# (DONE) Brix to FG calculation

Discussion in 'Feature Requests' started by CapnBry, Jan 9, 2013.

1. ### CapnBry New Member

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I use a refractometer to do all of my "specific gravity" fermentation measurements because it takes so little beer to do a measurement. The alcohol in solution makes the standard Brix to SG calcuation not work, as it changes the refraction index of the solution, but it is good enough to tell when the beer has fermented out. However, I'm always curious what the actual specific gravity is while performing these readings.

There are various sites that outline methods of estimating SG, if you know the starting Brix:
http://onebeer.net/refractometer.shtml
http://brew.stderr.net/refractometer.html
http://morebeer.com/view_product/18739/ ... Dual_Scale (Documents -> Refractometer Alcohol Correction Spreadsheet)

It would be pretty sweet it Brewer's Friend also had such a calculator... available from the Android app as well.

2. ### Kaiser Member

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#2
3. ### CapnBry New Member

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Re: Brix to FG calculation

I would say that mixed results would be a generous way of putting it. I generally just use the refractometer because pulling a 200mL sample for the hydrometer every day just wastes so much beer. The refractometer may not provide an exact correlation to a SG but it still goes down during fermentation enough to say that fermentation is complete when you get 3 days of readings where the measured Brix is the same. When I rack from the primary I then take an actual hydrometer reading. I use the Brix -> SG calculation along the way mainly as a curiosity.

4. ### Kaiser Member

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Re: Brix to FG calculation

If there is an inherent inaccuracy in Brix to sg conversion for beer I'd like to understand why that is. From the looks of it there should not be that much error but I haven't had the time and motivation to look into the existing data more closely.

Kai

5. ### perogi New Member

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#5
Re: Brix to FG calculation

Adding this to the application would make me uninstall BeerSmith once and for all.

I used Sean Terrill's calculator and compared it with my previous brews and it was extremely close.

perogi.

6. ### LarryBrewer Active Member

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#6
Re: Brix to FG calculation

So the idea is to record brew log events in Brix, and have it automatically account for any alcohol (since it knows the OG as well). That shouldn't be too hard.

Then we could update the existing stand alone ABV calculator and update the Android app too.

Here is the equation from the MoreBeer spreadsheet:

Brix (Plato) = -676.67 + 1286.4*SG - 800.47*(SG^2) + 190.74*(SG^3)
SG = 1.001843 - 0.002318474(OB) - 0.000007775(OB^2) - 0.000000034(OB^3) + 0.00574(AB) + 0.00003344(AB^2) + 0.000000086(AB^3)
where SG = Specific Gravity, OB = Original Brix, AB = Apparent Brix (Brix Readings During Fermentation)

7. ### perogi New Member

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#7
Re: Brix to FG calculation

LarryBrewer,

Thank you very much for replying to my post.

A few questions and some feedback.

Are you going to assume that Brix and Plato are identical? From BeerSmith, it appears that the relationship between them is about 1.024682293 (Brix/Plato ).

What I've needed to do in BS is this:
1) Use refractometer to get Brix.
2) "Manually" translate to Plato (enter values in a tool).
3) For the FG, the interface uses the OG in Plato and the new value in Brix to determine FG.

In summary - it's a PITA.

Maybe have an additional value for Brix in the dropdown so user can enter the value? Having it automatically translate into Plato (and maybe SG) would be really helpful. Please note that I'm in the middle of fermenting my first batch using BF and entered my first sample today so I'm not sure what happens later in the process.

Thanks!!
perogi.

8. ### LarryBrewer Active Member

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#8
Re: Brix to FG calculation

We will use the most accurate equation for Brix -> Plato, Kaiser will have that.

Yes, adding Brix would make sense, perhaps labeled as 'Refractometer Brix'. When entering anything other than OG, the system will factor in the previous OG reading for purposes of recording the actual gravity (since the alcohol throws it off).

The brew log screen works the same for all log entries, and the ABV calculation doesn't understand refractometers at this point - so you will need to adjust it outside BF for awhile longer.

Don't worry, it won't be a PITA after we're through with it!

9. ### perogi New Member

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Re: Brix to FG calculation

Thanks!!!! I REALLY appreciate it!

p.s. I'm not getting notifications for replies to forum posts. I did get a reply to an earlier email. I checked my spam mail but no go.

