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IBU calculation and boil length



I noticed that when I made 2 identical recipes with the only exception being that the boil time was different (since I had the box checked to set boil volume with the equipment the boil volume was also different) I came out with a recipe that has two different IBUs.  Both hop addition were made at the same time as shown below 60min from the end of the boil and 15 min from the end of the boil and the final gravity of the boil (original gravity for the beer) was the same.  I calculated the IBU for each recipe with all three bitterness calculations and below are the results.  I notice if I unchecked the box to set the boil volume with the equipment and adjusted it to match in each recipe the IBU’s ended up very close if not the same.  My question now is does this program use the boil volume to calculate the SG of the wort for the entire boil in the hop utilization calculations?  If so and you use a boil time of 180 min which gives you a boil volume of 8.5 gal the SG will be much lower than a recipe with a boil time of 60 min which gives a boil volume of 6.34.  This means that even though I specify that the hops will only be boiled for an hour it is using the SG of the wort 2 hours before I put the hops in giving me a much better utilization of my hops in the program which reports higher IBU’s than actual.  Let me know if there is a reason for this or if this is just a bug that needs to be worked out.  I can get my correct IBU's by fudging the boil volume but wanted to let someone here know so you could look into this.

Below is my hop schedule from BeerSmith:
1.50 oz        Simcoe [11.90 %]  (60 min)             
1.00 oz      Amarillo [8.20 %]  (15 min)

Below is the difference in IBU’s per boil time.
            60 min Boil  |  180 min Boil
Tenseth  52.0 IBU  |  60.9 IBU
Rager      74.2 IBU  |  80.2 IBU
Garetz    41.5 IBU  |  44.5 IBU
  The answer is pretty simple - the boil volume will vary according to the length of the boil and also the boil SG will vary with the length of the boil.  Both of these affect hops utilization.

I understand that if the hops are in for the entire length of the boil but for both of these recipes the hops are only in for 60 min.  I am not very experienced in the long boil brews but I would think that hops in for 60min should be the same for both no matter how long the wort has been boiling.  I love BeerSmith and I dont want to sound like I am talking bad about it but I just wanted to bring this up incase it is something that can help improve it for later versions.
The longer boil and longer boil time changes the boil volume, which therefore changes the specific gravity of the boil.  Since the specific gravity of the boil is used to estimate the IBUs (it affects how much hop oils are absorbed) it will change the IBU level even if the hops are boiled the same time since they are not being boiled in the same gravity worts.

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
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
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
Boil Time: 60 minutes
Boil Volume: 7 gallons
Boil Volume at 60 minutes = 7 gallons
Gravity of Boil at 60 minutes = "A"

Case 4
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.


I know this is an ancient post, but while searching the web I found this and have the exact same problem.

I would understand it if my boil volume didn't change, but even with additional boil volume to compensate for the added boil off, the IBUs are still higher. As bspisak explained, if the gravity at 60 minutes is the same for each batch, then why would the IBUs increase if the hop addition was going in at 60 minutes?
It comes down the model being used and limitations on how the model describes the parameters of the equation and the specific parameters being used.

Let's put the IBU calculation into perspective.

It is a model created to mimic the IBU test results of one particular system.  If you are using the Tinseth model, then it would be the system that Glenn Tinseth used to develop the data.  If you are using Glenn's process, the model would be pretty close to the actual test results.  Oh, and that is if you are only using dried hop cones.  Glenn never did any tests using pellets.  Luckily, someone else did a comparison and came up with an offset of around 10% better utilization if you are using pellets over cones.  Glenn also did not employ quick chilling of the wort, so if you are chilling the wort down quickly, that throws the numbers out the window as well. His best guess is that you might be within 10% of the target using his model and employing approximately the same chilling regime that he did. 

Denny and Drew at Experimental Brewing did a test with a number of their brewing minions on the bitterness and actual IBU numbers vs predicted model targets.  They also interviewed Glenn (as did Brad on his podcast) and much of what I stated above comes from those sources.  I very highly recommend reading/listening to the study and podcasts listed below.