Interesting...I would speculate that each hops utilization model ignores the "batch size" since no one can know how much the brewer will dilute the beer, (intentionally or not). By that I mean Tinseth and Rager don't know whether I'm full-boil brewing 3 gallons of dopplebock, or partial-boiling 60% of a five-gallon bock that might have the same gravity in the pot.
I thought the models were largely based on boil time and sugar density, which Tinseth calls a "bigness" factor that has an inverse relationship to utilization, i.e., the higher the sugar concentration, the less IBUs extracted, all else equal. Below is a snippet of Tinseth's math: for a 60-minute boil, he assumes half the IBU's in a 1.130 monster compared to a 'typical' 1.050. But for that 1.050 wort, he assumes little is gained from 60 minutes to 90 or 120 minutes.
The major difference then, between models, are their assumptions and math. My ESB has a certain IBU that I could learn if I sent it to a lab. The models "estimate" the IBUs at 52 (T), 47 (R), and only 35 by Garetz, all for the same beer, and they each could be wrong.
My "Aha" moment was realizing these hop models are similar to economic models and all three could be wrong, and what matters most is your impression of "what's in your wallet?" It is helpful, however, to know (or best guess) the hops model used in a recipe you are trying to copy. If Rager, then build your recipe file using Rager to better copy the IBUs and the IBU:gravity ratio of the original brew.
Boil Original Gravity
Time 1.030 1.040 1.050 1.060 1.070 1.080 1.090 1.130
(min)
0 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
3 0.034 0.031 0.029 0.026 0.024 0.022 0.020 0.014
6 0.065 0.059 0.054 0.049 0.045 0.041 0.038 0.026
60 0.276 0.252 0.231 0.211 0.193 0.176 0.161 0.112
90 0.295 0.270 0.247 0.226 0.206 0.188 0.172 0.120
120 0.301 0.275 0.252 0.230 0.210 0.192 0.176 0.123
