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I've done 2 full boils and probably 10 partial boils since I've started homebrewing. In both full boils, my OG has been significantly higher. Most recently, by .02 (expected 1.062 measured 1.082). None of the partial boils were off by this level.

Any thoughts? Am I extracting more points from the specialty grains with 5g vs. 2g?

Recipe: http://www.northernbrewer.com/documentation/beerkits/BelgianDubbel.pdf

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Can you post the recipe as an example? 20 points is a very large margin. –  JoeFish Jan 18 '12 at 16:15
    
Added link to recipe. –  Bob Banks Jan 18 '12 at 16:28
    
I just realized that recipe is for a partial boil. Can you tell us the volumes of water you used? I'll update my answer as appropriate. –  JoeFish Jan 18 '12 at 16:51
    
How much wort did you collect at the end of your full boil? Did you add any water to bring it up to 5 gallons? –  roto Jan 18 '12 at 17:04
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Between 4.5 and 4.75 gallons. I measured 1.082 prior to top off. –  Bob Banks Jan 18 '12 at 17:07
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2 Answers 2

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I plugged your recipe into Beersmith, which comes up with a max OG of 1.064, assuming 100% efficiency extracting sugar from the 1/2 lb of specialty grains.

How did you get 1.082? Guesses:

  • Boiled the wort down too far and concentrated it. You'd have to boil off a whole gallon to get to 1.082, though.
  • Hydrometer is bad - calibrate in 60F distilled water to see
  • Misreading the hydrometer or miscalculating temperature adjustement
  • Bubbles sticking to the hydrometer, raising it up in the test tube
  • Weight/volume of ingredients off
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Those are all valid reasons but those things should be equal (except for the 1st) if I'm doing a full vs. partial boil. I've never been this far off with a partial boil. Only with the 2 full boils. Seems odd to me that I'd have these problems on 2 out of 12 boils and both of those were full boil. –  Bob Banks Jan 18 '12 at 17:10
    
Indeed. Did you also check the gravity after adding top-up water to 5 gallons? –  JoeFish Jan 18 '12 at 17:13
    
Yes, I added 1/2g and it was 1.072. –  Bob Banks Jan 18 '12 at 17:51
    
Then I'm at a loss. That is just about the right reading after diluting. Nothing in the full boil process should change how your sugars are utilized. At least not that I know of. –  JoeFish Jan 18 '12 at 18:44
    
I don't have any explanation either, but it will be interesting to see what the final gravity is. A starting gravity of 1.084 would yields a terminal gravity no lower than 1.020 (75% apparent attenuation), I think (for an all malt brew). If the terminal gravity is significantly lower than this, we might want to conclude that the original gravity reading was wrong. –  Tobias Patton Jan 19 '12 at 15:08
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When I go to my local HBS to get a kit made up, I have noticed that they do their best not to short me on ingredients but typically over it, sometimes by quite a bit. I assume that you got your kit from northern brewer but I can’t imagine that they don’t do something similar. Did you happen to weigh out you ingredients before you boiled?

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Nope, but I'll do that on my next couple batches to see. –  Bob Banks Jan 24 '12 at 21:34
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