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Thread: OG Discrepancy: Post Boil vs. Post Knockout

  1. #1
    Join Date
    Jan 2017
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    Mountainside, New Jersey, United States
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    OG Discrepancy: Post Boil vs. Post Knockout

    I hope someone can share some insight on this situation and still without resolve, I implore any and all of you to help me:

    SCENARIO 1 (and the focus of my post) -
    Using a refractometer, I cut runoff once the gravity was 1.077.
    After 85 minutes (of a 90 minute boil), I took a pre-sugar gravity and I measured a gravity of 1.080 (w/ same refractometer).
    I took a post-sugar gravity at flameout which measured 1.087-1.088 (w/ same refractometer).
    Knockout into fermenter 66-68F
    Pulled sample from fermenter ~45mins after knockout and completely to my surprise, only measured 1.078-1.079 with a hydrometer.

    SCENARIO 2 (used as a comparison) -
    We recently brewed a much lower gravity beer (target OG of 1.050) and had the following metrics:
    - Pre-boil: 1.048 (w/ refractometer)
    - Pre-sugar (55mins of a 60min boil): 1.050 (w/ refractometer)
    - Post-sugar: 1.054 (w/ refractometer)
    - Post knockout (still waiting ~45mins after knockout to take hydrometer reading): 1.052

    QUESTIONS:
    What is the nature of the discrepancy with the higher gravity wort as compared with the lower gravity wort...and which reading should I trust? For a high gravity beer such as this (I was actually targeting an OG of 1.088 which is what I got post boil but pre-knockout), would the higher gravity wort stratify that much after ~45mins post-knockout?

    NOTES/OBSERVATIONS/COMMENTS/SPECS:
    -10bbl brewhouse and knockout into a 20bbl fermenter...yield of high gravity wort was 9-10bbls into fermenter and yielded about 11bbls of the lower gravity wort (into a separate fermenter...but I assume that is understood).
    -Dry Yeast pitched for both beers...no starter or slurry used (sprinkled on top of the wort through the manway when fermenter was filling)
    -The sightglass on the fermenter had a very noticeable color gradient. The fluid at the top was much, MUCH clearer than the rich, golden color observed at the bottom

    Thank you in advance for any and all feedback.

    Cheers,
    Brent

  2. #2
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    Jameson, SK, Canada
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    I don't know nearly enough to speak about gradients, but I do know that the refractometer is a rough tool, not precision.

    If your running a system that big, why not get a thief wort chiller from Stout tanks?? I got one and I really like it. Pull a sample, champ it in, start the chill and about 70 seconds later you have a sound hydrometer reading at the right temp.

    -J.

  3. #3
    Join Date
    Sep 2012
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    45

    Wort Chiller

    Look at your wort chiller. You could have chill water leaking into the wort stream. This happened at a neighboring brewery to us.

  4. #4
    Join Date
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    Water is getting into your wort during the transfer. The HX is a likely villain, but a leaking water-washed seal on a pump could also be the culprit.

    Make sure the cooling water side of your HX has an unrestricted outlet, so the pressure of the water is lower than that of the wort. ANy leaks will go from the wort to the water, instead of vise-versa.

    Check the seal on your transfer pump and be sure there is no leakage there. Hooking up a small tank of water dyed with food coloring to gravity-feed the seal wash should allow you to see if any water is getting in this way.

    You've got to find and fix this problem, as any cold water getting into your wort is also introducing bad critters you don't want in your ferment!
    Timm Turrentine

    Brewerywright,
    Terminal Gravity Brewing,
    Enterprise. Oregon.

