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Post mash gravity vs pre boil gravity


Master Brewer
Mar 11, 2017
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Today our post mash gravity matched the calculated one in BS3. The recipe called for the addition of dextrose in the boil. We added the dextrose just as the boiled started. Gave it a good stir and took another sample to measure preboil gravity. For some reason now the gravity is significantly lower than the preboil gravity BS3 predicted. Any wisdom?


Grandmaster Brewer
Mar 16, 2013
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New Hampshire, US
You don't state how you are measuring the gravity.  If you were measuring hot wort and using a calculator to adjust the reading to the calibration temperature of the hydrometer, then you may have an error based upon the actual wort temperature versus what you used for the adjustment.  Depending upon your system, if you measured the gravity of the wort and then started recirculating wort which was in pump hoses, this volume may have made a difference. 

It could maybe be that the dextrose was not fully dissolved at your second reading, though that would not explain why it would be lower than your initial reading.

Either way, I would measure you post boil gravity and the use a gravity point balance to help determine which reading might be in error.