from what i've seen, the typical equation to calculate IBU is:

IBU = AAU x U x 75 / V

where AAU = weight of hops (oz) x % alpha acid

U = utilization, which is a function of boil gravity and time of hops within boil

V = recipe volume

what i'm having a hard time determining is what is meant by "recipe volume". for example, say you are working on an all-grain recipe (for a 5 gallon batch) and your initial boil volume is 7 gallons. you boil that down to 6 gallons before you add the hops. after the hops addition, the final volume (of wort plus hops) is ~5.8 gallons. you throw the whole mix (wort and trub) into a fermenter and wait a week. when you rack to the secondary, you've got ~4.8 gallons of liquid in the secondary.

so, out of all those volumes, what is the "recipe volume"? is it the final volume of wort (excluding the trub) that you put in the primary fermenter? or is it the liquid volume you rack OUT of the primary fermenter?

the way i see it, once the alpha acids find their way into the wort, they're there to stay. so the recipe volume is really just the final volume of wort that makes it way into your secondary/keg. of course, that's not accounting for any spillage or the little bit of wort left behind when you rack off of the trub.

what say you texags?

It's the total post-boil volume. The easiest way to do this is just use beersmith. Or get your own LC-MS.

look below, the next guy will have your answer.

[This message has been edited by 62strat (edited 1/20/2011 10:12a).]

+1 on beersmith.

http://www.homebrewtalk.com/wiki/index.php/IBU

"IBUs = 1.65 * 0.000125^(SG - 1) * ((1 - e^(-0.04 * t)) / 4.15) * ((AA * m * 1000) / V)

...where SG is specific gravity, t is the time of the hops in the boil, AA is the alpha acid rating of the hops (i.e. 7.0% alpha acid means AA = 0.07), m is the mass of the hops in grams, and V is the final volume of the wort in liters. "

or
"U = 0.1811 + 0.1386 * tanh[(t - 31.32) / 18.27]
G = (SG - 1.050)/0.2 if SG > 1.050, otherwise 0
IBUs = (U * AA * m * 1000) / (V * (1+G))

...where tanh() is hyperbolic tangent, SG is specific gravity, t is the time of the hops in the boil, AA is the alpha acid rating of the hops (i.e. 7.0% alpha acid means AA = 0.07), m is the mass of the hops in grams, and V is the final volume of the wort in liters. "

[This message has been edited by 62strat (edited 1/20/2011 10:13a).]

just making sure you got that.

[This message has been edited by 62strat (edited 1/20/2011 10:12a).]

quote:

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It's the total post-boil volume.

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so post-boil wort only

at what temperature? pre or post-cooling?

couldn't you just rack off your boiling pot into the fermenter and get the volume that way?

[This message has been edited by SuperDave03 (edited 1/20/2011 10:57a).]

temp shouldn't matter?

you're splitting hairs I think.. what's the difference in volume of your pot post boil and that of what you put in your fermenter? a couple ounces?

post cooling. IBU is a measure of the concentration of the alpha-acids. The AAs are extracted throughout the boil and concentrated as the wort evaporates during the boil. If you leave behind a half gallon of wort with the trub, the CONCENTRATION of AAs dissolved does not change, but you will make errors in your calculations if you omit this left behind volume and your calculation will be artificially high due to low volume.

This is not that hard to figure out. We're talking general chemistry here.

of course i'm splitting hairs

that's the whole point of calculating the IBUs

so, in summation:

V = volume of brewing kettle prior to chilling (including hot break and hops)

Boil Gravity = average of boil gravity immediately prior to initial hops addition and immediately prior to cooling

quote:

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This is not that hard to figure out.

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dude, i'm just asking your (and everyone else's) opinion

i don't consider hot break and hops solids to be part of the wort. just curious how you all account for that and the change in volume due to temperature in your calcs.

I think boiling to room temp is like a 5% decrease in volume.

I'm so confused right now that I don't know if what I've been drinking all of these years is beer or not! I think I'll go open another and try to figure it out!

My target TV is 12OZ with a ABV of around 5%!!!

superdave.. it's post chilling. you want your final volume. when is your final volume? not pre chilling, cause it's still boiling and losing vapor.
you want the amount of volume of liquid you have. it's concentration, which needs a final volume.

thank goodness for consistency

As someone who would really like to start home brewing I really only have one question after all that...

Where does the conveyor belt come in to the equation?

a monkey can brew beer. you don't have to be smart to make beer. you have to be smart to sell beer.

the conveyor is moot.

yeh brewing beer is definitely as complicated as you wish to make it. it can be as easy as throwing some premixed stuff into boiling water, and as complicated as you've seen here, but in all aspects of brewing.. not just IBU calculations.

[This message has been edited by 62strat (edited 1/20/2011 5:45p).]

Alright guys, I've got a specific question related to IBU's. This is a total noob question, but I picked up a Mirror Pond extract kit from Austin Homebrew. The recipe calls for 3 gallon boil. If I boil 4 or 5 gallons, but use the same amount of hops, I'm going to get a hoppier beer, right?

How close are the clone kits from AHB? I'd rather err on the side of hoppier.

the utilization term is partly a function of boil gravity.

hop utilization decreases with increasing wort gravity. the higher concentration of sugars makes it more difficult for the isomerized alpha acids to dissolve.

if you dilute the recipe provided during the boil, you should get higher utilization

quote:

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This is not that hard to figure out. We're talking general chemistry here.

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Dave is a ChemE, iirc.