Beersmith 2 IBU calcs

Hi guys

Have posted this in the beersmith forums but not getting any love so thought I'd try here.

I'm having some trouble with IBUs in Beersmith 2.

My hop schedule for 10L is*
15g***Centennial 60min
7.5g***Centennial 15min
7.5g***Simcoe 15min
7.5g***Centennial 5min
7.5g***Simcoe 5min
with Simcoe at 14.1% and Centennial at 8.2%

I know it makes things a bit tricky being in metric but thought I better leave it in the units I'm working in.

I'm getting 87.4 IBUs. While if I use the "Hop Bitterness" tool in beersmith I get 53.8, which is similar to what I get using other online calculators.*

Based on what I can find in the various forums I gather it's probably something to do with my equipment setup but everything in there looks correct to me. All I have really changed is the batch size from 5gal/19 litres down to 10 litres, and did this before starting the recipe.

Any ideas would be appreciated!
Cheers

So it turns out the IBUs are getting skewed by me entered my DME at a late boil time as opposed to the full 60min. The IBU changes massively when you alter the DME boil time due to built in hop utilisation factors.

Anyone have any thoughts on this? Should I just enter all DME as 60min boil even if I will add some of it late? Which gives a more accurate IBU reading?

no you shouldn't. bitterness is all about perception. although ibu is an indication of bitterness, there are other factors that come into play. if you really want to experiment (and since you're only doing 10L batches), this time add all the extract at 60 min. then do the exact same recipe next time, but add 2/3 of the extract at flameout. you will see a remarkable difference in the perceptible bitterness (i.e. the one with the late additions will likely be perceived as more bitter).