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Re: Sugar calculations


On Thu, Mar 7, 2013 at 9:12 PM, mik firestone <mikfire@xxxxxxxxx> wrote:

> In a small word, the brewnote efficiency calculations are rather messed
> up. I'm using the postboil gravity when I should be dealing with the
> preboil. I'm trying to unwind this right now but I keep running into weird
> things.
> I am specifically looking at Recipe::recalcBoilGrav() and trying to figure
> out why the boil gravity (aka, SG) is worried about trub/chiller losses.
> Trub/chiller loss shouldn't have anything to do with the preboil gravity,
> should it?

You'd think it wouldn't have anything to do with it. However, there is a
nit to be picked...

BeerXML says the recipe efficiency is brewhouse efficiency, which is
"to-the-fermentor" efficiency. Meaning, if I have 5.00 kg grain with 76%
yield (3.80 kg sugar), and my brewhouse efficiency is 70%, I get 5.00 kg *
0.76 * 0.70 = 2.66 kg into the fermentor. Now, how would you go about
finding how much sugar is in the kettle? Since the recipe's efficiency is
"to-the-fermentor", we can't use that number directly; we need
"to-the-kettle" efficiency. Since you are always losing some amount of
sugar at each step of the brewing process, it must be the case the your
"to-the-kettle" efficiency is somewhat higher than your "to-the-fermentor"
efficiency. How much more?

eff_tokettle = eff_tofermentor / (1 - trubChillerLoss/finalVolume)

To extend the previous example, suppose my final volume is 20 L and I lose
1 L in the transfer from kettle to fermentor.
eff_tofermentor = 0.70
trubChillerLoss = 1 L
finalVolume = 20 L

sugar_kettle = 2.66 kg / (1 - 1L/20L) = 2.80 kg (73.7% efficiency to kettle)
sugar_lost = 1L/20L * 2.80 kg = 0.14 kg
sugar_fermentor = 2.80 kg - 0.14 kg = 2.66 kg (70.0% efficiency to

I highly doubt any other software is picking this nit, but it definitely
exists. You can see for this typical example that all your OG-related
numbers would be off by about 4% if you don't calculate the kettle
efficiency to be different from the brewhouse efficiency.

Philip G. Lee

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