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)