Insane boil/OG experience

[Brewer #89247]

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.

 

[J A]

I vote for bad reading on the pre-boil gravity. If the volumes were right and you're confident of your post-boil readings, your pre-boil had to be 1.049.

 

[Brewer #89247]

Took three different readings pre-boil, and that is dead on what I was supposed to hit for my efficiency and set up per BF and Beersmith. But, possibly my refractometer is jacked up somehow.

 

[Ozarks Mountain Brewery]

don't worry about the pre-boil too much, too many things can change that reading, the original gravity and the final reading in the carboy is more important than the pre-boil

 

[J A]

Yep, that^^^
I take pre-boil refractometer readings but only as an estimate to confirm that I'm in the ballpark for 60 minute hop addition. It's always way high on my refractometer and somewhat high using a cooled sample with a hydrometer. I just don't try to nail it down in real time. Pre-boil volume and post boil volume and gravity gives me accurate efficiency number and actual OG for computing ABV after FG is reached. Anything else is interesting info, but not critical for the purposes of brewing accurately.