Why does changing pot size throw calculated IBU's out of whack?

I'm playing around with a trial version of Beersmith, and it's quite a powerful tool: it can move mountains or take off both your legs at the knees with equal facility. I may be in the process of doing the latter. 

I've entered the following basic stuff in the recipe design section:

Type: extract
Equipment: pot (4 gal / 15.1L) - Extract
Batch size: 10 liters
1.5kg pale liquid extract (8.0 SRM)
15 gr. hops 11%AA - 60 mins
1 pkt. US-05 yeast.

Beersmith calculates that this will give me an OG of 1.045, 19.4 IBUs, 6.4 SRM, 4.5% ABV. Which seems fine.

BUT... When I change the equipment from a 4 gal. to a 3 gal pot (Pot ( 3 Gal/11.4 L) - Extract) suddenly my bitterness drops down from 19.4 to to 1.7 IBUs! The batch size is still the same (10L) which still fits into an 11.4L pot (OK, you will want more head space for the boil, but still.)

What's going on here? How can reducing the pot volume by less than 4 liters reduce my IBUs by more than 90%?  The rest (OG, color, ABV) stays the same. Hop utilization is set to 100% in both the 3 and 4 gallon profiles (I checked). I also made sure that the batch size is still 10L (adjusted back to 10L after changing equipment profiles).

I also don't have a clue why the pre-boil gravity that Beersmith comes up with is so high (1.116 for the 4 gal. pot, 1.384 for the 3 gal. pot). That may be due to a botched entry I made, of course, but I'm not sure what I could have done wrong and where.

Could anyone unconfuse me? All suggestions would be appreciated. Tnx!

frankvw said:

1.5kg pale liquid extract (8.0 SRM)

Beersmith calculates that this will give me an OG of 1.045, 19.4 IBUs, 6.4 SRM, 4.5% ABV. Which seems fine.

I also don't have a clue why the pre-boil gravity that Beersmith comes up with is so high (1.116 for the 4 gal. pot, 1.384 for the 3 gal. pot). That may be due to a botched entry I made, of course, but I'm not sure what I could have done wrong and where.

Click to expand...

The default profile for that pot has top up water. Have you changed that?

The very high boil gravity is limiting the IBUs you can get. The top up water would dilute that by the direct proportion (in this case about 90%).

brewfun said:

The default profile for that pot has top up water. Have you changed that?

Click to expand...

No I haven't. But it shows the same in both profiles. I'm attaching a PDF with screenshots of both the 3 and 4 gallon profiles.

brewfun said:

The very high boil gravity is limiting the IBUs you can get. The top up water would dilute that by the direct proportion (in this case about 90%).

Click to expand...

That makes sense. But for a 4 gallon pot I get a boil gravity of over 1,100 already, and it's over 1,300 for a 3 gallon pot... So that means for the 4 gallon pot my IBUs are way off, too?

Here is the thing with BeerSmith: it is a program based upon calculations that respond directly to user input.

If you wanted to change the equipment profile within a recipe, the way to do it and keep the same parameters as you had before is to start by creating custom profiles which match your equipment settings and volume losses.  Next, when you want to use the "Scale Recipe" feature to allow the program to adjust the ingredients to maintain the same target parameters as the original recipe.  Scaling your recipe does not only mean changing batch size but also changing a recipe for different equipment parameters. 

Of the two you have posted, they not only have different top off water as Brewfun has pointed out, but different boil off rates as well.  Given this, the starting volume of water would change between the two recipes if you change the equipment profile and this changes all the other calculated targets in the recipe. 

Beyond that, the easiest way to diagnose your high gravity issue is to export the recipes with the different equipment profiles and attach them to your next method.  It is easier to diagnose when you have the whole recipe which we can open within BeerSmith and look at how the ingredients are entered and how the calculations play out.