Author Topic: SG OG BG FG,... why is my post boil reading lower than pre boil?  (Read 3586 times)

Offline Wort-H.O.G.

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Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
« Reply #15 on: May 08, 2013, 10:15:13 AM »
If measurements over 100f are worthless, how do you get a valid gravity pre boil?

I use my ATC Refractometer

+1  quick and easy
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Offline klickitat jim

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Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
« Reply #16 on: May 08, 2013, 06:13:19 PM »
Everything is brix until I get it I to the fermentor then hydrometer

Offline hubie

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Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
« Reply #17 on: May 09, 2013, 06:51:36 AM »
I'm not certain, but I bet the relationship between density and temperature is not linear.

It isn't.  I've coded up my own hydrometer correction and I've done a fit to the water density vs temperature data out of the CRC Handbook.  I'd put up a plot if I could figure out how to post it, but by eye it looks linear out around 100C, but it rolls off as you drop in temp.

Offline joe_feist

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Re: SG OG BG FG,... why is my post boil reading lower than pre boil?
« Reply #18 on: May 09, 2013, 09:41:16 AM »
I do remember reaing (just don't have the sourse) that hydrometer temperature corrections are not linear and the further you get from the calibration temp, the more error is introuced.

+1 to several comments to cool the sample. It's quick, cheap, easy and more accurate. Never thought of the cocktail shaker, but I like it. I'll typically set my pyrex measuring cup in a pan with cold water/ice and swirl it around. That 3/4 cup or so of wort cools pretty fast.
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