IBU Calculations Off

Hi, can someone please explain this discrepancy to me?

Recipe Design
Batch Size: 11 gallon
Pre-boil Vol: 14.95
Pre-boil Gravity: 1.084
Est. OG: 1.114
Trub Loss: 0 gallons
5 oz EKG, 10%, 60 min: 63.5 IBU

Hop Bitterness Tool
Batch Vol: 11 gallons
Boil Vol: 14.95 gallons
Boil Gravity: 1.084
5 oz EKG, 10%, 60 min: 63.4 IBU

Tinseth Javascript Tool (http://www.realbeer.com/hops/bcalc_js.html)
Wort Gravity: 1.074
Final Volume: 11 gallons
1st Hop: 5oz, 10%. 60 min: 63 IBU

I got BeerSmith to agree on IBU assuming the Hop Bitterness Tool's Original Gravity means pre-boil gravity. However, the utilization is way off from the Tinseth tool. The Tinseth tool is asking for overall "Boil Gravity" which I take to mean the  average gravity during the boil. The only way I can get the Tinseth tool to agree, is to plug in a pre-boil gravity of 1.074. Given the parameters above, the average gravity should be 1.099 (not 1.074) during a 60 minute boil. Putting 1.099 into the Tinseth tool gives me 51 IBUS. This is more than 10 IBUs off from BeerSmith.

Can someone please tell me what I'm doing wrong?!

You are worrying about the wrong thing. The actual bitterness is likely to be 20 to 50% less depending on the age and storage of the hops and YOUR actual utilization.

The calculation of IBU is tentative at best.  All the models out there (Tinseth, Garetz, Rager) are based  upon a strict set of conditions from one person's process only.  That relationship to your process may be right on or it may be way off, depending upon so many variables that they would be difficult to track them all down.  Even Glenn Tinseth has stated in interviews that it is only a 'best guess' model for people using it.

Your best route is to select a model and stick with it.  Compare the bitterness of your brews versus commercial examples of the same style and published bitterness.  Establish if you think the numbers you have are right on, too high or too low and adjust your expectations (and therefore, target) accordingly.

TAHammerton said:

You are worrying about the wrong thing. The actual bitterness is likely to be 20 to 50% less depending on the age and storage of the hops and YOUR actual utilization.

Click to expand...

I'm not worried about the actual bitterness or actual IBUs. I'm in the process of migrating myself away from using BeerSmith's IBU calculations and I'm mostly interested in the hop utilization percentage so I can carry it over to my method of doing it by hand. I have many recipes that I've brewed many times and would like to transition the BeerSmith hop utilization number without having to "retune" to my new process. I honestly don't care what the number is, as long as I can get consistent results with my new method.

I need to understand how BeerSmith produces the utilization number so I can have confidence that using that number in my own calculations will produce expected results. My question still stands.

Oginme said:

The calculation of IBU is tentative at best.  All the models out there (Tinseth, Garetz, Rager) are based  upon a strict set of conditions from one person's process only.  That relationship to your process may be right on or it may be way off, depending upon so many variables that they would be difficult to track them all down.  Even Glenn Tinseth has stated in interviews that it is only a 'best guess' model for people using it.

Your best route is to select a model and stick with it.  Compare the bitterness of your brews versus commercial examples of the same style and published bitterness.  Establish if you think the numbers you have are right on, too high or too low and adjust your expectations (and therefore, target) accordingly.

Click to expand...

Thank you for your answer. I'm comparing the same method (Tinseth) in all 3 scenarios. I don't expect them to line up exactly, however they're off by a large margin. 10 IBUs is barely perceptible on the palate,  but from a mathematical standpoint it shouldn't be this far off.

When I went to the on-line calculator and plugged in your initial numbers sg = 1.084, final volume = 11 gal, 10% AA, 60 minute boil, 5 oz, I get a number of 58 IBU.  Now I don't know what the % utilization number is for the on-line calculator, but in BeerSmith it can be set or adjusted in your equipment profile.

