Pre-Boil Gravity and OG not in line...

[Whisler85]

I used "Designing Great Beers" to make myself a recipe for a roasty, toasty stout with an OG of about 1.082. I calculated this for 5.5 gallons post-boil, from which i calculated that my 7-gallon pre-boil gravity should be 1.064. I read the gravity at 130 degrees F to be about 1.050, which, with the correction factor for the temperature, meant i was dead-on! Needless to say, my brewing partner and i had a huge round of high-fives when we discovered how right we were (its only our sixth all-grain batch.)

But, after the boil, we took another gravity reading and found it to only be around 1.075, significantly short of our TOG of 1.082...

What did we do wrong? It isn't a volume issue, i don't think, since if anything we only got about 5 or less gallons of yield. Is this a hops issue? Could taking the gravity reading at 70 degrees have had an effect?

Help!

 

[korndog]

I used "Designing Great Beers" to make myself a recipe for a roasty, toasty stout with an OG of about 1.082. I calculated this for 5.5 gallons post-boil, from which i calculated that my 7-gallon pre-boil gravity should be 1.064. I read the gravity at 130 degrees F to be about 1.050, which, with the correction factor for the temperature, meant i was dead-on! Needless to say, my brewing partner and i had a huge round of high-fives when we discovered how right we were (its only our sixth all-grain batch.)

But, after the boil, we took another gravity reading and found it to only be around 1.075, significantly short of our TOG of 1.082...

What did we do wrong? It isn't a volume issue, i don't think, since if anything we only got about 5 or less gallons of yield. Is this a hops issue? Could taking the gravity reading at 70 degrees have had an effect?

Help!

Did you stir the wort thoroughly before taking the pre-boil reading? Are you sure you had 7 gallons? What was your boil time? Did you use software to calculate?

 

[ajf]

Did you take the OG in the fermenter or in the boil kettle? If you took it in the fermenter, you will have to take into account the volume that never made it there because of trub, dead space, wort left in chillers, hop absorption etc. Also, if you pitched a starter, you will have to take the volume of the starter (with low gravity) into account .

-a.

 

[Gordie]

One other thing to add - even with the temp correction formulas in the brewing software and the instructions that come with the hydrometers, the margin of error the further you get from calibration temperature is, from what I'm told, basically logarithmic. From 130 deg the correction factor may be no better than a rough estimate. Chilling your sample down to at least 80 deg will get you better reliability.

Gordie