10. ### Kaiser Member

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Re: Brix to FG calculation

It seems that different brewers have different factors for converting Brix readings to Plato. For me Plato = Brix * 1.04. That doesn't agree with the one from BS, which seems to be Plato = Brix * 0.976. Maybe that's why the post ferment refractometer readings never worked for me.

To fix this there needs to be a Brix/Plato factor in the equipment setting and that factor needs to be accounted for in the formula.

I also second the idea that one should be able to enter refracto Brix in the log entries. I think I mentioned that you Larry.

Kai

11. ### perogi New Member

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#11
Re: Brix to FG calculation

Hey Kai - curious about the BS conversion for Brix to Plato. I did a few test Brix values and the ratio was 1.024682293 (see my earlier post). Do you recall if you calibrated BS to your Refractometer?

Thanks!
perogi.

12. ### LarryBrewer Active Member

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#12
Re: Brix to FG calculation

Not sure where the equations regarding converting Brix to Plato are coming from? According to the Wikipedia, Brix and Plato are nearly identical and can be used interchangeably:

http://en.wikipedia.org/wiki/Brix
Treating Plato and Brix as makes the most sense to me at this point. Besides, the system would round out any differences because we are not going out to the 3rd decimal place.

Here is the equation we are planning to use:
http://seanterrill.com/2011/04/07/refra ... g-results/
FG = 1.0000 – 0.00085683*(Brix RI, OG) + 0.0034941*(Brix RI, FG)
Where Brix RI is the adjusted Brix Refraction Index (based on a wort density factor).

13. ### Kaiser Member

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#13
Re: Brix to FG calculation

Brix and Plato are the same when you deal with a sucrose solution. But wort and a sucrose solution, that have the density refract light slightly different. That's why there is this ominous wort correction factor. Note that in brewing we use the density of the wort as a measure of its extract content.

When brewers talk about Brix, they generally refer to the Brix reading of a refractometer. Since the refractometer was not calibrated for wort the Brix reading needs to be corrected before it can be used.

What I'm looking for is the ability to select "Brix" or, to remove the confusion, "Brix reading" as a unit and under the hood a correction to actual Plato and finally sg is done if the latter is used as the unit for the actual data record.

Kai

14. ### LarryBrewer Active Member

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#14
Re: Brix to FG calculation

That makes sense.

In the context of a refractometer reading, we need two corrections:
1) Correction for wort density, aka Brix Refraction Index (Brix RI), which needs to be a profile option. This applies to ALL refractometer readings, and refractometer readings can only be entered in Brix RI. Don't trust the SG scale on you refractometer if it has one.
2) Correction for any alcohol present, which requires the OG (Sean Terrill's equation appears to be the most accurate: http://seanterrill.com/2011/04/07/refra ... g-results/)

We need to make sure the documentation is clear that there is some inherent inaccuracy in using refractometers because of the wort density factor - a) it is actually non-linear, and b) varies somewhat wort to wort.

http://braukaiser.com/blog/blog/2012/03 ... r-blindly/

Here's an old thread in HBT, but the first one that comes up in google, about a guy who has some serious frustration with using his refractometer, and didn't get much of an answer.
http://www.homebrewtalk.com/f84/beersmi ... tor-87922/
I want to make sure we nail this, and make it as easy as possible for users.

15. ### bilhelm96 Member

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#15
Re: Brix to FG calculation

I agree, I have a dual scale refractometer and the SG scale is almost useless. My refractometer has Auto Temp Correction (ATC) and I managed to calibrate it using the SG scale based on my hydrometer reading after taking the OG on one brew. Since then I always take OG and FG with both the hydrometer and refractometer. The Onebeer.net calculator has been giving me good results matching the FG refractometer Brix reading compared to my hydrometer reading. I like the refractometer for taking intermediate samples because I don't want to waste beer on samples.

16. ### LarryBrewer Active Member

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#16
Re: (NEXT RELEASE) Brix to FG calculation

The next release will support refractometers on the brew log entry page!

17. ### LarryBrewer Active Member

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18. ### perogi New Member

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Awesome!!!!