Post-boil OG mystery

[element533]

Greetings, beer nerds! Long time lurker here with an OG mystery. See if you can help me figure this one out.

• Pre-boil OG 1.051 @ 95 F = 1.056 SG standard
• Pre-boil volume 6.8 gal @ 140 F = 6.7 gal standard
• Post-boil volume 5.9 gal @ 190 F = 5.7 gal standard

I calculate this as 1 gal of boil-off, which means I should see a post-boil OG of 1.066 (56 x 6.7 ÷ 5.7 = 66). However, I measured a post-boil OG of 1.069. What gives?

Some more info:

• Gravity measurements are +/- 0.002 precision
• Volume measurements are +/- 0.13 gal precision
• Temp measurements are +/- 2 F precision
• Temp correction for volume is done using temp correction for SG formula
• 15 lbs of grain, 3 ounces of hops, FermCap, Whirlfloc, Yeastex
• Both pre- and post-boil were stirred vigorously in the kettle before taking the sample

Questions:

• The post-boil sample had a large amount of suspended hop trub - could this influence the gravity reading?
• Is it possible the hop addition caused an increase in volume, making the boil off rate appear lower than it really was?
• Do the protein and other suspended solids "take away" from the volume in the kettle for the purposes of gravity calculations?

 

[Big Monk]

Is there a problem? 3 points is not that big of a deal in my opinion.

I would have measured gravity closer to calibration temp but other than that it sounds like a success.

Displaced liquid from solid matter may have given you a larger preboil volume.

 

[element533]

Is there a problem? 3 points is not that big of a deal in my opinion.

I would have measured gravity closer to calibration temp but other than that it sounds like a success.

Displaced liquid from solid matter may have given you a larger preboil volume.

I just want to rule out if I'm being an idiot or whether this is a standard, repeatable aspect of my setup. If I know where the extra 3 points came from, it's easier for me to make predictable beers. I've struggled with efficiency issues and unpredictable hop utilization.

Regarding the gravity measurement, I measured at three separate temperatures and all the corrected values came to the same pre-boil OG of 1.056. The post-boil OG measurement of 1.069 was at calibration temp.

 

[BlueHouseBrewhaus]

I really think 3 points is within your margin of error. Given the variables of gravity readings, temperature measurement and corrections, volume measurement, and solids content all compounded, 3 points variation doesn't seem unreasonable.

 

[element533]

I really think 3 points is within your margin of error. Given the variables of gravity readings, temperature measurement and corrections, volume measurement, and solids content all compounded, 3 points variation doesn't seem unreasonable.

Fair enough, but it's still odd that the pre-boil OG didn't read high. I also took volume, temp, gravity measurements for first wort and sparge, and all the numbers add up so to speak except for the post-boil OG. Seems suspicious that it would be the only one to be off.

In any case, it's 3 points high, which I'll take!