IBU calculation way off for a Briess Recipe

[Grumpy Old Git]

I'm new to Beersmith and before I use the software in anger next weekend I thought I'd used a Briess Recipe to see how my setup performed.

I selected their 'Mighty Fine Barley Wine' (http://www.brewingwithbriess.com/Recipes/beer/display/one_might_fine_barleywine) and set up my equipment with their profile (basically steep grains in 3G of water, remove and add DME and boil, top up to 5 in the bucket...) as this is close to how I brew indoors on a radiant stove top. The blurb states this recipe has "...been built to age with over 100 IBUs of hops"

So I tried - and although I match OG and Color, the IBU is waaaaaaaaaaaaaaaay off.

Parameter	Briess	Me
OG	1.105	1.107
Color	15	15.8
IBU	90	37.7

So, as a noob I'm assuming the issue is somewhere between seat and keyboard and most likely the equipment setup. I've attached the recipe and equipment setup - any help/feedback gratefully received as I can't work out why if the rest matches but the IBU doesn't even come close.

[Oginme]

First, you are doing a partial boil and then topping off with almost half of your batch size.  Hop Alpha acid isomerization is dependent upon the gravity of the wort, with the higher gravity impeding the isomerization of the hop acids.  Since you are limited in your kettle size, then I would recommend adding half of your extract up front and then adding the remainder with 5 or 10 minutes left in the boil.  This will help with utilization of the early bittering hops and should be reflected in the IBU number

Next, I didn't spend much time on the Briess site looking up the way they calculate the IBU numbers but they may be using a different IBU model than you are using.  If you are in default BeerSmith settings, it is set to use the Tinseth model.  You can change this through your 'options' settings from Tinseth to either Rager or Garetz, but without knowing which model the recipe follows, this would be just cherry picking to get the numbers to match.  IMHO, you are better off just sticking with the Tinseth model as I have found it to be more responsive than the other models and better matches the apparent bitterness in my brewing process.

Lastly, I never sweat the IBU numbers from recipe to my system.  Too many people don't specify the model they use nor do they specify the volumes through their system to allow me to make adjustments.  So, I match each addition for AA units (amount of hops times %AA of the hops) and brew on.  Once I have brewed a recipe with my system, I can adjust the bitterness up or down the next time I brew it depending upon how I want the bitterness to come out in the finished product.

[Grumpy Old Git]

Hop Alpha acid isomerization is dependent upon the gravity of the wort, with the higher gravity impeding the isomerization of the hop acids.  Since you are limited in your kettle size, then I would recommend adding half of your extract up front and then adding the remainder with 5 or 10 minutes left in the boil.

You nailed it  - the excessive gravity was depressing the isomerization something awful. I changed the recipe to add 3lb up front, and then add the remaining 9lb 5 mins from the end just to pasteurize it - net result is an IBU of 97.1 which is close enough for me.