Finding pre-boil gravity after the fact

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ThreeWiseMonkeys

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Gents,

Is there a simple way of guesstimating what the pre-boil gravity was after you have taken a post boil sample. I had my head up my ass when I was brewing tonight, and didn't take a pre-boil reading. I ended up hitting my target gravity of 1.050, and I know that I boiled off about a gallon to end with 5.5 gal., but I'm missing the pre-boil gravity reading to calculate my efficiency. Any ideas?
 
Only if you know your pre-boil volume. In which case the equation is: Concentration#1 X Volume#1 = Concentration#2 X Volume#2 (where #1 refers to pre-boil and #2 is post-boil values). IF you don't know your pre-boil volume, you can't figure it out.
 
5.5 gallons at 1.050 is roughly equivalent to 6.5 gallons at 1.042, but that number would probably swing between 1.040 and 1.045 depending on your guess at the boil volume, accounting for wort shrinkage after cooling, etc.

I think.
 
you can still calculate your efficiency with post-boil gravity and volume. (since just water is lost during the boil, all the sugars extracted are still there)
 

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