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 Home Brew Forums > Determining OG from the Pre-Boil Gravity?
02-12-2007, 12:13 AM   #1
jcarson83
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 Determining OG from the Pre-Boil Gravity?

Anyone know how to estimate the original gravity of your wort by taking a reading before the boil off?

EX:
My 7 gallons of wort is 1.065. If my final volume is 5.5 gallons what is my OG?

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02-12-2007, 01:56 AM   #2
ajf
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Assuming you boil down to exactly 5.5g, and transfer the whole 5.5g to the primary, then your OG would be (1.065 - 1.000) * 7 / 5.5 + 1.000 = 1.083.
Unfortunately, the transfer to the primary will not normally be 100% efficient, and to estimate your OG you will have to estimate the amount of wort lost during the transfer. Assuming you lose x gallons, the formula would be modified to
OG = (1.065 - 1.000) * 7 / (5.5 + x) + 1.000, so if you lose 2 pints it would be
(1.065 - 1.000) * 7 / ( 5.5 + 0.25) + 1.000 or 1.079

-a.

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02-12-2007, 11:14 AM   #3
newfie
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hey ajf , i'm not sure what gets left behind will change the grav. when you boil off to the 5.5 gal volume, won't the wort all have a grav of 1083 (including the 2 pts). So the 5.5 gals(less 2 pts) will have a grav of 1083 as will the 2 pts left behind?? Otherwise, the grav of the 2 pts left behind is going to be pretty darn high ( apply conservation of mass theory here).
hey maybe i'm seeing this wrong.... i'm interested in sorting it out though ....
brewfie

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02-12-2007, 12:56 PM   #4
ajf
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Yes, if you boil down to 5.5g, you'll have a gravity of 1.083, but if you leave 2 pts behind, you will end up with only 5.25g in the fermenter (still at 1.083)

-a.

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02-12-2007, 01:41 PM   #5
jcarson83
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Thanks for the help guys. Those things really bug the hell out of me until I figure them out (or someone else does.

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