So, if I check "Set Boil Volume Based on Equipment" and change the "Boil Time", the calculated "Boil Volume" will increase based on my "Evaporation Rate". I assume this is the boil volume at the start of the boil. Therefore, when I get to the 60 minute mark, the 60 minute boil volume should be the same in both cases. Since the amount of total sugars in the wort doesn't change, the gravity should also be the same in both cases at the 60 minute mark. Thus, the IBU calculations should be the same, right?

Assuming a Batch Size of 6 gallons, an evaporation rate of 10%, equal grain bills, and a 60 minute hop addition:

Case 1

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Boil Time: 60 min

Boil Volume at Start of Boil: 6.67 gallons

Boil Volume at 60 minutes = 6.67 gallons

Gravity of the Boil at 60 minutes = "X"

Case 2

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Boil Time: 90 minutes

Boil Volume at Start of Boil: 7.06 gallons

Boil Volume at 60 minutes = 6.67 gallons

Gravity of the Boil at 90 minutes = "Y"

Gravity of the Boil at 60 minutes = "X"

Therefore IBU1 == IBU2, however the program shows them different.

Another thing I notice is that when "Set Boil Volume Based on Equipment" is NOT checked, changing the "Boil Time" does NOT change the IBUs for a given "Boil Volume".

Case 3

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Boil Time: 60 minutes

Boil Volume: 7 gallons

Boil Volume at 60 minutes = 7 gallons

Gravity of Boil at 60 minutes = "A"

Case 4

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Boil Time: 90 minutes

Boil Volume: 7 gallons

Boil Volume at 60 minutes = 6.65 gallons (assuming 10% / hour evaporation)

Gravity of Boil at 90 minutes "A"

Gravity of Boil at 60 minutes "B"

Therefore IBU3 != IBU4, however the program shows them the same.

Brian