  5. #5
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    Jan 2017
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    Mountainside, New Jersey, United States
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    Quote Originally Posted by MikeyB View Post
    Look at your wort chiller. You could have chill water leaking into the wort stream. This happened at a neighboring brewery to us.
    Quote Originally Posted by TGTimm View Post
    Water is getting into your wort during the transfer. The HX is a likely villain, but a leaking water-washed seal on a pump could also be the culprit.
    Make sure the cooling water side of your HX has an unrestricted outlet, so the pressure of the water is lower than that of the wort. ANy leaks will go from the wort to the water, instead of vise-versa.
    Check the seal on your transfer pump and be sure there is no leakage there. Hooking up a small tank of water dyed with food coloring to gravity-feed the seal wash should allow you to see if any water is getting in this way.
    You've got to find and fix this problem, as any cold water getting into your wort is also introducing bad critters you don't want in your ferment!

    Thank you for this insight. However, if this is in fact the case, why is the lower gravity wort of which we yielded a higher volume post knockout didn't suffer the same (or more) drop in gravity points?
    Said another way, why would this issue only effect higher gravity wort of which we yielded 1-2bbls less (and hence a shorter knockout overall)...shouldn't the lower gravity wort suffer more at the hands of a leaky heat exchanger?

    BTW - This was our 8th brew on the system and
    1) have not had any issues with infection/off flavors with any previous batch
    2) all gravities of previous batches matched pre and post knockout
    3) SCENARIO #2 in my OP is the 9th brew on our system where the numbers worked out on either side of the H.E.

    Any other ideas?

  6. #6
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    Quote Originally Posted by threeviews View Post
    Thank you for this insight. However, if this is in fact the case, why is the lower gravity wort of which we yielded a higher volume post knockout didn't suffer the same (or more) drop in gravity points?
    Said another way, why would this issue only effect higher gravity wort of which we yielded 1-2bbls less (and hence a shorter knockout overall)...shouldn't the lower gravity wort suffer more at the hands of a leaky heat exchanger?

    BTW - This was our 8th brew on the system and
    1) have not had any issues with infection/off flavors with any previous batch
    2) all gravities of previous batches matched pre and post knockout
    3) SCENARIO #2 in my OP is the 9th brew on our system where the numbers worked out on either side of the H.E.

    Any other ideas?
    I would lean toward the use of the refractometer, even if previous batches tracked just fine. In my experience with using refractometers they (even with so called "ATC") can be very finicky if trying to match up to a cold sample taken with a hydrometer. I will use a refractometer to monitor running and kettle full gravity, but will make sure to pull a sample to put in the sample chiller for a hydrometer reading to both confirm the kettle full gravity and post boil gravity. Meaning, I will stop the runnings from the LT based solely on a refractometer, but at kettle full I will both take a refractometer reading and also a hyrometer sample. I want to make sure that by the time I come to a full boil my hydrometer sample is ready and I can then make any adjustments prior to hop additions. I then rely on my historical boil off rate to get me where I need to be post boil. I then take a refractomer reading, but confirm end of boil with a hydrometer prior to finishing hops. I find that my refractometer will half of the time match my hydrometer, and the other half of the time the numbers are drastically different. I have the greatest chance of my two instruments being closer if I let the sample destined for the refractometer cool down first before applying it to the refractometer as opposed to dropping hot wort on the unit. As far as the fermenter sight glass, that is very normal to have a diffence in color if the sight glass after a tank cleaning and was common in all of the breweries I have ever worked in at least..... Anyway, I would say as long as someone didnt forget to empty the residual liquid from the FV prior to knockout, then the difference between the hydrometer and the spotty refractometer is the problem. I would do a few brews using a hydrometer as my main tool. You can always get a small stainless cup like for frothing milk for lattes and then take a hot sample in it. Then put the stainless cup in a ice bath and stir. Your sample temp will drop rapidly so you can take a hydrometer reading....

  7. #7
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    You were measuring the wort consistently up until knockout with a refractometer. Then switched to hydrometer after KO and found a huge difference, it seems to me one of those two instruments is out of calibration. Try measuring your post sugar flameout reading with hydrometer too.

