Please help me understand my brewing numbers

[Froyd]

Trying to understand my numbers... please help!

I brewed a brown ale with a target 1.045 mash gravity (74%eff). Recipe recommended 153F mash, but I went a little warmer to suit my taste (154-156F). I sparged and managed to hit the target volume of 6.75 gallons dead-on and my gravity reading was pretty close: 1.044.

So far, so good, however, after the boil, with the volume having dropped from 6.75 to 6.25, and then to approx 6 gal. after cooling, I took my a gravity reading and it read 1.043. Thinking I made a mistake with the refractometer, I gave the cooled wort a good stir to ensure there was no stratification, confirmed my temp was 70F (close enough to 68F) and took a reading with my high-precision hydrometer...1.043 again.

How's that even possible? This is only my second all-grain batch, but if I understand things correctly, by boiling off water (my volume went from 6.75 to 6 after boiling and cooling) my gravity should INCREASE, not decrease. Am I missing something, or do you think I somehow took poor readings post boil/cooling?

 

[iamwhatiseem]

It is impossible for gravity to lower after boiling. Impossible.
So that means either before/after reading (or both) is an error.
Lot's of possibilities, improper temps, didn't stir the wort before sample. Etc.
Cheers!

 

[rburrelli]

How did you take your pre-boil gravity reading? You mention a hydrometer. Was there any temp compensation involved?

 

[McKnuckle]

Preboil readings are notoriously inaccurate. Stratification is a major factor, as is temperature correction either with a refractometer (small sample) or a hydrometer.

Since you have volume measurements and a reliable OG, use the dilution/evaporation calculator on the brewer’s friend site to determine your actual preboil gravity value.

 

[Froyd]

How did you take your pre-boil gravity reading? You mention a hydrometer. Was there any temp compensation involved?

That was a refractometer reading. Pipette sample was cooled in cold water.

 

[Froyd]

Preboil readings are notoriously inaccurate. Stratification is a major factor, as is temperature correction either with a refractometer (small sample) or a hydrometer.

Since you have volume measurements and a reliable OG, use the dilution/evaporation calculator on the brewer’s friend site to determine your actual preboil gravity value.

Thanks. I'll take your advice on the calculator. I'm trying to figure out my efficiency for formulating future recipes, so I'm disappointed my mash gravity reading was off. Glad the calculator can help work backwards to get me my number.

 

[McKnuckle]

My mash tuns have spigots, and I use recirculation during the mash, so my samples are pretty well mixed.

What I do is to decant about a quarter cup (or less) from the spigot into a stainless steel milk frothing pitcher, which I put right into the fridge. After it has cooled over 5-10 minutes, I stir it and collect about a 1/4 teaspoon. I deposit a couple drops from that onto the refractometer lens. I wait a couple of seconds for the temp to equalize on the glass, and I read the gravity in Brix. Then I will repeat with a newly drawn teaspoon and get a second, or even a third reading.

Based on these readings, I'm usually pretty confident that I'm within a point of the actual pre-boil gravity. I use a 1.04 correction factor to convert the Brix to SG.

Brewer's Friend refractometer calculator