Why is my AAU calculating off?

[brackbrew]

I'm plugging a recipe from BYO into BeerTools pro. It's a Northern England Brown Ale that calls for 3.5 AAU East Kent Golding Hops for 60 min, and 3.5 AAU East Kent Golding Hops of 5 min, supposedly giving me an IBU rating of 26. When I plug all the ingredients into BTP, it only gives me an IBU rating of 15.2! Anybody know why? Both the recipe and BTP are assuming a full wort boil, so I thought it would be a lot closer.

Should I adjust the recipe until BTP computes an IBU of 26?

 

[SumnerH]

They two guesses are probably using different methods of estimating IBUs (e.g. Tinseth vs. Rager). Set them to use the same formula.

That's not all that far off, really; there are (real world) recipes where one formula will give 18 IBUs and another will give 74!!!

Even if you pick a good formula (e.g. Tinseth), do full boils, and have a fairly normal hopping schedule, you shouldn't expect IBU estimates to be much closer than +/-30%.

On top of tat, brewing software often makes guesses (e.g. "pellets have 10% higher utilization", "hop bags have 7% lower utilization", etc) that are just kind of based on how the software developer was feeling or what one home brewing book they read thought.

Pick just one formula, use it as a guide, use the same formula the same way every time, and you'll get a feel for what it means in terms of what things wind up tasting like.

 

[brackbrew]

Anyway I can figure out what method of calculation the recipe author used? Do you know which method is the most common?

 

[SumnerH]

Rager and Tinseth are the 2 most common.

 

[brackbrew]

When I change the BTP utilization calculation to the "Rager simulation," the IBU's jump from 15.1 to 18.2. I guess that's a bit closer; when I use the "Tinseth simulation" it drops to 13.

Guess I'll plug a couple more recipes in using that and see how closely they match up.

Thanks!