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I usually take a hydrometer reading immediately after sparge so that I can calculate total points of gravity (i.e. gravity reading * volume), and from there calculate what volume of wort I should target post-boil to hit my target OG. For my pre-boil gravity reading, my runoff temp is usually in the 140*F range. I use a temperature correction formula from Homebrew Digest to correct my readings.

I then make sure that I end my boil with the calculated volume of wort, either boiling longer to evaporate off more water, or adding water during the boil to get back up to the target volume. When all is said and done, post-boil, post-chill, my OG is often quite different from the target.

Any ideas of things that could be contributing to this error?

I have a couple of thoughts:

  • the high temp correction formula isn't accurate
  • my runoff is stratified into high-gravity on the bottom of the bucket and low-gravity on the top of the bucket, and my sample for the hydrometer reading is being taken from one of these layers (maybe I should stir the runoff before sampling)
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Why would you ever add water during the boil? Isn't it easier to just add cold water post boil in the fermentor? Also, are you using something like a sight glass to determine your volume, or is it involving a degree of guestamation? –  hartski Jul 11 '12 at 2:48
    
@hartski, I add water during boil to avoid any contamination issues. For this reason, many technique books talk about adding cold, pre-boiled water post-boil to make up volume. I find that I never end up having this cold, pre-boiled water on hand, so i just add water as I boil. I've marked the inside of my brewpot in 1/2 gal increments, and I use this for volume estimation. –  mac Jul 11 '12 at 17:57
    
I understand the contamination concerns...I guess I just never had a concern or issue with that myself. Also seems like your markings should get you pretty close to your actual volume estimation. –  hartski Jul 11 '12 at 19:51
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2 Answers 2

You've covered the two main likely causes, with stratification being the most significant cause of error. Another potential error is in the volume measurement - if this is out, then all the calculations to relate pre and post boil gravity will be out also. Related to that is the 4 percent shrinkage that happens as wort is chilled from boiling to pitching temperature, so the volume you see post boil, while still at boiling temperature, is 4 percent grater than what you end up with.

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Good point on the shrinkage. 4% could definitely contribute to the error that I'm seeing. –  mac Jul 11 '12 at 1:14
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The only other thing I can think of that isn't in mdma's answer is that your hydrometer is miscalbirated. Check that it reads 1.000 in distilled water at the temperature that the hydrometer is calibrated at.

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+1, yes, hydrometer, check that first –  mdma Jul 11 '12 at 1:36
    
or better, microwave some distilled water up to 140f take a reading, and run it through your formula. –  Dale Jul 22 '12 at 21:18
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