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Last week I brewed an Arrogant Bastard clone. My pre-boil gravity was off by .001 so I was pretty happy with that... But my OG (post-boil) was off by .009! I thought that because my pre-boil gravity was so close to the original (estimated), my OG will result as well in a closer number to the estimated OG.

My boil was vigorous and I boiled for 90 min (as the recipe stated) so I have no idea what could have interfered in reaching my estimated OG.

PS. My gravity measurements were taken at the same temp.

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Did you hit your intended volume? –  Denny Conn Aug 1 at 15:47
    
Yes, volume was spot on –  Jorge Gautier Aug 1 at 15:48
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Can you tell us the pre and post boil volume and gravity? –  Denny Conn Aug 1 at 20:19
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Sounds like your volume measurements are off. 7 gallons of 1.062 would yield 6 gallons of 1.072. Getting 5 gallons of 1.071 shouldn't be possible without taking a lot of sugar out of solution. –  Neil Aug 4 at 15:42
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It had to go somewhere didn't it? 7g at 1.062 should boil down to 5g at 1.087. That's just math. So either 1) your gravity readings were off 2) your volume measurements were off or 3) you lost some (boil over)? –  Chris M Aug 4 at 18:11

3 Answers 3

I've had this same thing happen to me before. My pre-boil gravity was one thing and the post boil gravity was actually LESS than the pre-boil. I do all grain and I've found this happens if I take my pre-boil gravity after collecting my wort WITHOUT stirring the wort in the kettle.

What happens in my case is that my first runnings sit in the bottom of the kettle with the lower gravity second runnings settling on top. I would take the gravity reading from the valve at the bottom of my kettle and get a huge gravity reading. Then, after boiling & getting everything mixed up, the reading afterwards would be accurate, but lower than the pre-boil reading.

So, I stir up my wort really well after sparging now to make sure that all the sugars are evenly distributed through the wort.

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My guess is that you took your gravity readings with a refractometer?

Refractometer measurements are very sensitive to extra drops of water that stick around in your pipette or on the refractometer itself.

For instance, lets say you pull 4 drops of wort into a pipette that has one drop of water, your reading will be off by 20%!

Good practice for gravity readings by refractometer is to "rinse" the pipette with wort (throw that wort away), then take a second sample and measure that on the completely dry refractometer prism.

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I used hydrometer, not refractometer –  Jorge Gautier Aug 11 at 14:16
    
Are you taking all readings with the liquid at the same temp? –  Wolfger Sep 8 at 14:49

I would pay close attention to the temperature at which you took the hydrometer reading. Hot wort can throw off a reading. Also as other posters have suggested, a quick stir before a hydrometer reading can make a huge difference.

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