Pre boil Gravity Calculation error

Discussion in 'Recipe Editor' started by bazzamckazza, Aug 5, 2016.

  1. bazzamckazza

    bazzamckazza New Member

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    there seem to be an error in the way BF calculates Pre boil gravity

    if I have a 21ltr batch size (fermentor target) and a 5 ltr boil off with 2 ltr kettle dead space for a final gravity of 1.045 I'll have a pre boil volume of 28ltr with a gravity of 1.037 but BF shows a pre boil of 1.034.

    it seems that BF assumes your fermentor volume is the same as your post boil kettle volume and doesn't factor in kettle dead space. I.e it assumes you will boil down to 21ltrs not 23ltr.

    this has caused reasonable confusion on a number of brews for me, I've made pre boil adjustments thinking I was going to over shoot my final gravity. since kettle losses are entered why can't it factor this in when calculating pre boil gravity
     
  2. Ozarks Mountain Brew

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    Ive had many discussions about this very issue and all I can say is its usually in your profile settings not set up correctly or a water volume issue, Ive tested and tested this so called issue but I hit my preboil gravity on the numbers every time I brew
     
  3. bazzamckazza

    bazzamckazza New Member

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    not sure what i've set up in correctly in my profile. I set up a test profile as stated above and it show a pre boil of 1.034 and FG of 1.045 if you use the separate boil off calculator under tools enter 28 ltr and 1.034 and target volume of 23ltr (post boil volume) you'll get post boil gravity of 1.041 not 1.045 the recipe calculates. clearly one of these two is wrong. if you tip all of your kettle into your fementor and have zero kettle losses then your pre boil gravity will be correct.

    please test both full recipe calculator and boil off calculator with the settings i used and let me know if you get a difference
     
  4. Ozarks Mountain Brew

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    one thing to note the individual calculators on the tools tab are not tied into your profile and are generic settings, use the recipe for readings, make sure you set up your recipe to be tailored to a profile and pick that profile in the recipe under the more tab
     
  5. J A

    J A Well-Known Member

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    I've tried everything to figure out the discrepancy between what I measure and what's predicted. I've calculated the same recipe with very different equipment profiles and nothing changes.
    I've taken to getting a pre-boil kettle volume before I start heating and getting a gravity sample just before boil starts and after stirring a lot during heating. I end up coming a little closer to predicted numbers, but when I adjust the efficiency to account for the the actual pre-boil reading, it gives me an OG that's higher than what I actually get as measured with a post-boil gravity sample. I've managed to get it pretty close, but never quite there.
    I end up just tweaking volumes to reflect the measurements I made and don't really worry about anything but fermenter volume and actual gravity reading for OG. I'm still never exactly sure what's happening in the mash and boil, but with accurate OG, fermenter volume and FG, I know about what the ABV is.
     
  6. Ozarks Mountain Brew

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    I adjust my boil amount to the kettle, but you hit another possible issue is the efficiency setting, if your not hitting your numbers and you know your equipment settings are correct, "which took me many batches to get perfect" than that could be an issue. people get caught up in these numbers and formulas and trust me people don't brew just as the numbers say most of the time, you have to account for all kinds of losses in the real world, the numbers cant account for everything and not everyone is perfect, point is more than not its not the numbers its human error :D
     
  7. bazzamckazza

    bazzamckazza New Member

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    sorry but i definitely think this is a calculator error. Maybe another way to show it more clearly is to use one profile with 5ltr boil off and 2 ltrs of kettle losses. then compare this to a second profile with 7 ltr of boil odd and zero kettle losses. if you use any recipe set to fermentor target you will note changing between profiles the final gravity and per boil remain unchanged. obviously with different boil off rates isn't possible to achieve the same final gravity without changes in per boil gravity. it looks to me as if BF cant' make adjustments for kettle losses when calculating pre boil gravity, I'm not sure how this can be user error.

    I have my profiles pretty well dialed in with consistent volumes and efficiency. I'm guessing if your pre and post boil measurements your are either set target volume to kettle or your kettle losses are zero.
    please rather than just dismiss this as user error test what i have suggested and tell me what results you get
     
  8. J A

    J A Well-Known Member

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    Completely disregarding the recipe calculator I continue to get SG/volume measurements that just don't make sense. I brewed yesterday and my pre-boil was 6.75 at 1.05. Fermenter was 5.5 at 1.054. On any dilution calcuator I can find, that doesn't add up. I've tried to figure out where the discrepancy is, but I just can't.
    Even accounting for thermal expansion, I can't figure out why the pre-boil seems higher than it should be. I trust the post-boil numbers more because everything is cooked and cooled.
     
  9. Ozarks Mountain Brew

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    Ill move this to the technical forum, might help you out
     
  10. Meddo

    Meddo New Member

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    #10 Meddo, Nov 29, 2016
    Last edited: Nov 29, 2016
    So this still hasn't been fixed? This is a simple bug that results in a big error in predicted OG and IBUs.

