Beersmith 2.0 - IBU calculation vs Batch Size

Let me pre-face this by saying, I have seen lots of questions in relation to batch size and haven't found the answer I'm looking for.  I also have a HERMS setup in the equipment profile to account for losses in trub, chiller, tubes, etc.  I typically boil 7 gallons and end up around 6 gallons post boil.  Into the ferementer, I usually end up with 5.25 gallons or just under.  As I've read a million times, Batch Size is what actually goes into the fermenter.

However, I'm having issues with IBU calculations because it uses batch size on the DESIGN tab to figure IBUs out.  For instance, I used Beersmith's IBU tool (tool alone and not in the recipe interface) to calculate 7 gallons pre-boil, 6 gallons post boil with an OG of 1.070 and a AA% of 10 at 60 minutes.  With the Rager formula it gives me an IBU of 37.3.  Now if I just change the Batch Volume to 5.25 gallons, the IBUs go to 42.6 - as if i boiled off 1.75 gallons.

The above is true when you are in a recipe and mess with the Batch Size.  With Rager as the default and batch size set to 5.25 (beer into the fermenter), I get 42.6 IBUs for 10% AA with 60 minute boil with OG of 1.070.  If I change it to a 6 gallon batch size, then it is 37.3 which I believe to be more accurate.

I honestly feel like I should get rid of all the losses in my equipment profile and just set my batch size to 6 gallons to get the right IBUs.  IBUs should not go up because you lost some wort in the chiller and to trub.  It should be based upon the actual liquid you are left with at the end of the boil.

Can anyone shed some light on this?  I'm sure it has been talked about but I have missed it.

crowbarkarl said:

I honestly feel like I should get rid of all the losses in my equipment profile and just set my batch size to 6 gallons to get the right IBUs.  IBUs should not go up because you lost some wort in the chiller and to trub.  It should be based upon the actual liquid you are left with at the end of the boil.

Click to expand...

You're on the right track, because IBUs are exactly like OG....... A measure of concentration in a discrete volume of liquid.  Two ounces of 10% AA% hops can create an infinite number of IBUs depending on how much the AA is diluted, right?  So measuring those IBUs in 6.0 gallons versus 5.25 gallons would certainly be different. 

Batch volume is a critical measure in BSmith and lots of stats depend on it, but in my case, my boiler usually has 5.2 gallons in it post-boil, but I transfer only 4.0 gallons into a carboy for fermentation.  My gravity and my IBUs must be based on the 5.2 gallons of liquid in order to be the beer I want.  I choose to leave it behind  and must tell BSmith that I want 5.2 gallons of target wort as my batch volume, and I account for it by telling BSmith that I lose 1.2 gallons to losses.