Calculated IBU are far too low

Sorry if this has been asked before, I couldn't find any answers after searching the forum.
At the moment I tend to brew partial mash and when I put in all of my ingredients into BeerSmith the calculated IBU's come out far lower than expected. I'm just drinking an IPA made with Amarillo and Citra hops which BrewSmith calculated to be 33.2 IBU. I know from tasting it it is much higher than that and when I plug the same numbers into other software  (Brewtoad and BeerEngine) it shows around 77IBU which I feel is much closer to what I'm tasting.

Can anyone please suggest what I'm doing wrong with BeerSmith to give such crazy bitterness figures.

Thanks
Matt

Does it look better if you change the units to/from Rager/Tinseth/Garetz?  (I think only the first two are in BS, not at my computer)

Post recipe.bsm file so people can review it?

Likely the other programs are using Rager instead of Tinseth.  Rager gives you much higher estimates.  You can change the equation used to estimate IBUs from Options->Bitterness.

Brad

Thanks for your suggestions, I have tried to attach the BrewSmith recipe if you wouldn't mind taking a look.

Edit:
Changing from Tinseth to Rager  bumps the bitterness for this brew up to 60 which is closer to what I was expecting and also seems to tie in more closely with the IBU quoted in some of my brewing books. Thanks for the help. 

The issue is the boil size. You're only partial boiling, which limits the upper end of the IBUs possible.

Before I go further, there is a huge difference between measured IBU (which is only bitterness) and hop flavor/complexity gained by large and frequent hop additions. IBUs have nothing to do with flavor or aroma.

In any boil size, the upper limit of IBU saturation is around 100 (+/-10%). Even the hoppiest of commercial beers top out at a measurable 80 or so IBUs. So, your partial boil is maybe 110 IBUs for a few minutes at the end of the boil, then you dilute that with water. The percentage of dilution is the same percentage of IBU's reduced at full volume. In your case, that's about 70% dilution.

When I remove the dilution, the calculation shoots to 90 IBU.

However, all is not lost! Hop alpha acids and oils like to bond with melanoidins in the wort. They are then released during fermentation an can be partially fermented themselves. This results in a changed aroma and flavor profile that is far more associated with "hoppy" than just bitterness.

So, I think that all together, you might actually have 35 to 40 IBUs, but a massive amount of flavor and aroma. This is fine, since the perception limit of bitterness is 50 to 65 IBU, according to the most recent research. 

One last thing is that bitterness is more than a single perception. The texture of the bitterness can go from very smooth, even at the highest levels, to very coarse, even at lower levels. Make two identical worts and bitter to 80 IBU, say, one with Horzon and the other with Nugget. The Horizon will be "bitter, but smooth" and the Nugget will be "harshly cloying, coarse and unpleasant." This is a direct result of the  CoHumulone content, and some other yet-to-be-understood* qualities of hop varieties.




*A paper put out for review by University of Oregon this past fall, states that CoHumulone does not seem to be the sole determining factor in bitterness texture and perception. Some level of oxidation and/or growing conditions seems to be at work. This could change the understanding gained more than 2 decades ago about CoH as the cause of coarseness.

What a brilliant reply, thank you very much.

Matt

I fixed a typo in my previous post.

I scaled that recipe up to commercial size and it has about 22 lbs of dry hop in 15 barrels. This is in line with my levels. Something I've noticed is an enhanced bitterness perception from before to after. This seems to be caused by the plant tannins of the hops adding a touch of astringency. I have no doubt that this can happen on the homebrew scale, too.