Quick question -- I don't take the SG of what ends up in my boil kettle because by the time my sparge is done I'm pretty much at a boil. I do take a gravity reading of the first and final runnings however.
Can I average the first and final runnings to get the efficiency into the boiler?
It seems to add up in Beersmith -- for example I plan for and get 80% efficiency. My first runnings are 1.053 and my final runnings are 1.025. The average is 1.039 -- I put this into Beersmith and it comes out with exactly 80% eff into the boiler.....so that's got to be right, correct?
With fly sparging, the gravity drop is theoretically linear (or close) but I wouldn't want to chance it. If preboil is something you want to record, stir the wort in the kettle and pull a sample. It's the only accurate way assuming you have a method of measuring volume as well.
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I'll have to start putting that sample aside like you guys are suggesting and compare it to the first/final runnings avg.....This is just curiosity on my part -- I actually don't worry about boiler eff.