From the HBD Archive
From: bszymcz%ulysses@relay.nswc.navy.mil (Bill Szymczak)
Subject: Correct IBU Corrections
Date: 1992-05-06 14:32:26 GMT

In HBD 871 there is a clip from a note from srussell which
mentioned that in Jackie Rager's article in the Hops ZYMURGY
special issue that to correct for specific gravity of the wort
you should divide by

1 + 5(G-1.050) if G > 1.050
1 if G < 1.050

After reading the Rager article myself, I found some obvious
errors. (I'm new to subscribing to HBD and don't know if these
have been already discussed.) In the ZYMURGY issue
the correction factor given by Rager is 1 + GA, where

GB - .050
GA = --------
0.2

if GB (gravity of boil) > 1.050, and GA=0 otherwise. If this
formula were correct the value of GA would jump from 0 to 5 as
soon as GB hit a value of 1.050, and you would need about six
times as much hops with a gravity of 1.0501 than a gravity of
1.0499. Replacing the value of .050 by 1.050 (as srussell
correctly did) gives more reasonable values. Even worse is the
example Rager computes on page 54 of the ZYMURGY issue, where
the equation

1.096-.050
GA = ---------- = .24 ?????????
0.2

appears. It seems to me that

1.096-.050
---------- = 5(1.096-.050) = 5(1.046) = 5.23
0.2

If the value of .050 which was repeated in the example is
replaced by 1.050, then GA = .23, which is getting closer to
Rager's value of .24. No wonder we're all confused about IBU's.

Besides these obvious errors I have found Rager's article very
useful. The formula given by Frank Tutzauer/Tom Hettmansperger
in HBD 871 also seems very convienient since it eliminates the
need for a utilization table and accounts for gravity at the
same time. Does this formula agree with the table listed in
Rager's article and the "corrected" gravity correction formula?
(Or equivalently, do the formulas given in Charlie II agree with
Rager's?)

I apologize for this being a little outdated, but it was originally
sent last week and apparently lost during one of the "down" days.

Bill Szymczak

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