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Old 01-14-2011, 04:21 PM   #1
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Default GU (Gravity Units)

Maybe I'm looking into this too much, but I'm a "units go with numbers and numbers go with units" kind of guy. I read "Designing Great Beers" by Ray Daniels (excellent book btw), and I love the BU:GU ratio concept, but I have one problem with it. Since IBUs are there own units, it's implied that if IBU=70 the beers has 70 Bitterness Units (BUs). My question is, are GUs the same or do they actually have units?

This all came about because we get the GU from the OG of the beer, for example OG=1.050 --> GU=50. But OG has units of kg/L. Did someone one day just decide to create GUs by taking the last 2 digits of the OG? Or is there a conversion step that I'm missing? If it is the former, it seems pretty far-fetched considering that we use GUs to calculate mash efficiency, and predict OGs (which have units).

I guess what I'm really asking is, what is the history of Gravity Units?

Thanks to anyone who can clear this up.

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Old 01-14-2011, 10:31 PM   #2
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IIRC, Ray Daniels invented the BU:GU (or at least was the first to include it in a published work). The whole purpose behind the ratio is just a means to put a numerical value on how balanced a particular recipe is. It is not hard science and the numbers wouldn't mean anything of real value to a chemist... just another visual/numerical tool for a homebrewer to get an idea of how balanced his recipe is.

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Old 01-14-2011, 10:37 PM   #3
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Originally Posted by BigB View Post
IIRC, Ray Daniels invented the BU:GU (or at least was the first to include it in a published work). The whole purpose behind the ratio is just a means to put a numerical value on how balanced a particular recipe is. It is not hard science and the numbers wouldn't mean anything of real value to a chemist... just another visual/numerical tool for a homebrewer to get an idea of how balanced his recipe is.
Yes, that's my understanding as well - just a relative ratio to look at balance between gravity and bitterness...
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Old 01-14-2011, 11:51 PM   #4
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You're confusing two things, I think.

GU in the BU:GU ratio is based solely off the two or three numbers at the end of the OG and don't have anything to do with predicting what kind of extract you'll get. This ratio is only used when comparing IBUs to the batch's final OG and the "gravity units" in this context don't mean anything else other than converting the OG into an easier format to compare with bittering units.

The term "gravity units" is also used to describe the number you get after you add up all the points per gallon you should get from your fermentables. For example, if you're using Maris Otter which I think is 36ppg, this means that it'll contribute 36 gravity units to your beer if sugars are extracted 100% efficiently. If you have 50 gravity units, that's 1.050 OG in 1 gallon, or 1.010 OG in 5 gallons. If you have an OG of 1.050 in a typical 5 gallon batch, that means your fermentables gave you 250 gravity units. This number is used to help calculate efficiency.

I'm pretty sure he uses the same phrase to describe both ideas in the book, which can be confusing.

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Last edited by indigi; 01-14-2011 at 11:57 PM. Reason: clarity
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