Difference between measuring mash efficiency and original gravity exactly?

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ClemTiger0408

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This is puzzling me. Clearly I am doing something wrong that I don't understand.

I brew AG through BIAB.

Before the boil, I always take a sample and a gravity reading before the boil. I have a refractometer and still let the sample cool. I will take my reading and put in the results to this efficiency calculator: http://www.brewersfriend.com/brewhouse-efficiency/

Of course I substitute the volume for the current pre-boil volume (typically around 7.1 gallons for 1.6 gallons of boil-off). I have had readings as high as upper 80s and 90%. Woah, I'm so good at brewing!

Then I will go through with the boil and take my OG reading. Typically, this reading comes out to be 70-75% using the same calculator (adjusted for new volume of 5.5 gallons).

Believe me, I know that 70-75% efficiency is not bad, even pretty good! I just don't understand how I am misreading the first reading or the mash efficiency. What am I doing wrong?
 
What makes you think you are misreading the first efficiency? If there is a difference between the two, it just means you have system losses (dead space, hops absorption, etc.).
 
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