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  • low post boil gravity

    So after my first two brews on a new 20bbl system I am perplexed.
    Batch 1 - 23 bbls pre boil at 14.5P
    21.75 bbls post boil at 14.5P

    Batch 2 - 24bbls pre boil at 11.9P
    23 bbls post boil at 11.5P

    How is that even possible? Using brand new temperature adjusted hydrometers and refractometer. Getting a good rolling boil at 212.5F, condensate dripping out of the kettle stack condensate line, and seeing good amounts of steam outside coming out of the kettle stack.

    Any thoughts would be greatly appreciated.

  • #2
    Probably due to layering, particularly when taking the first sample. My guess is the sample point is in the half of the vessel that contains the strongest wort (typically the bottom half if running wort in through / into the top of the vessel), and that until you have had a rolling boil for several minutes, length of time depending on exact design of heating system and the kettle dimensions itself, you will not get thorough mixing. It is possible that you only get through mixing towards the end of boil. I some extreme cases, it has been known for portions of wort not to be boiled at all. Try checking the sample at say 5 minute intervals to see what the gravities are like. If you have a combined kettle whirlpool, run the pump during wort runoff / heating up and see if that makes any difference.
    dick

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    • #3
      Piggy backing off of Dick's response, the point at which you take your pre-boil sample can effect the measurement. In order to help ensure that I get a "clean" sample, I wait to take my pre-boil at around 207 degrees. Typically at this temperature, the kettle is starting to roll and I can be more assured that I am not taking a stratified sample. Also, how long are you boiling? Have you calculated your evaporation rate? When our new system was installed, the contractors undersized our steam traps and we had condensate in our kettle's jackets. This severely impacted our boil-off. Are your post-boil volumes consistent or do you see a correlation between the post-boil gravity and volume?

      If you are working with steam, I would look at the traps and whether or not the steam returns are of sufficient size and pitch back to the boiler/condensate return. Your kettle stack vent can have a dramatic effect on boil-off. If your evaporation rates are poor but your heat source is sufficient, I would look at installing a stack fan.

      Sorry to introduce a few more variables, but pre-boil samples can be tricky if not taken consistently coupled with a consistent boil.

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      • #4
        I taked my preboil gravity at 5 min into the boil to ensure even circulation.

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        • #5
          Probably due to layering, particularly when taking the first sample.
          Ditto. We wait ~5 min into the boil to take a sample as well.

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