    I just spent an afternoon confused as hell AGAIN as to why I hit the pre-boil volume and SG and the post-boil volume spot on, but post-boil SG was 8 points lower than predicted.

    I have done tests changing efficiencies, recipes and equipment profiles and can confirm that, as mentioned above, the error is due to the calculator treating the kettle deadspace as additional boil-off volume. The calculator assumes that the wort sugars and AAUs are concentrated rather than lost in the step from post-boil kettle volume to fermenter volume.

    This is a simple bug and should be fixed immediately - not allowed to sit idle for the four months that this thread has been open.
     
  11. Yooper

    Yooper Administrator
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    We are working on it! It's not really a "simple bug" unfortunately.
     
  12. OkanaganMike

    OkanaganMike Active Member

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    Not trying to derail this thread but where do you folks find your preboil gravities and amounts? I've never seen these and can't find them now I'm looking.
     
  13. Meddo

    Meddo New Member

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    #13 Meddo, Nov 29, 2016
    Last edited: Nov 29, 2016
    OK I'll have to take your word for it that it's not simple, despite appearances. The calculation and the error certainly seem simple, even if the software implementation may not be.

    If that's the case however there needs to be some sort of announcement or warning notifying users that the recipe planner is giving incorrect predictions, and warning us to use the kettle target only during recipe design. Especially since, from memory, BF defaults to the fermenter target which is giving the wrong predictions. Alternatively, this option should be disabled until fixed - better to have no answer than an incorrect answer.

    I understand that mistakes and bugs happen and was quite happy to just shoot an email when I worked out what was going on a couple of days ago. My disappointment is really now finding this thread and realising that BF has been aware of the problem for at least four months but still allowed us to use the recipe designer without warning that a key calculation is giving incorrect results.
     
  14. Yooper

    Yooper Administrator
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    We are working on it right now, but the gist of it, as described by our guys, " is that one fundamental decision the original author made is that Recipes are completely independent from Equipment. So the calculations on the recipe page are simply based on what you put in for your target batch size, boil size, ingredients, etc. Anything to do with your kettle characteristics aren’t taken into account.

    It’s only once you get to a Brew Session that those things are used. I think the idea is that a recipe should be agnostic of the brewer/equipment used to replicated it – i.e. it represents a particular beer/finished-product, not necessarily the specific intricacies used to get there. It’s honestly a bit of a weird distinction, but it enables the ability for users to clone a brew and not get completely different numbers on the recipe since they have a different equipment profile. If they want to replicate it, they can create a brew session and start to calculate what it’ll take for their equipment to produce it. Changing things to allow the recipe builder to pull in Equipment Profile data/options would be a big undertaking." (But- we are willing to do it after seeing if that is for the best, of course.)

    So for better or worse, the net result is that the recipe builder doesn’t have the capacity to assume any of the gravity of the brew is reduced due to wort being removed (through dead space, etc.). One way around this might be to use the Target: Kettle option, and put in the desired pre-drain kettle volume (so fermentor volume + dead space). That should give the correct values for pre-boil gravity in theory.

    So, we are looking at more than one ramification of the change, once we really know the effects. We are working on this, and it's important to us!
     
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  15. Yooper

    Yooper Administrator
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    I will be testing this tomorrow, so feel free to comment as you have the time! I appreciate your willingness to provide suggestions and help.
     
  16. Meddo

    Meddo New Member

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    Hi, thanks for the response and explanation Yooper, and sorry for my late reply.

    I have to say that personally I was unaware of the distinction between the parameters used in the recipe builder and the brew session, and to me it still doesn't seem obvious from looking at the software although there are a few hints if you go looking for them. To my mind there doesn't seem to be any gain to this since there's nowhere in the brew session to actually adjust the recipe based on equipment parameters from what I can see? For instance, I designed a recipe at 74% brewhouse efficiency. In actual fact, by back-calculating through my equipment profile after the brew, the best I can achieve with that grain bill is 70% (at 100% conversion efficiency) due to high loss volumes. This isn't highlighted at any point of the process, and even if I had realised this prior to brewing there is nowhere in the brew session to work out the actual grain bill required to hit the recipe OG.

    There also certainly appears to be a fault in relation to the calculated efficiencies in the brew session based on volumes. I've dummied up a few variations of recorded volumes and gravities in a session to try to identify where the fault is (please ignore the comments fields).

    The first dummy session here (Figure 1) was made to try to maintain the same efficiency and gravity through the pre-boil, ending kettle, and brewhouse steps. As can be seen, to achieve the same pre- and post-boil efficiency, I had to increase the water volume (by the same volume as the kettle deadspace in the equipment profile) at the same gravity to maintain the efficiency. Clearly this is a fault, as per the BF FAQ definition of ending kettle efficiency:

    Ending Kettle Efficiency: The percentage of total sugars in the kettle after the boil. Theoretically the same as Pre-Boil Efficiency, provided accurate measurements at both points (minus rounding error).

