Calculating IBU according to Palmer's instructions. Is my attempt a success?

[Elysium]

I am trying to understand how the IBU is calculated and this is my attempt to do so:

According to Palmer the IBU is AAU * U * 75/ Vrecipe
AAU (alpha acid unit) is the alpha acid (AA%) content multiplied by the weight of the hops. So, if I use 11% Mosaic hops and 0.19 oz of it, then the AAU is 2.09
The U is the utilization factor which is at 0.231 according to Palmer’s chart if I boil hops for 60 mins at 1.053 gravity (see the chart attached).
Vrecipe is the final volume that goes into the fermentor. In my case it is 2.37 gallons.
So, the IBU is (2.09 * 0.231 * 75/ 2.37) which is 15.22.

Now…I’d like to dilute the 2.37 gallons to 3.96 gallons so I need to multiply the IBU of 15.22 by 0.598 (2.37/3.96) which gives 9.1.

So, boiling 0.19 oz of mosaic (11%) hops for 60 mins at 1.053 preboil gravity will give me 9.1 IBU in a 3.96 diluted batch, right?

 

[Biscuits]

Your calculations look pretty close. I also made an excel spreadsheet using Palmer's formulas, but at OG of 1.055 (my spreadsheet takes the average hop utilization of 1.050 and 1.060 wort) w/ 11% AA Mosaic, I am coming in at 14.62 IBU's w/ 2.37 gallons. However, what goes into your fermenter isn't what you should calculate from, you should be using the final volume in the kettle. So, if your post boil volume is 2.5 gallons, your IBU's are really 13.86. Given your formula for dilution is correct, you would really have 8.29 IBU's.

 

[Elysium]

Your calculations look pretty close. I also made an excel spreadsheet using Palmer's formulas, but at OG of 1.055 (my spreadsheet takes the average hop utilization of 1.050 and 1.060 wort) w/ 11% AA Mosaic, I am coming in at 14.62 IBU's w/ 2.37 gallons. However, what goes into your fermenter isn't what you should calculate from, you should be using the final volume in the kettle. So, if your post boil volume is 2.5 gallons, your IBU's are really 13.86. Given your formula for dilution is correct, you would really have 8.29 IBU's.

Thanks for the reply.

I have just modified the spreadsheet and I got 4 IBU less in the calculation with the afterboil size. Thanks for bringing that to my attention.

Oh...by the way....the hop utilization factor is something I gotta understand. What do you do when you have OG like 1.045 which is between 1.04 and 1.05? I mean the table only account for specific round numbers. So at 1.04 it is 0.252 and at 1.050 it is 0.231 for a 60min boils....how do you calculate it for example for an OG of 1.045? 0.252-0.231/2? I mean that would make sense, but it would be problematic with OGs like 1.041/1.042 or 1.043.....
with numbers that end in 1.xx5, that's easy to do, but the rest might get complicated.

Sorry for making it so confusinhg.

 

[ZebulonBrewer]

It's linear interpolation. It's usually 'good enough' for stuff like this that sometimes doesn't scale linearly. Think of it in percentages. 1.045 is 50% of the way between 1.040 and 1.050, so it's also 50% of the utilization factor difference from one to the other.

I'll get a link and add it in a bit

https://en.wikipedia.org/wiki/Linear_interpolation

 

[Biscuits]

Thanks for the reply.

I have just modified the spreadsheet and I got 4 IBU less in the calculation with the afterboil size. Thanks for bringing that to my attention.

Oh...by the way....the hop utilization factor is something I gotta understand. What do you do when you have OG like 1.045 which is between 1.04 and 1.05? I mean the table only account for specific round numbers. So at 1.04 it is 0.252 and at 1.050 it is 0.231 for a 60min boils....how do you calculate it for example for an OG of 1.045? 0.252-0.231/2? I mean that would make sense, but it would be problematic with OGs like 1.041/1.042 or 1.043.....
with numbers that end in 1.xx5, that's easy to do, but the rest might get complicated.

Sorry for making it so confusinhg.

It's linear interpolation. It's usually 'good enough' for stuff like this that sometimes doesn't scale linearly. Think of it in percentages. 1.045 is 50% of the way between 1.040 and 1.050, so it's also 50% of the utilization factor difference from one to the other.

I'll get a link and add it in a bit

https://en.wikipedia.org/wiki/Linear_interpolation

Like ZebulonBrewer said, a 'good enough' estimate is, just that...good enough. My spreadsheet is a little bit more than what Palmer provided, but I don't split hairs any further...if my OG is 1.042, I just use the Utilization for 1.040, if it is 1.043, I use 1.045...

 

[Elysium]

Like ZebulonBrewer said, a 'good enough' estimate is, just that...good enough. My spreadsheet is a little bit more than what Palmer provided, but I don't split hairs any further...if my OG is 1.042, I just use the Utilization for 1.040, if it is 1.043, I use 1.045...

That actually make perfect sense. Thanks.