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 Home Brew Forums > IBU Calculation question...
12-24-2010, 08:31 PM   #1
pacebrew
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 IBU Calculation question...

Something interesting here... It seems that the IBU math formulas generally use the final Volume when calculating the IBU content. I have a bone to pick here with this.

Shouldnt we be using the post boil content? THink about it. The hops are immersed in the wort while it boils, all the alpha acids leave into the boil. In the boil there is an IBU/Boil.

now, because this is all mixed, the solution coming to the fermenter should maintain this ratio, so you are getting x gallons of wort from the boil kettle. But the IBU formulas are using the final gallon amount, wouldnt this underestimate the bitterness of the beer by instructing us to use less?

especially if you are leaving a gallon in the boil kettle as loss....

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12-25-2010, 02:25 AM   #2
Nateo
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Any "formula" is going to have problems. Maybe your hops were a few months old when you bought them, maybe they were stored improperly at some point, maybe the sample they used for the AA% from the harvest differs from your specific hops, etc. Most of those formulae don't even take into account the bittering potential of beta acids.

So, any formula is just a rough guess, ultimately. I don't see why anyone should take exception with cutting one specific corner, like post boil volume vs. final volume, without taking exception to all the other shaky variables that go into that calculation.

That said, a rough guess is better than no guess at all, and I can get into the bitterness range I'm happy with using the standard way, flawed though it is.

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12-25-2010, 02:58 AM   #3
WrongCoastBrewery
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Think of the equation as a general way to calculate IBUs. Its more of an umbrella equation than specific. Granted, to be scientifically exact, you would need to have every variable accounted for. Most homebrewers wont go into that detail.

I never leave behind that much wort anyway. When I pour to the fermenter, I strain and get every last drop. I lose more to evaporation than transfering.

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12-25-2010, 03:02 AM   #4
CPooley4
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As I understand it the final volume is all you should be concerned about. The utilization of your hop additions is based upon the gravity, your boil time, and final volume. My recommendation (and what you seem to be concerned about) would be to use an average gravity for each addition and that should put you as close as you're going to get without a lab review.

Tinseth's calculation allows you to account for the average gravity. You can use the standard calculation as well and just adjust your utilization figure to the average gravity for each addition.

From Tinseth's website:

To calculate IBUs, the formula is simple:

IBUs = decimal alpha acid utilization * mg/l of added alpha acids

To calculate the concentration of alpha acids you add to the wort:

mg/l of added alpha acids = decimal AA rating * grams hops * 1000
-------------------------------------
volume of finished beer in liters

or for those of you not using metric units:

mg/l of added alpha acids = decimal AA rating * ozs hops * 7490
-------------------------------------
volume of finished beer in gallons

You can look up the decimal alpha acid utilization in the utilization table below or calculate it directly using the Bigness factor and the Boil Time factor.

decimal alpha acid utilization = Bigness factor * Boil Time factor

The Bigness factor accounts for reduced utilization due to higher wort gravities. Use an average gravity value for the entire boil to account for changes in the wort volume.

Bigness factor = 1.65 * 0.000125^(wort gravity - 1)

The Boil Time factor accounts for the change in utilization due to boil time:

Boil Time factor = 1 - e^(-0.04 * time in mins)
--------------------------
4.15

The numbers 1.65 and 0.000125 are empirically derived to fit my data. The number 0.04 controls the shape of the utilization vs. time curve. The factor 4.15 controls the maximum utilization value--make it smaller if your kettle utilization is higher than mine.

I'd suggest fiddling with 4.15 if necessary to match your system; only play with the other three if you like to muck around. I make no guarantees if you do.

The really cool thing about these new equations is that they are easily customizable. I believe the basic form is correct; by playing with the different factors, different brewers should be able to make them fit their breweries perfectly.

Here is an example of how I have mine written in excel:

(1.65*0.000125^((((E25/B7)-(E25/E13))/2+(E25/B7))/1000))*((1-2.71828182845904^(-0.04*D27))/4.15)*((E27*C27*7490)/B7)

Probably a bit confusing. If you need further clarification let me know.

Good luck.

cp

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12-25-2010, 06:41 PM   #5
ScrewyBrewer
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Quote:
 Originally Posted by pacebrew Something interesting here... It seems that the IBU math formulas generally use the final Volume when calculating the IBU content. I have a bone to pick here with this. Shouldnt we be using the post boil content? THink about it. The hops are immersed in the wort while it boils, all the alpha acids leave into the boil. In the boil there is an IBU/Boil. now, because this is all mixed, the solution coming to the fermenter should maintain this ratio, so you are getting x gallons of wort from the boil kettle. But the IBU formulas are using the final gallon amount, wouldnt this underestimate the bitterness of the beer by instructing us to use less? especially if you are leaving a gallon in the boil kettle as loss....
pacebrew when you plug in your recipe's volume size (2.3, 5 or 10 gallons as an example) that's the number used for the rest of the calculations.

Just like when you add in all your hops before adding in your malts and fermentables the IBU calculations will be high. Then as you add in the fermentables the IBU numbers will get lower, since we're offsetting sweetness with hops.

I use the calculation results as a 'guide' related to my brewing equipment, type of brew I'm doing (extract, steep or mash) and volume loss during the boil.
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12-26-2010, 04:41 AM   #6
KeithMoonsLiver
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Every one of the IBU formulas is off, some of them significantly so. My advice is to just stick to one formula and through experience gain an idea of how bitter each calculated IBU value is. Then you can adjust each one to taste, rather than trying to get a number within a certain range. What you calculate as a 40 IBU beer might not have anywhere near 40 mg/L iso-alpha acids, but it's not like anyone is going to be running your beer through a spectrometer or gas chromatographer; that calculated value is just a way to predict the taste based on past experiences.

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04-13-2011, 02:55 AM   #7
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When Tinseth says "The factor 4.15 controls the maximum utilization value--make it smaller if your kettle utilization is higher than mine.", can anyone explain what he means? Just curious what exactly kettle utilization is and how I would figure mine. Diameter vs depth? Thanks. Cheers!

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04-13-2011, 05:09 AM   #8
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Kettle utilization usually refers to batch size. If your batch is over, say, 20 gallons, then your utilization will be higher than a 5 gallon batch. I've looked, but havent been able to find utilization factor equations.

One big source of differing utilization would be your wort chilling capacity. Hops will contribute bitterness if they're over 140*. If it takes Tinseth 0 minutes to get below 140* and it takes you 30 minutes, then your "flameout" hops are actually -30min hops, and your -60min hops were really -90min hops.

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04-13-2011, 08:40 PM   #9
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I seem to remember Ray Daniels going into kettle utilization in Designing Great Beers, but I may be thinking of something else. If I'm thinking about the right thing, he basically had to brew up some batches and have them tested for alpha acids to calculate his own kettle efficiency. Unless you're planning on going down this road, I think you've just got to take Tinseth's factor.

I always used the idea that KeithMoonsLiver mentioned previously. Pick a value and equation and stick with it. Makes it tough to convert to other people's recipes, but will help you a lot in designing your own.

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