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Old 09-15-2008, 12:48 AM   #1
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Default Post boil gravity doesn't match post boil gravity

That is correct.

Today I measured the post boil gravity before chilling the wort by cooling down a sample in a large stainless steel bowl and then using a hydrometer. The gravity I measured doesn't match the gravity I got when I measured the same wort after chilling with an immersion chiller.

post boil and pre chilling ~ 16.0 Plato or 1.064
post boil and post chilling ~ 17.0 Plato or 1.068.

That is to large to be caused by looking at the hydrometer in different ways. I even compared both samples at the same temperature.

What's going on here? Is it the precipitation of cold break. But 1 Plato difference is a lot. That is 1% of the weight of the wort. No water was added by a possibly leaking immersion chiller as the volumes pre and post chilling matched once corrected for temperature

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Old 09-15-2008, 01:03 AM   #2
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Temperature affects the reading. Says so right on the hydrometer instructions.

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Old 09-15-2008, 01:06 AM   #3
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Quote:
Originally Posted by Stinkonamonkey View Post
Temperature affects the reading. Says so right on the hydrometer instructions.
Yes, but both samples were at the same temperature. Just to make sure that I don't have an error in the temp correction table that I'm using.

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Old 09-15-2008, 01:08 AM   #4
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Did you do a partial boil or a full boil?

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Old 09-15-2008, 01:12 AM   #5
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I have always been curious whether break and hop material affects the hydrometer reading. I usually forget until I have the carboy full, so I measure with the last from the kettle which is sometimes very uh... thick. I let it settle and make sure the hydrometer isnt resting on a compost pile in the bottom of the tube.

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Old 09-15-2008, 01:13 AM   #6
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Kai - Just a thought, but perhaps your wort wasn't properly mixed during one of the samplings?

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Old 09-15-2008, 01:14 AM   #7
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Quote:
Originally Posted by Kaiser View Post
What's going on here? Is it the precipitation of cold break. But 1 Plato difference is a lot.
I can't think what else it could be. I assume that your measurement practices are sound, since I know you're pretty experienced with the scientific side of brewing... Having not added or subtracted anything from the wort between the two samples, it has to be some physical change that occurred during chilling, and cold break is the very obvious culprit.

Considering how much cold break you can get with proper chilling, I'm not surprised that it would produce some noticeable change, and the numbers you've quoted don't seem COMPLETELY unreasonable to me as being attributable to cold break, but I certainly don't know enough to be sure.
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Old 09-15-2008, 01:14 AM   #8
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Full boil and all grain.

I think I have a possible explanation: evaporation. The sample I pulled before chilling must have lost about 5% of its water while it was cooling. Seems like an awful lot for something that is not boiling, but it was a large bowl to speed up cooling.

But the pre-chill gravity and volume make sense with the pre-boil gravity and volume, whereas the post-chill gravity and volume don't make sense with the pre-boil volume. (everything temperature corrected of couse).

Cold break is reported to be only 40-350 mg/l, but to cause the difference I'm seeing it must have been 10,000 mg/l.

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Old 09-15-2008, 01:16 AM   #9
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Quote:
Originally Posted by anth View Post
Kai - Just a thought, but perhaps your wort wasn't properly mixed during one of the samplings?
that's what I was thinking too.
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Old 09-15-2008, 01:37 AM   #10
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I do not understand how a full boil not be mixed well? I guess it settles a bit during whirlpool etc, but I would not expect it to make a 4 point difference?

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