I've been experiencing this weird phenomenon lately where my gravity readings before and after boil don't jive.
Example: Brewed a pale ale this weekend
10# 2-row, 12 oz Crystal 60, 8 oz Carapils
My goal was for 7.10 gallons pre-boil, boil down to 6 gallons, transfer 5.5 to carboy.
60 minute mash @ 153. I mashed with 3.4 gallons of water. I used Five Star 5.2, a fairly good crush (.24mm gap on the barley crusher) and I am using a 10 gallon rubbermaid cooler as a mash tun and a stainless braid.
(Batch) Sparged with 5.6 gallons of water. Collected a little over 7 gallons. Stirred up the wort and checked the gravity twice - 1.030 @ 133 degrees, which calculates to 1.044. Great. 75% efficiency there (was targetting 1.041)
60 minute boil. Post boil volume was around 5.3 gallons. Whirlpooled and racked slightly more than 5 gallons to my fermenter. Measured gravity was 1.046 @ 79 degrees, which equates to 1.048.
Huh? How'd that happen? Boiled off almost 2 whole gallons... and only gained 4 gravity points?
This has happened probably 3 out of the last 4 batches I have done recently.
Theories I have:
1) The pre-boil gravity reading is waaaaay off via the temp correction formulas. Really? How else would I go about doing it, other than buying a refractometer, which I thought has to be temp-corrected too? Also - that would mean indicate efficiency is waaaay crappy, and I don't know what else I could do (sparging with lots of 170dF water, using the 5.2 ph buffer stuff, doing a fine crush) to correct that.
2) Sugar really is evaporating during the boil, or there are little gnomes and/or fairies stealing it while I am not looking.
3) [Far-fetched] I am usually using whole leaf hops.. Maybe they are really absorbing a ton more than I expect. This weekend's brew had 3.5 oz of flowers in the kettle.
Any other ideas/explanations would be, umm, much appreciated.