Partial Mash Pre/Post-Boil Gravity descrepancy

Hey guys,

Several days ago I brewed my first partial mash. Not knowing where my efficiency would end up, I took a pre-boil reading. In doing so, I ended up with a discrepancy that I've spent several days trying to come up with an explanation for and can't. I know I might be missing something simple so I figure I'd throw this up here and see what you guys think.

I'll try to keep it succinct but if there's any essential details I'm missing let me know!

My mash was 3lbs munich + 1lb specialty grains, single infusion for 45 minutes (started at 154f, finished at 151f), and a single batch sparge that was a little low at 164deg.

@ 1.375lb/qt I ended up with around 2.3gallons, which I topped off to 3gallons for the boil.

After topping off @ 3 gallons, I stirred the wort and took a sample. After letting that sample cool to room temperature (putting it in the freezer for part of it), I took a gravity reading and got 1.027, short of my ~1.033 target.

I ran those numbers through beersmith, giving me 56% efficiency. Based on that, I calculated that I'd have to scale down to 4.5gal to hit my target (1.047)

The boil went smoothly, and I added 3lbs of Pilsen DME with about 25 minutes left.

After cooling my wort, I put it in my fermenter with enough top off water to get to ~4.5 gallons. After aerating it, I took a gravity reading and found it to be 1.050.

I began adding water and ended up with about 4.8 gallons in my fermenter before I hit a 1.047 gravity.

That equates to an efficiency of 65%, a pretty big discrepancy over what I calculated based on my pre-boil gravity.

The only possibility I've come up with is a post I read where someone mentioned getting inaccurate gravity readings from wort cooled to room temp in a fridge/freezer. Other than that I'm at a loss.

Any ideas?

P.S. I've previously checked the gallon marks on my fermenter (the stock markings were a bit off), and made my own markings using a measuring pitcher, etc. I'm reasonably sure they're correct but I may double check after this batch is done.

 

My first thought is that you have three liquids of different density in your wort at preboil: the first runnings, the second runnings, and the top off water. I found that no matter how much I stirred after combining the runnings, I wasn't happy with the mixture of the runnings in the kettle, and I was getting unreliable preboil gravity readings. So, I've started grabbing my preboil SG sample right as I'm coming to the boil when the runnings are more fully incorporated.

 

Thanks for the response. This could definitely be it. It's pretty counterintuitive for me to think liquid that's at least stirred a bit isn't pretty well homogenized quickly. I'll give your method a shot next time.