Partial-boil IBU Calculation

[noggins]

I'm using tastybrew to calculate a recipe. When the boil volume is set to 7, which I would normally use for a 1 hour boil, the IBU is 8, which is what I'm aiming for. When I change the boil volume to 4, which I may need to do today due to lack of propane, it changes the IBU to 40. I have the batch size set to 5.5 on both calculations, I just can't figure out why the IBU would be so much higher for less water if it's being watered down after the boil? I understand if the final volume were 4 the IBU would be higher, but it's not...

 

[noggins]

bueller?

 

[noggins]

oh well, I guess I'm doing a split boil today.

 

[CPooley4]

It would all come down to hop utilization in the boil which is dependent upon the average boil gravity. You will get higher hop utilization rates with a lower gravity wort (essentially getting more IBU's out of your hops) with the lower boil gravity.

Example: 1.050 avg boil gravity for 60 minute boil = .231 utilization factor; 1.060 avg boil gravity for 60 minute boil = .211 utilization factor

However, that is in complete contrast to the issue you are experiencing. With a smaller volume you should have a higher avg boil gravity which should equal lower IBU's due to the lower utilization factor.

Assuming a 1.050 gravity beer of 5 gallons:

1.050 = 50 gravity points
50 x 5 gallons = 250 gravity points

If you regularly do a 7 gallon boil that means your boil off rate is 2 gallons per hour. If we want to calculate your avg boil gravity in this scenario we would take 250 (total gravity points) and divide it by 6 (halfway point between 7 and 5) to get an avg gravity of about 42. For simplicity sake let us assume 1 oz of 10 alpha acid hops being used for a 60 minute boil.

To calculate the IBU's in this scenario we would:

AA's x utilization x 75 / final batch volume = IBU's
10 x .248 x 75 / 5 = 37 IBU's

Now, if you were creating the same beer, but only boiling 4 gallons rather than 7 we would then need to figure out your avg boil gravity for that volume. To do so we would still figure a 2 gallon per hour boil off rate. That means when you are done boiling you'll have 2 gallons of wort so we'll use 3 gallons for the avg. 250 divided by 3 = 83 gravity points.

AA's x utilization x 75 / 5 = IBU's
10 x .172 x 75 / 5 = 26 IBU's

Sounds like either the calculator is wrong or you are using it improperly. If you have any questions please feel free to ask.

cp