Strange gravity issue

[prrriiide]

I have a real head-scratcher...

I have my efficiency in BeerSmith set to 80%, which has historically been pretty close.

The last several brews (4-5), the pre-boil gravity has been off - low - anywhere from 5-10 points via refactometer (calibrated with distilled water). I've adjusted with a longer boil and/or DME additions to correct. But...by the end of the boil, the grav is high by the same amount. Yesterday's brew was 5 points low at pre-boil, so we boiled until the wort was at target pre-boil grav. At the end of the the boil, it was 5 points high.

Pre-boil volume was spot-on. Post-boil was, of course a little low due to the extra boil-off. If I had left it alone, I think it would have hit almost exactly.

WTH is going on here?

 

[IslandLizard]

If pre-boil gravity is a bit lower than predicted, look at a better crush, and perhaps your mash pH and water composition.

From what you're describing, it looks like your boil-off rate is a little higher than estimated. Also, hot liquids have a larger volume than cold ones, keep that in mind (for water/wort, ~4% between boiling and room temps).

When you measure gravity of a boiling wort, watch out for any evaporation between sampling and transferring, as that will skew your reading. Either scoop some out and let it cool down a bit, or better yet, use one of those plastic sample pipettes with the bulb at the end and let it cool down in the bottom of the bulb by sticking it in a cup with cold water for a minute (probably less).

Another thought, the volume markings on your kettle may be off.