AAU to IBU not making sence...

[Blueflint]

First off, I am new to brewing. I have brewed 6 batches so far. I am in the middle of a brown ale (in secondary now) and the way I figured the IBU and the IBU that hopville come up with are quite different. My recipe is:
http://hopville.com/recipe/1452819/home-brew/honey-brown

I used 1 ounce of 3.5 AAU at 30 minutes from the end and 1 ounce 3.5 AAU at 5 minutes from the end, so the way I figure it...

3.5 AAU x .21 = .735
3.5 AAU x .10 = .350

total = 1.085

1.085 / 5 gallons = .217

.217 / .01335 = 16 IBU

My hopville IBU comes to 8.9 IBU...

What am I missing...which one is correct?

Any insight would be appreciated.

Thanks, Tony

 

[Blueflint]

Anyone?

 

[billl]

It is different utilization numbers. Are you using your OG as a 3 gallon boil for your calculation. ie multiple by 1.66.

 

[Blueflint]

OK, just read about utilization due to wort density (boil gravity) at http://www.howtobrew.com/section1/chapter5-5.html/ This throws all kinds of wrenches into this, specially since wort density is changing every time you add either LME or DME too. So the rough calculation I cam up with this time would be 9 IBU...so...looks like Hopville calculations are correct.

Thanks, Tony

 

[billl]

FYI - the "workaround" for utilization is splitting the extract additions up. eg only half to start and the rest at the end of the boil. It also keeps the extract from darkening as it boils longer.

 

[ludomonster]

Boil gravity doesn't have as much of an impact as that table implies.