Author Topic: Gravity calculations in BS3  (Read 692 times)

Offline thebeershack

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Gravity calculations in BS3
« on: June 03, 2022, 09:02:00 AM »
I'm an experienced brewer, but a BS3 noob, having a hard time understanding BS3's gravity calculation, as it seems to consistently estimate a higher post-boil gravity than the standard calculation would predict. I do have my equipment profile properly set, including the boil-off rate. Let's take a look at today's batch: BS3 predicts 1.059 post-mash gravity with 9.6 gallons going into the kettle. My boil-off rate is 1.5 gallons/hour, and today is a 1 hr boil.
so: 59x9.6=566.4 points going into the kettle. A post-boil volume is 8.1 gallons, so the estimated post-boil gravity: 566.4/8.1=70, or 1.070 OG. Yet BS3 is giving me three more points with an estimated post-boil gravity of 1.073. This gravity would be produced by a 7.75gallon final volume, or a 1.85 gallon boil-off rate. I do have the "Use boil-off as hourly rate" box checked. What else is BS3 factoring in to produce this discrepancy? I want to account for this (small) difference during recipe creation.

Offline BOB357

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Re: Gravity calculations in BS3
« Reply #1 on: June 03, 2022, 12:47:40 PM »
You're comparing apples to oranges. You need to compare accurate pre and post boil gravity and volume measurements. Using a prediction for pre boil and actual OG post boil doesn't work, especially if you're not accurately measuring volume too. Once you've used accurate measurements for a few batches, adjust equipment profile values as needed and then adjust brewhouse efficiency.
Bob

Offline Kevin58

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Re: Gravity calculations in BS3
« Reply #2 on: June 03, 2022, 03:18:34 PM »
Quote
Once you've used accurate measurements for a few batches, adjust equipment profile values as needed and then adjust brewhouse efficiency.

This is important right here. You may have done the proper volume and weight measurements to set up an equipment profile but it is not complete until you make adjustments along the way. Take accurate measurements of every brew day and use these to tweak your initial profile until you have it dialed in.
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Online GigaFemto

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Re: Gravity calculations in BS3
« Reply #3 on: June 03, 2022, 05:47:41 PM »
The temperature may be the difference. The pre-boil gravity is at boiling temperature, while the post-boil gravity and OG are at room temperature. There is a 4% difference based on the thermal expansion/shrinkage. If you take 1.070 and multiply by 1.04 you get 1.073, exactly the number you are looking for.

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Offline thebeershack

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Re: Gravity calculations in BS3
« Reply #4 on: June 04, 2022, 09:13:51 AM »
Thanks for the replies. I'll think on this. But I'm not sure any of this answered my question.

Offline BeerSmith

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Re: Gravity calculations in BS3
« Reply #5 on: June 06, 2022, 09:24:30 PM »
Hi,
  The OG is based primarily on the brewhouse efficiency, as well as the volumes.  Changing boil volumes will change the volumes but not the brewhouse efficiency which is a gross number that reflects all losses in the system.  So that may explain the slightly higher OG.

Brad
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Offline vector33beer

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Re: Gravity calculations in BS3
« Reply #6 on: July 23, 2022, 07:06:06 AM »
I have a gravity question not directly related to the original post. The Estimated Post Boil gravity is 1.053. The way I have the program set up, it understands I need to add water to bring up volume to what I need in the fermenter. My question is at a higher (almost double) volume the program is giving me the same final target (1.053) but shouldn't it be lower (or the post boil higher)?  I assume my target OG is at full volume and I was adjusting during the boil to try to hit the GP I needed for final OG (I missed my pre-boil target so added DME).  I'm new to brewing and I'm trying to be sure I understand the math behind the program.

Thank you!

Offline Oginme

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Re: Gravity calculations in BS3
« Reply #7 on: July 23, 2022, 09:23:40 AM »
Reread the post by Brad above yours.  BeeSmith calculates the final gravity by using the volume into the fermenter and the brew house efficiency.  It does not matter where the water volume comes in to the program, bit back calculates the amount of sugar which needs to be extracted to satisfy the initial equation of:  [Potential in Grain] x [BHE] / [volume in fermenter].

If you shift the volumes around to move water out of the mash or sparge, you will need to adjust the brewhouse efficiency to account for the is change in extraction due to the lower water in the mash and sparge.

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