  8. #8
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    Location
    Chicago, IL
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    95
    Since the fear of a leaking heat-exchanger has already been raised, you might want to put this fear to bed through testing. Disconnect all wort lines from the wort side of the heat-x. Run water through your cooling side with the wort inlet and outlet exposed. Initially when you turn on the water, you may see residual water get pushed out of the wort side but this is just the change in pressure on the plates squeezing out the remaining water trapped in the heat-x. Keep running the cooling water for 10 minutes or so and see if you have a constant stream of water coming out of the wort inlet and/or outlet. If you do, it would tell me you have a seal issue. If you don't see any water flow aside from the initial water when you started the flow of cooling water, your seals are fine and it should rule out the heat-x as the source of dilution.

    I hate refractometers... They are a great tool for quick ballpark values but they are not consistent. You can take three samples and have three different results. The next time you brew, I would suggest collecting a large enough sample of your pre-boil, pre-sugar, and post-boil, to be able to measure these samples with a hydrometer once the samples are cooled. Keep the samples covered as they there will be some evaporation while cooling. This will concentrate the sample. Use the refractometer as you have been and see if there are any discrepancies before any additional processes are added to the mix.

    Are you using calibrated hydrometers that have been certified? Homebrew hydrometers are pretty much useless. Do you whirlpool when adding sugar or just toss it in the boil? You may be seeing stratification if the boil isn't vigorous. You may want to whirlpool the kettle when making the sugar addition. Same goes for the post-boil sample. I would grab this sample during your whirlpool as to avoid any chance of stratification.

    Cheers,

    James

  9. #9
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    Where does one get a non-useless hydrometer?? I'm not being a dick, I'm asking....

  10. #10
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    Pasadena MD USA
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    Non-Useless Hydrometer

    Cynmar hydrometers have always been good for me. Try their "Plato" hydrometers which are sold in three ranges: 0-8.5, 7.5-16, and 15.5-24.

    Spreading the range between three hydrometers (which are also physically larger than standard homebrewing hydrometers) spreads out the scale so the reading precision is much greater. Accuracy also seems to be quite good for a reasonable price even if you need to buy all three.

    BSG also sells three very similar (possibly identical) hydrometers which they list as "brix saccharometers"

  11. #11
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    Quote Originally Posted by Jer View Post
    Where does one get a non-useless hydrometer?? I'm not being a dick, I'm asking....
    Foxx Equipment has some nice ones with built in temperature correction thermometer that I really like. About $60 ish for the set of three if I remember correctly.

  12. #12
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    Interesting, thank you. I have been frustrated with the scales on the homebrew ones, I guess the whole "there has to be a better way" just hadn't hit me yet!!!

    -J.

  13. #13
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    Refractometers are effected by suspended solids, so if there is anything floating in your beer, even tiny starch particles, it will be incorrect.

  14. #14
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    Quote Originally Posted by Yellowbeard View Post
    Cynmar hydrometers have always been good for me. Try their "Plato" hydrometers which are sold in three ranges: 0-8.5, 7.5-16, and 15.5-24.

    Spreading the range between three hydrometers (which are also physically larger than standard homebrewing hydrometers) spreads out the scale so the reading precision is much greater. Accuracy also seems to be quite good for a reasonable price even if you need to buy all three.

    BSG also sells three very similar (possibly identical) hydrometers which they list as "brix saccharometers"
    +1 for the Cynmar hydrometers. I'll also throw a shout out to this digital refractometer. Yes, it is pricey, but I have also found it to quite accurate when measured against the Cynmar ones. I use it mostly to monitor runoff and hit my pre-boil gravity. It is way more accurate then any handheld refractometer I have ever used and is specifically in plato, rather than brix.
    Dave Cowie
    Three Forks Bakery & Brewing Company
    Nevada City, CA

  15. #15
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    Apr 2012
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    Quote Originally Posted by Jer View Post
    If your running a system that big, why not get a thief wort chiller from Stout tanks?? I got one and I really like it. Pull a sample, champ it in, start the chill and about 70 seconds later you have a sound hydrometer reading at the right temp.

    -J.
    Neat! And here I am, just using ice from the ice maker like a sucker.
    Russell Everett
    Co-Founder / Head Brewer
    Bainbridge Island Brewing
    Bainbridge Island, WA

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