch has already gone down as far as possible (as it's a year+ old and fermented/aged with STA-1 pos sacc and brett), so why would blending in another component with the same STA-1 pos sacc and brett blend bring it down further (thus adding residual CO2)? Logic would dictate that it's already as low as it's going to go.

And I see, now that it was intended as a calculator for prediction pH and acid: