Today was batch 100....was pretty pumped until I experienced something I never have at home or at the brewery I work part time for. Pre-boil gravity - hit 1.054 with 7.5 gallons (great, no issues, conversion was good to go) Target OG - 1.073 at 5.25 gallons After about 55 min into the boil.....went out and checked gravity with my refractometer, nothing new. Came inside, let the small sample cool....OG was only 1.061!!! I also had already boiled down to 6 gallons, which is bad considering I have about .75 gal dead space, so I'm already at target finished quantity. What in the world with this happening!? (I boiled down to hit about 70 points, but as you can already guess, I only had 4.75 gallons to get into the fermenter. Epic fail. Without getting into the efficiency weeds....bottom line up front....if I was at 54 points to start, and even if my boil off has increased dramatically due to colder weather, I used the rear end of an F-15 as my burner, or a full moon, I honestly cannot imagine how the density only reflected 7 points different in nearly an hour.