Yesterday I brewed an american brown ale with 12.4 lbs of grain. I mashed in with 18 quarts of water at 154F and fly sparged with 17 quarts of water at 170F.
The last couple of brews have come home at around 78% efficiency.
Yesterday's OG reading was 1.054 (should've been 1.066) for 61% efficiency. DANGIT!!!
There are two variables that I can identify:
1. I mashed thinner than I usually do. I typically go 1.25 qts/lb but went 1.5 qts/lb for this brew.
2. My grain bed got dry when I walked away for a few minutes. I thought I had my flow rate nailed, walked away for 10 minutes, and came back to find the bed dry and the little plastic tupperware lid I float on top to disperse my sparge water slightly pressed down into the grain. There was still wort flowing. I opened up my HLT valve and got the water level back to 1-1 1/2" above the grain. I monitored the flow rate and kept it in check the rest of the way.
I think it was #2 that got me. From what I have read, a thinner mash should give you better efficiency if anything. And I think the dry bed compacted the grains throughout the mash tun, keeping me from rinsing all the sugars in there.
What do you guys think?