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 Home Brew Forums > gravity and volume - linear ?

11-12-2007, 04:47 PM   #1
kappclark
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 gravity and volume - linear ?

I know brewing sw does it, but how would you calculate/correct the gravity of 4.5 gals to 5 gals ?

If I have 4.5 gals @ 1060, then what is gravity @ 5 gals (same temp)

I figure that .5 gallons represents 1/9 increase in volume ... does that mean gravity goes down by 1/9 ? Is the rel strictly linear ?

If that were true, then a 100% increase in volume should give a 100% decrease in gravity ? (Obviously 9 gals would still be thicker than water, so the math is a little fuzzy)

I am just trying to determine the corrected gravity if my final volume were the correct amount of 5 gals...

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Bill Clark
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11-12-2007, 04:55 PM   #2
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Yes it is linear.

60 X 4.5g = 270 "gravity points" total

270/5.0g = 54 or 1.054

And a 100% increase in volume doubles the volume so halves the gravity...

GT

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11-12-2007, 04:59 PM   #3
kappclark
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Yep - Just about says it all !

Thank you for the quick reply ! Looks like I would have hit OG after all !

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Bill Clark
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11-12-2007, 04:59 PM   #4
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I think you're trying to make it harder than it really is. Your volume x the last two numbers of your gravity = the "gravity points" for your wort. Divide by your target volume to get the new gravity reading.

4.5g x 60 = 270 gravity points

270 / 5g = 54 or a specific gravity of 1.054

Hope this helps.

ETA: Beaten to the punch!

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11-12-2007, 05:37 PM   #5
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Well, it's linear, but not in the way you're thinking. Another way of thinking about it (which may or may not make more sense) is as a weighted average, considering water is SG 1.000:
(4.5*1.060+0.5*1.000)/5 = 1.054
This is a slightly more general approach which would also work for mixing wort with something other than water (like another wort with different SG), not that you need to do that real often...

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11-12-2007, 06:03 PM   #6
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Yeah, you didn't say that you were adding water. I assume that you are topping off?

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11-13-2007, 12:28 AM   #7
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Not topping off, but in the long run, I would have been topping off, had I known I was .5 gallons short ...

I am not too concerned about it ... which brings up the other topic of how can you be sure to get your target boil volume ??

I suppose this is where the "art" of brewing comes in ...

I should have topped off after pouring into the fermenter, but I did not have any pre-boiled water, so I just let it ride..

Thx to all who answered this for me...

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