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Today I attempted an American IPA (Extract) full boil.

I calculated the OG to be around 1085 using beersmith. The recipe included just over 11lbs of fermentables (9.15lbs LME, 1.10lb DME, 1lb corn sugar).

Somehow the OG came out low, around 1070-1072.

The only thing I can figure is that because of the massive amount of hops I added there was a significant amount of sludge in the bottom of the brew kettle. It was impossible to filter all of this out to get it to the fermentor, resulting in me leaving it behind and topping the fermentor off w/ between 1/3 to 1/2 gallon of water to reach 5 gallons. Could this cause the lower Gravity reading? Otherwise I am stumped.

Thanks, Bob

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please write 'because' instead of 'b/c'. –  mdma Aug 11 '12 at 0:47
    
If you are doing a full volume boil the amount of hop mased sludge left behind will have no bearing on the OG reading pre the addition of water. I would have just gone with less beer at what I wanted rather than water it down to get the 5 gallons. Now you've diluted the hop character that much as well. –  brewchez Aug 12 '12 at 12:43
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3 Answers

Most likely the water you added wasn't mixed well. Wort is heavier than water so it will sink to the bottom of the fermenter. When you take your reading you get the watered down wort from the top.

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It is possible that the hops absorbed a portion of the sugar-saturated water. My money is on the water top-up though. Adding more water will dilute the overall gravity of the solution.

This similar question lists a few possibilities if you used grains at all.

EDIT: I keep coming back to this question as I have been having the same trouble. I use Beer Calculus for my expected OG calculations and they currently have a new system in Beta. After re-entering a couple of old recipes I'm finding the expected OG is much closer to what I am getting out of my recipes. Perhaps try your recipes in their beta version and compare with your results.

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The other possibility could be temperature related. If you took a reading when the wort was 100 degrees (for instance), the adjusted OG would be 1.078. At 120 degrees, it'd be 1.082. (http://www.brewersfriend.com/hydrometer-temp/)

That, combined with a poor mix after top off could mean you were significantly off in your measurement.

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The gravity reading was 1070 @74F - which should put it around 1072. –  Bob M. Jun 8 '11 at 22:36
    
The gravity reading was 1070 @74F - which should put it around 1072. I stirred the topped off wort pretty well to aerate it, I would think this would have mixed it fine. After all when doing a partial boil you add 2-3 gallons of water, mix with the same method and that works –  Bob M. Jun 8 '11 at 22:42
    
Whole hops or pellet? I've never seen that sort of absorption from pellet, and never used whole, but understand they can soak up more goodies. –  Ell Jun 10 '11 at 20:32
    
" I stirred the topped off wort pretty well to aerate it, I would think this would have mixed it fine"...you;d be surprised. I've seen it happen too many times. Unless you used something like a Mixstir for 3-5 minutes, it probably wasn't as well mixed as you think. –  Denny Conn Aug 12 '12 at 16:06
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