Why do post boil gravity and OG values not match?

Discussion in 'Recipe Editor' started by Brewer #53034, Oct 8, 2019.

  1. Brewer #53034

    Brewer #53034 New Member

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    I'm confused why these aren't the same values.

    upload_2019-10-8_18-29-44.png
     
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  2. Mark Farrall

    Mark Farrall Well-Known Member

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    Is your target set to fermenter or kettle? If it's fermenter and you've got losses set up in your equipment profile you may see a difference. It's a problem that's had attempts to fix, but it's connected to so many things that it's a much harder problem to fix than expected.

    Personally I just use kettle as my target and add the losses to the batch size rather than the equipment profile. Not a big deal to me anymore. Brewing a bunch of different styles I have losses that are related to the recipe as well as the system, so putting the losses into the batch size is at least consistent.
     
  3. Brewer #53034

    Brewer #53034 New Member

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    The recipe is here: https://www.brewersfriend.com/homebrew/recipe/view/892817/all-pils-test-batch

    It's set to the fermenter.

    Maybe it's not important but I don't understand that logic. A loss in volume from the kettle to fermenter would not raise the OG it just changes the volume. I think it's a fair assumption that the sugar is evenly distributed throughout the volume of the wort so a loss in volume from the kettle to fermenter would just be that; a loss in volume. Instead of having 1.17 gallons of 1.045 wort (as there is in the kettle) I would have 1 gallon of 1.045 wort (due to say .17 gallons left in the kettle or transfer device), not suddenly 1 gallon of 1.053 wort.
     
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  4. Brewer #53034

    Brewer #53034 New Member

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    This bug would also explain why it seems like I'm always off target on my OG as I've always compared to the "OG" value and not the "Post boil gravity".
     
  5. Brewer #53034

    Brewer #53034 New Member

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    I'm sorry I did not see the existing threads on this bug already. My apologies.
     
  6. Pricelessbrewing

    Pricelessbrewing QA Software Tester
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    #6 Pricelessbrewing, Oct 9, 2019
    Last edited: Oct 9, 2019
    The og formula is correct.

    Post boil gravity should match OG, always. Unless sugars are added after transferring wort to fermenter or an infusion of water is added after the boil (water top off).

    I would just ignore the postboil OG for now.
     
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  7. Brewer #53034

    Brewer #53034 New Member

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    OK, should I assume "pre boil gravity" is wrong then too?
     
  8. Pricelessbrewing

    Pricelessbrewing QA Software Tester
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    Preboil seems accurate.

    Formula for preboil, in the absence of sugar additions or water additions during the boil.

    Preboil grav = ((( og - 1) 1000 * ) * batch size ) / preboil

    Ie ((( 1.053 - 1) 1000 * ) * 1 ) / 1.73

    = 53 * 1 / 1.73 = 30.64 ~= 31 => 1.031 preboil gravity
     
  9. Brewer #53034

    Brewer #53034 New Member

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    Cool thank you for verifying that for me.
     
  10. Pricelessbrewing

    Pricelessbrewing QA Software Tester
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    Hey, just thought I'd follow up with you again. I've reviewed the pre/post boil gravities again and it does appear there's an error not accounting for the kettle losses in both formulas.

    In your example they weren't large enough to contribute significantly enough for me to notice, but I've commented in the thread here outlining the issue. https://www.brewersfriend.com/forum/threads/suggested-fixes-and-features.12138/page-3
     

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