First partial mash - OG question

I did my first partial mash today and man was it a lot more fun! There was a bit of gravity magic happening, which I hope I can attribute to not mixing the top off water well enough. What do yall think?

My 3 gallons of strike water was at 150, mashed for 75 min, then I sorta sparged at 170 with another gallon. The second pot I have wasn't big enough to hold the 8 lbs of grains and a gallon of water, so I poured it through the grain bag as best I could then soaked them in the remaining quarter of a gallon.

Long story short, I used 4 gallons for the mash/sparge, so I assume that I was left with about 3.5 gallons for the boil after the grains soaked up some.

The boil gravity of the 3.5 gallons was 1.054 which comes to about 64% efficiency.

I did my boil as usual, then did a late LME addition of 4.5 lbs. After topping off and shaking, the gravity ready 1.054 again! What?

Doing a bit of math, I found that I should have ended up with an OG of 1.068 given my boil gravity and assuming 34 ppg for the LME.

Is it reasonable to assume that I was taking a reading of top off water? Maybe I missed something in my math?

Did you cool the preboil gravity sample? If it was over 90 degrees, the readings (even with temperature correction tables) are greatly inaccurate. The sample must be cooled to 90 degrees or less (in a freezer or a pitcher of ice) and then the reading taken and a correction table used.


Also, it's important to take exact measurements of the runnings to figure efficiency. Maybe the amount was about 3.5 gallons- but if it was less, that would make a big difference in figuring the efficiency.

All your numbers appear to be accurate up until your final O.G. reading. I would agree 1.068 would be accurate regardless of the actual reading.

Yooper said:

Did you cool the preboil gravity sample?

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Yes. It sat in the plastic tube for about an hour while I did my boil.

Yooper said:

Also, it's important to take exact measurements of the runnings to figure efficiency. Maybe the amount was about 3.5 gallons- but if it was less, that would make a big difference in figuring the efficiency.

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With just a kettle, how do you determine the exact amount of initial boil volume? I know I topped off with 2 gallons, so .5 gal boil off seems about right. I'd be afraid to sharpie the inside of my kettle, but maybe that's a solution? Maybe mark the outside and use a ruler on the inside to determine the liquid level?

oh the fun problems we have brewing