IBU Calculations - Is there a problem?

[AHarper]

After working with the new Pre / Post Boil volumes I thought I'd check out my latest brew figures - just to be sure.

There are differences between the IBU Calculator Tool and the results displayed in the Recipe Builder - given the same values in each.

I have the screen shots of 1) Recipe Editor View, 2) Recipe View (just another way of looking at the figures) and 3) The IBU Calculator Tool.

WHy are the two calculations different given the same values in each?

1)

2)

3)

 

[Yooper]

I know it's different, but here's why-
Yes, the boil volume and batch size are the same, so at first glance you'd think they should match.
However, the stand alone calculator doesn't take your losses into account. That is- where did the missing wort GO? In your first example:
Pre: 28L
Post: 25L
Batch: 20L

So, the software "knows" you had 3L of boiloff/shrinkage. We have to assume that you had 5L of loss in the chiller/trub/tubing/etc.

In the second,
Pre: 28L
Batch: 20L

The calculator assumes you had 8L of boiloff/shrinkage since the post-boil volume is not specified. That means a greater IBU because you concentrated the wort that much.

I hope that makes sense.

I put in a ticket for this, for the stand-alone calculator to better account for losses somehow to get a more accurate accounting.

 

[J A]

I just tried for 20 minutes to follow the logic of why the same pre-boil volume with the same gravity, using the same equipment profile with the same boil-off rate would give different IBU values for the same hop additions? Why would post-kettle losses make a difference in the IBUs? Why would less of the exact same beer be less or more bitter?
I had hoped that I could just work around this new mess with the beta version but I'm intensely disappointed to see that that version has been converted, too.

 

[Nosybear]

I just tried for 20 minutes to follow the logic of why the same pre-boil volume with the same gravity, using the same equipment profile with the same boil-off rate would give different IBU values for the same hop additions? Why would post-kettle losses make a difference in the IBUs? Why would less of the exact same beer be less or more bitter?
I had hoped that I could just work around this new mess with the beta version but I'm intensely disappointed to see that that version has been converted, too.

I think someone misapplied a formula. If I brew to x IBUs (mg/l iso alpha acid), then don't use half the wort, the wort still has x IBUs. Post-boil losses do not affect bitterness, only efficiency.

 

[Pricelessbrewing]

@AHarper I should be able to take a look Monday the 2nd in the evening just to double check it by hand.