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Can you think of any scenario (apart from errors of the measurement tools) where gravity actually goes down during boil. Normally, due to the loss of water, gravity should always go up. During our last brew session, we measured lower post-boil gravity (1.058) than pre-boil (1.068) and we're quite puzzled.

How we took the samples:

Pre-boil: pipette from top of stirred boiling pot.
Post-boil: pipette from first drawings after whirlpool.

Any ideas?

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up vote 5 down vote accepted

Bottom line: its impossible for the gravity to decrease during boil. You only evaporate water, leaving an increasingly sugary solution.

So there is an error with your preboil postboil or both gravity readings.

Most common reasons for incorrect readings:

  • Incomplete Mixings
  • Failure to to a temperature adjustment
  • Related to the second - Uncalibrated hydrometer
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So the question is maybe: How to measure post-boil gravity if whirlpool is used after the boil. Maybe mix the wort after the boil, measure, then start the whirlpool? – markus Apr 29 '13 at 14:05
The whirlpool would not effect the reading. What temperature(s) did you take your gravity at? what temp is your hydrometer calibrated at? – Jared Meyering Apr 29 '13 at 15:27
We took that gravity with a refractometer from the chilled wort (about 20 degrees). – markus Apr 29 '13 at 15:38
Same temperature for both samples? – Jared Meyering Apr 29 '13 at 15:47
Both pre-boil and post-boil readings were taken with refractometer and sample temp. allowed to adjust to room temp by waiting. – markus Apr 29 '13 at 15:56

from where did you take the sample? I am guessing you hadn't stirred the wort before you took the sample from the bottom of the brew pot, where the gravity will be the highest.

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Please see my edited question. – markus Apr 29 '13 at 12:07

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