Hop Bitterness Calculation

Hello, I am a new time brewer and thought downloading this program would help me with beer recipes.  However, I am a little confused on how the BeerSmith2 calculates the IBUs.  If I just plug the numbers into the Hop Bitterness Tool using Tinseth I get one reading. For example.

Northern Brewer Hops  1 oz x 60 mins = 31.6 IBU
Amarillo Hops 1 oz x 5 mins = 6.8 IBU
Amarillo Hops 2 oz x 0 mins = 0 IBU

If I put the same hop profile in the new recipe program I get different numbers.  For example.

Northern Brewer Hops  1 oz x 60 mins = 50.5 IBU
Amarillo Hops 1 oz x 5 mins = 10.9 IBU
Amarillo Hops 2 oz x 0 mins = 0 IBU

Then as I add grains to the IBUs decrease until it comes out to 28.1 by the end of the recipe. I am actually following a recipe that was calculated to around 41 IBUs. 

I don't understand why there is a difference.  I am looking at an extract recipe using a 3.1 gal boil to achieve a final 5.0 gal output.

I think I get the formula IBU - AAU x U x 75/V.

So is the Hop Bitterness Tool using a set utilization rate and the recipe calculation is actually based on the boil gravity?

Not trying to make this harder than it needs to be just trying to understand how the program figures out the IBUs.

Thanks

Batch2 said:

So is the Hop Bitterness Tool using a set utilization rate and the recipe calculation is actually based on the boil gravity?

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Yes!  The higher the gravity during the boil, the your less hop utilization will be.  This is why many extract brewers put about 1/3 of their extract in at the beginning of the boil and 2/3 towards the end of their boil.  This way, they have a lower gravity boil to get good hop utilization and then up the gravity at the end of the boil to hit their target SG.  This is especially helpful when trying to brew an American IPA style beer, where you want a lot of hop bitterness.

The two hop addition schedules aren't the same. They have different AA% levels. This accounts for most of the difference.

The Tinseth formula accounts for more variables than are in the utilization tool. The recipe will have those variables in it.

Batch2 said:

So is the Hop Bitterness Tool using a set utilization rate and the recipe calculation is actually based on the boil gravity?

Click to expand...

Basically, yes. In your equipment profile, you can change the utilization to match your system.

As Scott stated, the Tinseth formula reduces hop efficiency with increased gravity.

Thanks for the answers.  I guess the best plan is to brew it, taste it, and adjust it if I need to.

and.............the best part of the plan itself is the tasting part!!!  8)