    Since the quantity of sugars is not changing through the boil in this ideal/theoretical scenario the SG is unchanged then the same volume of wort (sugars) should be required to deliver the same efficiency.

    upload_2016-12-2_7-30-55.png
    Figure 1. Ideal scenario of maintained efficiency and SG with required volumes.

    This is perhaps highlighted even more clearly in Figure 2 where I used the BF boil-off and dilution calculator to calculate what the actual ending kettle SG should be for the boil-off rate used in the applicable equipment profile. In this case, despite having the same quantity of sugars pre-boil and ending kettle, the efficiency has dropped by 10% when it should be identical.

    upload_2016-12-2_7-49-53.png
    Figure 2. Boil complete volume from equipment settings, boil complete SG from BF boil-off and dilution calculator using pre-boil inputs

    The problem here is that I cannot even use a dummy brew session in BF to work out what grain quantity will be necessary to hit the required OG and fermenter volumes of the recipe, nor can I use it to work out the correct efficiency to plug back into the recipe builder in the first place. Obviously I can do that through a spreadsheet tool or some other software, but that really shouldn't be necessary when all the tools appear to be in place in BF - just working incorrectly from what I can tell.

    Certainly it appears that in this scenario the fault is due to mis-treatment of the kettle deadspace when calculating volumes, efficiencies and gravities.

    ---

    As an aside, my preference for the brew session and recipe builder would be to expose the conversion efficiency as a parameter that can be defined/adjusted. Based on the recipe quantities and the existing equipment profile parameters all the subsequent efficiencies can be derived from the conversion efficiency (if the calculations did not have errors) - in this manner there will be no chance of specifying an ending kettle or brewhouse efficiency that is higher than what can actually be achieved for that recipe on that equipment, assuming the conversion efficiency entered is sensible.

    ---

    Apologies for the essay here and any frustration in my earlier posts. It's just disappointing to invest time and money (consumables) brewing a beer that was never going to be able to meet the recipe specifications due to software problems. A more experienced brewer may have been able to pick up on that prior to brewing, however I have been relying on the calculations on BF and assuming them to be reliable.
     
  17. Trialben

    Trialben Well-Known Member

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    That first brew session looks out of wack you can't get the same numbers for pre boil grav and brew house as per the volume you made that one up? Second brew looks more realistic.,is that 23lt into fermentor how much did you leave in the kettle? 10points off a 60 min boil is good in my book your boil off is healthy. I'm thinking because of your dead space this is your hit to efficiency. Try the somehow reduce dead space. Below is a snapshot of my current brew maybe that might help. I seem to have like 1.6 lt of dead space. Good luck mate :) image.png
     
  18. Meddo

    Meddo New Member

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    Hey Ben, both of those sets of numbers are made up numbers to show how the efficiency calculations do not match the input data. I'm not trying to improve my brewhouse efficiency here (I mean I am, but that's external to this discussion, and you're right that my deadspace is the main culprit...), just highlight the point that the calculations in the recipe builder and brew session are not working correctly.

    If you punch a grain bill into the recipe builder then to the brew session, and enter a logged volume and SG, then compare that to the stand-alone BF All-grain OG/FG calculator with the same grain bill you will get different answers (at least I'm getting different answers, I don't know if everyone else is experiencing the same error). 100% conversion efficiency for the same water volume and grain bill results in different SGs, or represented another way then the same volume, SG, and grain bills will give different conversion efficiencies.

    I've done the basic ppg calculations in a spreadsheet and they align completely to the answers given by the standalone BF OG/FG calculator, but the recipe builder / brew session differs significantly with the same inputs.

    The second screenshot I posted shows that the brew session efficiency calculations are not internally consistent either - converting from a pre-boil volume and SG to end-kettle volume and SG as determined by the standard BF dilution and boil-off calculator results in the reported end-kettle efficiency dropping by 10% compared to pre-boil, when it should actually be identical.
     
  19. Trialben

    Trialben Well-Known Member

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    It's above and beyond me meddo :rolleyes: I youse the BF water requirements as a guideline but still just brew by what my system requirements are given my boil time grist amount ect. I know I lose 4-5 liters from a 60 min boil and around 1-2 liters in dead space depending if I just don't dump her all into fermentor and let the yeast sort it out:p. In the end I loose on average and inch and a half in fermentor to crud so all in all if it turns out 22lt or 20 I'm happy. Near enough is good enough for me I'm just happy making fairly drinkable beer on a regular basis I'm not even sure if I've really even rid myself of me training wheels so to speak :). But I hope you find your solution to this brewing conundrum.
     
  20. Brewer #56740

    Brewer #56740 New Member

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    Holy crap I'm so glad I found this thread. I've been killing myself the last few months to figure out why any of my non-kit brews were missing OGs- it's the 1gal of kettle dead space that BF is treating as boil off. I think I knew intuitively because my grain bill wasn't increasing whenever i increased the kettle dead space, just the water.

    Yeah I guess that original assumption (that the recipe is totally separate from the equipment) probably wasn't a good one.
     

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