Oginme said:

When I went to the on-line calculator and plugged in your initial numbers sg = 1.084, final volume = 11 gal, 10% AA, 60 minute boil, 5 oz, I get a number of 58 IBU.  Now I don't know what the % utilization number is for the on-line calculator, but in BeerSmith it can be set or adjusted in your equipment profile.

Click to expand...

The % utilization I'm referring to the is one found using the Tinseth table (it's a function of gravity and boil time), not the overall +10% for using pellet hops.

If my pre-boil were 1.084, and my post boil was 1.114, then my average boil gravity would be 1.099. If I plug that into the online calculator, I get 51 IBUs. This is over 10 IBUs off from what BeerSmith is claiming the IBUs to be using the same formulas.

I see Wort specific gravity, not average gravity during the boil.  Typically, I would interpret this to mean the starting gravity of the boil.  With Glenn's on-line calculator, I do not know what the boil off rate is, so I don't know how he is figuring the initial volume of the boil but this could explain the difference between the 58 of the on-line calculator and 63 figured by BeerSmith.

When looking at the table, the utilization rate of the AAs is based upon the gravity of the wort WHEN the hops are added.  His calculation takes into account the changing gravity during the boil. It seems to me that you are trying to do some of the calculation yourself when entering the numbers by predetermining the midpoint of the gravity during the boil, which the calculator interprets as being the starting pre-boil gravity.

Oginme said:

I see Wort specific gravity, not average gravity during the boil.  Typically, I would interpret this to mean the starting gravity of the boil.  With Glenn's on-line calculator, I do not know what the boil off rate is, so I don't know how he is figuring the initial volume of the boil but this could explain the difference between the 58 of the on-line calculator and 63 figured by BeerSmith.

When looking at the table, the utilization rate of the AAs is based upon the gravity of the wort WHEN the hops are added.  His calculation takes into account the changing gravity during the boil. It seems to me that you are trying to do some of the calculation yourself when entering the numbers by predetermining the midpoint of the gravity during the boil, which the calculator interprets as being the starting pre-boil gravity.

Click to expand...

Now we're talking!

I'm assuming Glenn's calculator works as he describes in http://realbeer.com/hops/research.html. If so, then the boil gravity refers to the average gravity of the boil:

Use an average gravity value for the entire boil to account for changes in the wort volume.

Click to expand...

It appears that BeerSmith (and some other online tools) are using the pre-boil gravity rather than average gravity in its calculation, which I would argue is applying the Tinseth method incorrectly.

Given that [IBUs = (mg of AA)/L] and my example above for Beersmith:
11 gallons = 41.64L
63.5 * 41.64 = 2644.14mg of AA or 2.64 g of AA

since [(AA in g)=(g of hops)*(AA%)*(Util%)], we solve for Util% and get [Util%=(g of AA)/(g of hops)/(AA%)]:

2.64 / 141.748 / .1 = .186 or 18.6% utilization

BeerSmith bumps the utilization by 10% if using pellet (which I am). To make sure I'm comparing apples to apples, I remove that 10% from 18.6% which gives us 16.9% utilization. Still far from Glenn's tool which predicts a 15% utilization (assuming an average boil gravity of 1.099).

I understand exactly how Glenn's tool works because he describes the math in the link I pasted above. No where in BeerSmith or online can I find out how BeerSmith is coming up with it's utilization numbers.

Should I send an email to Brad?

I moved this to a more appropriate thread:

http://www.beersmith.com/forum/index.php/topic,13062.0.html

BeerSmith and other calculators can use the original gravity because they also calculate the final gravity and therefore can calculate the average gravity.  I will have to go back through my podcasts, but I am fairly sure that in his final calculator, Glenn used the pre-boil gravity and a typical boil off rate to calculate the wort gravity at the time of hop addition. 

I plugged in his formulas into a spreadsheet and compared the utilization with that of his calculator and they line up. I'm willing to be he's using the same formula in his calculator and the gravity is the "average" gravity.

I'd be interested in a link to the podcast you're referring to.