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Old 08-11-2008, 05:46 PM   #1
nchomebrew
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i hear some people talk about a pre-boil gravity. would it be the same as after the boil and the wort is cooled? my pre-boil temp will be alot higher obviously. i know there is a conversion factor based on temp, but wasn't sure if it was comparable.
it would be nice to measure it "pre-boil", so i can add a little dry extract if i'm under my og.

 
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Old 08-11-2008, 05:48 PM   #2
BigKahuna
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NO.
When you boil, you cook off some water (Steam) and thusly concentrate the sugars in the wort. Your Pre-Boil Gravity will always (unless you add back some water) be lower than your post boil (OG) gravity.


Quote:
it would be nice to measure it "pre-boil", so i can add a little dry extract if i'm under my og.
steal a sample of your wort 10 minutes before the end of the boil....cool it in a plastic cup and ice water...then take your gravity reading....you still have time to add some extract, and there won't be a significant change in 10 minutes.
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Old 08-11-2008, 05:58 PM   #3
Boar Beer
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Nchomebrew
If you take a reeding before boiling it will give you a good idea as to where you are starting. . I like to take a small sip of it at this time all so.
Some times if the reeding is too low i will stop sparging early.

 
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Old 08-11-2008, 06:11 PM   #4
nchomebrew
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aahh. ok, so is there some kind of conversion chart somewhere? say i have 6 gallons of pre-boil at a specific gravity, what would it be after it boils down, to 5 gallons. stopping the sparge early is a good idea too, but i'd like to make a full 5 gallons.

 
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Old 08-11-2008, 06:22 PM   #5
Bobby_M
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It's simple math... like this:

6.5 gallons of preboil wort that you measure at 1.043 has only so much sugar or total gravity. It's figured as 6.5 x 43 = 280 points.

If you think you'll boil off about 1.5 gallons in a 60 minute boil (typical) leaving 5 gallons post boil, then you just take that total number and divide by 5. 280/5 = 56 or 1.056.
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Old 08-11-2008, 06:34 PM   #6
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Quote:
Originally Posted by Bobby_M View Post
It's simple math... like this:

6.5 gallons of preboil wort that you measure at 1.043 has only so much sugar or total gravity. It's figured as 6.5 x 43 = 280 points.

If you think you'll boil off about 1.5 gallons in a 60 minute boil (typical) leaving 5 gallons post boil, then you just take that total number and divide by 5. 280/5 = 56 or 1.056.
In theory that should work but I've never been able to predict post boil gravity with any accuracy at all.

 
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Old 08-11-2008, 06:35 PM   #7
nchomebrew
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sweet, thanks bobby m. that'll help me. especially until i get my efficiency up.

 
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Old 08-11-2008, 07:38 PM   #8
nchomebrew
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if i do the pre-boil gravity, then i should still adjust for the temperature?
i believe it's .2 of a point per degree?

 
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Old 08-12-2008, 03:40 PM   #9
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Take your sample and throw it in the freezer to cool it to 65...

 
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Old 07-11-2013, 05:02 PM   #10
BostonianBrewer
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are preboil and specific gravity the same thing?

 
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