Can't get accurate pre-boil gravity readings

Discussion in 'General Brewing Discussions' started by J A, May 23, 2016.

  1. J A

    J A Well-Known Member

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    Okay, since I don't have enough brews down with the same equipment to trust projected efficiencies, I take pre-boil gravity to make sure I've got hop additions and know what to expect in terms of OG into the fermenter.
    I've run across this enough times that it's a pattern and not just a fluke...the reading I get after mashing and gathering wort is high by 5 points or so. If I plug the numbers into a calculator, I'm seeing a much higher OG than predicted. I do the boil, check the gravity, top up to fermenter volume if that's part of the plan and get the OG. That comes out much lower than the pre-boil gravity reading would predict. So my pre-boil gravity reading is consitently higher than it should be according to every other aspect of the process.
    I measure volume before boiling and I make sure to stir the wort a lot as it's heating to make sure it's mixed and I wait until it's nearly boiling to get a gravity sample. I cool it off (ice bath, etc) to close to the hydrometer temp.
    What else can I do to get a good reading? is waiting until it's actually boiling really going to make a difference?
    It seems to be a common issue and when it comes down to it, I'm getting the OG and FG numbers I'm using to build the recipe, so I can ignore it, I suppose, but it it would be nice to be able to figure it out.
    Any thoughts?
    Thanks ;)
     
  2. Ozarks Mountain Brew

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    make sure your putting in the mash thickness % in the recpie, it makes a difference and trust the numbers on the quick water calculator, but in order for it to all read correct you have to correctly put in your losses in the profile settings, it all works together, the wider the pot the more evaporation meaning more losses in the boil and mash and also along with the attenuation of the yeast works to predict the final gravity

    the preboil gravity is purely based on your recipe along with your profile settings and if its new equipment then the issue is most likely your profile losses
     
  3. J A

    J A Well-Known Member

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    Thanks...I was thinking along those lines, and tried changing some things to see what made a difference on the efficiency/ preboil and didn't have much luck.
    Bottom line is that everything proceeds perfectly from a pre-boil gravity and actual volume in kettle of substantially less than I actually got from the hydrometer. My reading seems too high and based on everything else that happens, it is.
    For instance, my volume of 5.5 gal (actual amount of wort collected in the brew kettle) and reading of 1.065 should have given me a gravity of 1.099 in the 3.625 post-boil volume I got. That would have topped up to 5 gallons in the fermenter at 1.072. That's what the recipe calculator said when I changed the efficiency to reflect the actual pre-boil gravity reading and what every boil-off/dilution calculation calls for.

    What I got from the 3.625 post-boil was 1.091 and topped to 5 gallons in the fermenter at 1.066. And when the efficiency rate is changed to give that OG result, it's the efficiency rate I expected in the first place and it shows that I should have been reading a pre-boil gravity of 1.060.
    So, regardless of the settings in the recipe calculator, the real-world, actual volumes and gravities I got after the boil make the original pre-boil 1.065 reading impossible. I don't get it. :?
     
  4. Ozarks Mountain Brew

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    its hard tunning a system doing a partial boil
     
  5. J A

    J A Well-Known Member

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    But even if I'd stopped at 3.625 gallons of 1.091 OG. the pre-boil reading wouldn't have made sense. The system aside, is there something about pre-boil wort/combined runnings that makes it prone to showing odd gravity readings?
    I haven't tried waiting for to get a sample or taking a sample after boil has commenced. Maybe I should try something different.
     
  6. Ozarks Mountain Brew

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    the boil gravity is based purely on the recipe and your settings, it cant know how we brew or mistakes we make but your not the first person to have issues with this so could be someone needs to look into it
     
  7. artbreu

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    If you've got the cash to spend a good refractometer is worth considering.
     
  8. J A

    J A Well-Known Member

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    Thanks for the help with it. Yes, a refractometer might make the difference.
    And, again, totally aside from the recipe calculator and efficiency settings, there's the physical fact of x amount of sugar content being concentrated (and/or diluted) into a different amount of water. The sugar content stays constant so the specific gravity of the new volume can be accurately predicted from the original volume and gravity.
    Every dilution calculator agrees that 5.5 gallons at 1.065 (my original reading) should yield 3.625 gallons at 1.099 and that's not what happened. The 3.625 gallons at 1.091 post boil (that I actually ended up with) yields 5 gallons at 1.066 which exactly agrees with all the dilution calculations. Working backwards, the gravity reading of the gathered wort should have been 1.060.
    The only reason to worry about pre-boil gravity is hop uptake and there's not a huge difference, so it's more of an academic question at this point. I just don't like not being able to measure it accurately or know why the process is working as it is.
    It's still making good beer and coming out at the numbers that match the style I'm doing, so no problem. ;)
     

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