Need help understanding pre-boil numbers and post-boil

Discussion in 'General Brewing Discussions' started by -MG-, Mar 16, 2013.

  1. -MG-

    -MG- New Member

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    I'm not sure if this is a software issue or not.

    I just took my pre-boil sample and it came out to 1.032, my target is 1.040.

    Taking 32 x 15 (gallons pre boil) = 480

    I boil off around 1.8 gallons an hour (electric setup).

    I estimate that I don't pick up around .75 gallons from the bottom of the kettle. Combine that with hop losses of .18 according to BF, and .25 in misc losses.

    But those are besides the fact. If you take 480 / 40 (my target) = 12. However, 12 gallons is the FINISHED product going into the fermenter. By my math I will have more like 13 gallons left in the kettle at the end of the boil leaving me short on my gravity number ~ 36.9.

    Do I have something calibrated wrong in the software? It seems to me more than a coincidence that my gravity number comes out to exactly 12 gallons and that is what I put in the system as going into the fermenter.
     
  2. The Brew Mentor

    The Brew Mentor Well-Known Member

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    Sounds like your combining 2 different equations.
    Here is what the equation looks like..
    OG x Volume = OG x Volume
    32 x 15 = OG x 13.2
    480 = OG x 13.2
    480/13.2=OG
    36.3636 = OG

    If you boiled down to 12 gallons, then the wort gravity would be 1.040
    You could add a pound of DME to get it close to your target. That would add about 44 total points. Or,3.33 PPG
    Hope this makes sense.
    Brian
     
  3. -MG-

    -MG- New Member

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    We're on the same page.

    But you made my point. Why would the system give me numbers based on boiling down to 12 gallons? (Assuming my efficiency was correct). 12 gallons is what is going in the fermenter and there are dead losses in the kettle.

    In this particular case I achieved higher efficiency than planed and I actually hit my 1.040 humber with 13 gallons in the kettle.
     
  4. LarryBrewer

    LarryBrewer Active Member

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    Please share the recipe and post it so we can take a look. It would depend if you had the batch size target set to fermentor or kettle. I will also take a look at your profile settings and see where the issue might be.
     
  5. -MG-

    -MG- New Member

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  6. LarryBrewer

    LarryBrewer Active Member

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    Took at look at it. Correct - if your pre-boil was 1.032 with 15 gallons, and your post boil volume is 13.2, so your OG will be ~1.036. Side note, here is our dilution and boil off calculator, which I used to arrive at the number above: http://www.brewersfriend.com/dilution-a ... alculator/

    Also correct - the hops absorption, misc losses, and kettle dead space don't change your OG, but they all count against your brew house efficiency. In terms of your profile setup, nothing alarming there, similar to my own settings. I have an electric kettle and my boil off rate is 5.5 qt/hr. Yours is closer to 7.2 qt/hr, which seems a little high but if that's what it is, then that's what it is.

    I think all that needs to happen is to lower the brew house efficiency on the recipe to around 76% and it will line up. Or, switch the batch size target to 'kettle', enter 13.2 as your batch size, and set the efficiency at 84%. When you switch batch size target to kettle, efficiency becomes "kettle efficiency" not brew house efficiency, so it is naturally higher. This FAQ explains it more, and has links to information about efficiency terms:
    http://www.brewersfriend.com/faq/#recipes16

    This chart may also be helpful in understanding efficiency. It explains how "efficiency" is really 4 different things:
    http://www.brewersfriend.com/brewing-efficiency-chart/
     
  7. -MG-

    -MG- New Member

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    Thanks for the explanation Larry.

    That helps clarify, it seems like both methods will get me to the end result. I'll probably switch the to boil kettle method, since this way I don't have to guess what the efficiency should be.

    I'll just set my amount for the BK to include my dead space, etc.
     
  8. LarryBrewer

    LarryBrewer Active Member

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    Sounds good. If you go to tools -> water requirements when your recipe is in kettle mode, it will show you a breakdown of the remaining volume losses and the estimated volume going into the fermentor.
     

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