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Help me understand Beermsith's gravity / volume estimate

Mothman

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I just noticed something that that is confusing me.

I just brewed a small batch brew, first time using this equipment profile,  (BIAB, no losses, everything from kettle goes in fermenter, if that matters).

The preboil gravity estimate was 2.16 gal at 1.043.

I hit the gravity right on, didn't bother measuring my actual volume.

Post boil estimate is 1.56 (hot)gal, at 1.062.

I only measured 1.057 or 58, and thought well, dang... oh well, and carried on, thinking I likely didn't boil off enough (first time using this small batch pot and boil off estimate may been off).

But then I realized... I don't think the math works out...  unless I don't understand the math?

Pre-boil:  2.16 x 43 = 92.88

Post-boil: 92.88 / 1.56 = 59.53

So shouldn't the post-boil gravity estimate be 1.56 gal at 1.060? 

It's only a 2 point difference, but I like to understand the math behind what's going on.

 
The difference is in the cooling shrinkage percentage you use. The post boil gravity is room temperature. The post boil volume is before chilling, therefore before the 4% contraction.
 
You are also better off using actual measurements for your volumes if you are trying to use this to tighten up your equipment profile.
 
brewfun said:
The difference is in the cooling shrinkage percentage you use. The post boil gravity is room temperature. The post boil volume is before chilling, therefore before the 4% contraction.

Ahh, ok that makes sense... 'hot' volume, but 'cool' gravity, as I'm cooling the sample.

Thanks.

Oginme said:
You are also better off using actual measurements for your volumes if you are trying to use this to tighten up your equipment profile.

Understood, I did make note of final volume, and my slightly under-achieving boil off accounts for the missing gravity, I was just confused by the math.

All clear now. :)
 
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