Lost 17 points of efficiency yesterday. Why?

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?

I don't flysparge, never have, but the logic in your #2 makes sense to me.

I think it was the dry grain bed as well.

Brewmasters Warehouse said:

I think it was the dry grain bed as well.

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+2

The thinner mash shouldn't be a problem.

ark1ebs said:

......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?

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I think you hit the nail on the head here.... My advice comes without guarantee, but that is where I would put my money ASSUMING your crush didn't change and you hit your temps pretty close.

Yeah, I had my bed go dry a few times during a session and took a hit in my efficiency. If it happens again, it's probably a good idea to "reset" the bed: close the outlet, add water, stir the bed, re-vorlauf (if needed) and continue. I target at least one hour for 7.5 gallons of wort.