Post-boil gravity was high. Not sure why. Can you guys help me troubleshoot?

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headwall

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Hi everyone. 1st post and I hope it is in the right place.

I just started fly sparging and tried my new system out the other day. I'm still pretty new to brewing in general, so I don't really know what I'm doing yet. Anyway, my target starting gravity for this all grain batch was 1.053. I used estimates for efficiency and evaporation and what not, and calculated that I needed 6.2 gallons pre-boil at 70% efficiency in order to hit my target starting gravity in 5.5 gallons post-boil. After continuous sparging, I ended up with 6.5 gallons of 1.047 wort (temperature corrected). My total gravity was thus a little bit higher than I needed so I calculated that I needed to shoot for a post-boil volume of 5.7 gallons instead of 5.5 gallons to hit my target gravity. During the boil, I noticed my evaporation rate was slightly higher than I had estimated, so I added .25 gallons of water. In the end, I hit my target post-boil volume pretty much spot on (5.7), but my specific gravity turned out to be 1.062 (again, temperature corrected) instead of 1.053. How could that have happened? Did I make a mistake in my calculations? The only thing I can think of is that I am not measuring my volumes accurately. I am using a measuring stick which I calibrated to my boil kettle using incremental volumes of water. I'm not sure what else the problem could be. Any ideas?

Thanks guys, this place is an incredible resource.
 
You could have got really good brew house eff i plugged your numbers into beer smith with your 70 eff than increased it i got to 85% and it got to 1.062 OG something to think on
 
Your math was correct: 6.5 gallons @ 1.047 is the same as 5.76 gallons @ 1.053

Did you mix well before taking your hydrometer samples? How close is your wort to the calibrated hydrometer temp when you measure it (I try to get within a few degrees or it doesn't seem to be very accurate).
 
Yeah I calculated my efficiency to actually be 75%. I did mix well before taking samples for gravity measurements, but I didn't take them at 60 degrees. The first reading was 1.040 at 104F and the second was 1.061 at 68F.

The thing I am confused about is that the total gravity seemed to change during the boil, which it should not have (right?). The specific gravity and volume measurements taken at two different times during the boil yield different total gravity numbers, and I was under the impression that total gravity would not change.
 
I was under the impression that total gravity would not change.

thats the wrong impression. the stuff that affects your SG readings will not evaporate during the boil. the water on the other hand will. as you boil the volume of water will of course go down but the amount of sugar will not. so as you boil your gravity will go up. if your calculating IBUs use an average of the boil gravities. any difference in IBU wont be detectable.
 
I should clarify. When I say total gravity, I mean the product of specific gravity and volume. So I understand that the wort concentrates during the boil, raising the SG, but at the same time, the volume is decreasing. This is how I was targeting my starting gravity.
 
oh ... SG * volume = some number .... yea i would think that would stay the same. maybe my assumption that compounds that affect SG don't evaporate is wrong.
 
Gravity readings above 80 degF are significantly lower than readings taken from the prescribed reading temp. By how much did you correct for you 104 degF reading?
 
Oh sorry, I should have read that more carefully. Well 7 SEEMS a reasonable correction but maybe it's not. I've heard some say that any reading that isn't at 60 degF is unreliable so maybe that's it?
 
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