Originally Posted by rodwha
Hmmm… I had figured out how to show the results of a created recipe on Brewtoad so that it does show tenths of alcohol percentages, and their numbers are a little different than yours. The numbers I originally gave were an estimate as I couldn't see what the results were specifically. I had to look at a slide.
It shows that recipe using Nottingham as having an OG of 1.033 and 3.4% ABV with DME and 1.028 and 2.9% ABV with LME, and 29 and 30 IBU's using Goldings, though there's a slide as far as AA's go.
I'm not sure why the difference as I see your math. Brewtoad has been fairly accurate for me.
It has to do with the values that Brewtoad is using to perform calculations, any deviations in how the calculations are being performed, and how rounding is being handled. That's why I do not use brewing software. I want control over how all of my intermediate and final values are handled. I also want to know the exact calculations that are being performed.
Let's look at the O.G. equation
1.75 x 0.046 / 2.5 + 1.0 ~= 1.032
The symbol "~=" is often use in mathematics to denote "approximately equal to" on text-based systems, which is different than "equal to." The actual value is 1.0322. Brewtoad is rounding the value up to 1.033, which is not how I would handle the trailing 2. I would truncate any value in the ten thousandths position that was less than or equal to 4.
I would have to see how Brewtoad is calculating IBUs to evaluate the differences.
Let's look at how ABV is calculated
ABV = O.G. - F.G. x 105 x 1.25
Working an ABV of 3.4% backwards yields an apparent attenuation of
3.4 / 1.25 / 105 / 0.033 x 100 = 79%, which is high for Nottingham when pitched into such a low gravity wort, especially a malt extract-based recipe.
I used an apparent attenuation of 75% in my calculation
(1.032 - 1.008) / 0.032 x 100 = 75%
With that said, the only way to determine the ABV of a batch is to measure the original and final gravities and plug those values into the ABV equation shown above (a hydrometer and an accurate thermometer are essential brewing tools). For example, I recently made an all-grain Pre-Pro Pilsner that had an O.G. of 1.062 and an F.G. of 1.012.
Apparent Attentuation = 1.062 - 1.012 / 0.062 = 80.6%, which is high, but not out of the question considering the grist, mash temperature, and the yeast strain.
ABV = 1.062 - 1.012 x 105 x 1.25 = 6.56%