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Home Brew Forums > Home Brewing Beer > All Grain & Partial Mash Brewing > Post boil gravity doesn't match post boil gravity
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Old 09-15-2008, 01:44 AM   #11
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aside from evaporation, my only other guess is a little bit of water from washing the hydrometer and cylinder didnt get shaken/dried out and it diluted one of your samples. Ive had this happen to me when I didnt dried it carefully.

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Old 09-15-2008, 01:47 AM   #12
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Quote:
Originally Posted by anth View Post
Just a thought, but perhaps your wort wasn't properly mixed during one of the samplings?
The odd thing is, that the pre-chill gravity would be the one that I could have messed up with insufficient mixing. But I did mix it well and it also makes sense if I look at the numbers:

pre boil gravity = 15.3 Plato; volume = 25 l
post boil/ pre chill gravity = 17.0 Plato; volume = 22 l

If I predict the post boil gravity from the preboil I get 17.4.

The post chill gravity of 16.0 Plato for 22l just doesn't fit into the picture.


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Old 09-15-2008, 02:44 AM   #13
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I don't think the difference would be caused by break material. The heavier solids precipitating out would leave a less dense wort - wouldn't it?.

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Old 09-15-2008, 07:38 PM   #14
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It could be your measurements are off some, ie temerature, gavity and volume, causing the final prediction to be off some too. Also, the wort should shrink about 4% from boiling temperature to 60 F.

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Old 09-15-2008, 10:44 PM   #15
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I'm inclined to think evaporation, but from your kettle, not the sample. If the sample was evaporating much, it would have had the higher gravity. How long between taking the sample & chilling the wort? Also, how do you cool the wort, CFC or chiller? The 20 minutes it takes me to cool a batch would allow a fair amount of evaporation.

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Old 09-16-2008, 04:00 AM   #16
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I'm inclined to think evaporation, but from your kettle, not the sample.
I put a lid on the kettle 2 min after taking the sample. So no significant evaporation. And even if there was evaporation, shouldn't I have seen a corresponding 5% drop in volume in addition to the 4% wort shrinkage? But I only saw a 4% smaller wort volume after the end of the boil.

This one is a real puzzler to me.

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Old 10-27-2008, 03:36 PM   #17
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This problem still exists for me. Last night I brewed a batch for which I used more water for chilling than usual as I wan’t able to move the IC as often as I generally do. I lost a whopping 6% efficiency between pre-boil and post chill:

4.2 kg grain bill:
25.8 liter at 100C pre-boil volume at 11.5 Plato -> 88% efficiency in kettle
21.9 liter at 7C post-chill volume at 12.0 Plato -> 82% efficiency in kettle

B/c this calculation was done with efficiency, it would not matter if the chiller leaked water I to the wort. Something it didn’t do since the pre-chill volume was about 4% more than the post chill volume. All extract measurements were temperature corrected.

I have the suspicion that the chiller allows for an exchange of extract from the wort into the chill water. Not sure how it does this w/o changing the volume in the kettle. Maybe water is pushed in at one place and pulled out from another though the venturi effect. But the chiller doesn’t leak when I just run water though. The outside stays dry. It has been suggested that there might be pin-holes that open when the chiller is hot.

In one case I even measured the gravity of the spent chill water and (corrected for temp) it seemed slightly higher than that of tap water. This would support the idea of the missing extract ending up in the spent water.

If that is truly the case, I should also be getting some “hose” flavors into the beer

Once we have snow I’ll brew one batch chilled by submersion in ice water only. That will remove the chiller from the equation.

This is puzzling and annoying me.

Kai

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Old 02-10-2009, 03:07 PM   #18
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I'm resurecting this old thread to give an update as I now think I know what was the problem:

The preboil samples were cooled in an open bowl which allowed for evaporation. Enough evaporation to increase the gravity of the sample by 2-4 points (0.5 – 1 Plato). This lead me to believe that my efficiency was higher than it actually was and hence the discrepancy to the post chill gravity.

I now always chill samples in a 100 ml baby food jar than I close with a lid and place in ice water. As a result my preboil and post chill measurements are now making sense.

Kai

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