FG from Refractometer

[sbsmann]

I broke my old trusty hydrometer recently so decided to buy a refractometer instead. I spent an hour today researching how to get an FG reading and I'm still not sure I understand if I am doing it correctly.

My post boil OG (from the refractometer) was 1.054 on brew day. I did not get the Brix reading, but kind of wish I did. My brew has been in fermentation for 4.5 weeks and the gravity readings for 3 days in a row has been 1.030 (or 7.5 Brix)

Certainly my FG is not really 1.030? I saw a calculator in my Beersmith app to convert FG from a refractometer and it gave me a converted FG of 1.015 (see attachment). Is that what I should be doing to get my FG? A 1.015 FG is actually about where I should be, so I am hoping this conversion tool is correct?

View attachment 1508103247856.jpg

View attachment 1508103267420.jpg

 

[Ludesbrews]

Not that I know much at this point (I am new!), but when I was gearing up for my first brew, the owner of my local Homebrew shop advised against a refractometer as a direct replacement for a hydrometer. He said that it usually takes accurate readings for the OG, but is often inaccurate when calculating the FG. Hope this helps, I’m interested to see what others have to say. Cheers.

 

[Smellyglove]

That should show you the ballpark-reading yes. If you click calibration settings I bet there's a box you can enter Brix WRI. Google Brix WRI to get an understanding of how these calculators work.

And the advice Ludesbrews got is correct. But if the hydrometer is broken it's broken. You can at least see if the FG is stable, which would keep you safe from bottle-bombs.

 

[Silver_Is_Money]

1.015 is likely pretty close to correct. Refractometers measure sugar in water, not in water plus alcohol, so since you have alcohol present you must use a program that corrects for this. My favorite calculator for this is at this link:

http://seanterrill.com/2012/01/06/refractometer-calculator/

 

[jpakstis]

I found both the Brewsmith and the Sean Terrill calculators great for getting that ballpark (probably within 1-2 points if gravity) FG reading, which for me is fine. That’s about 0.1% change in ABV.

 

[cbier60]

I've been using the Sean Terrill calculator for years, and it's spot on, but it is dependent on your OG reading. I expect that Beersmith has just used Terrill's calculations, although I don't get the exact same answers with the same inputs. One nice thing it will do is give you your OG from your final refractometer and hydrometer readings (obviously assuming both are 100% accurate). Unless you have high end digital measurement equipment, most readings are accurate within 1 or 2 gravity points; I feel like I can read my analog refractometer with higher resoluteness than a standard hydrometer. My conclusion is that the corrected values using the spreadsheet are at least as accurate as my ability to read a hydrometer.

You can at least see if the FG is stable, which would keep you safe from bottle-bombs.

Even if you're not comfortable trusting the reading for final gravity, it definitely lets you see that gravity is stable. Take both readings for a while if you need to convince yourself. Listen to this episode from Come and Brew It podcast. Another non-believer convinced that a refractometer reading is accurate if correctly adjusted.

 

[dmtaylor]

Every refractometer has its own correction factor. Terrill reports the average as 1.04, but for my own refractometer it is 0.98.

You really do need to know both the beginning and final Brix along with hydrometer readings for the first few batches if you want to learn the correction factor for your own refractometer. After that you can ditch the hydrometer with the understanding that it might be off by a couple gravity points, but pretty close anyway.

For your batch, if it were my refractometer, the original Brix would have been 13.1, and with final Brix of 7.5 would give a final gravity of 1.015. Like I said, this is using a correction factor of 0.98 for my own gauge. Others may differ as to whether the original estimated Brix was 13.1, 13.5, or whatever. This will affect the alcohol calculations. I would estimate alcohol at about 5.2% ABV. And...

Then there are multiple calculators for ABV. My own calculators don't agree with Terrill's, so I ditch his and calculate on my own.

Enjoy. For those who love mathematics like I do, performing these calculations is a "fun" chore on every batch.

 

[RM-MN]

I broke my old trusty hydrometer recently so decided to buy a refractometer instead. I spent an hour today researching how to get an FG reading and I'm still not sure I understand if I am doing it correctly.

You misspelled too.

While I love my refractometer for grabbing a sample during the mash or for pre boil/post boil I rely on the hydrometer for FG as it is the accurate tool for that job. If your intent is to just verify that your fermentation is done the refractometer will do a fine job of telling you that the gravity is no longer changing but even with the calculations there is plenty of room for inaccuracy.

In other news, I can often use my pliers to remove a screw but the screwdriver is the correct tool for the job.

 

[myndflyte]

Zymurgy published an article about this and this is the equation they came up with:

FG = -0.002349Bxi + 0.006276Bxf + 1, where Bxi is your starting Brix and Bxf is your final Brix.

 

[Silver_Is_Money]

One must tread with caution when using any Brix to SG conversion formula, as the Brix scale in our refractometers is actually the Plato scale. If the formula does not take this into consideration (and if the formula is correctly converting Brix and the refractometers are in error by using Plato, why on first glance should it?) it will be way off.

The 1.040 correction factor witnessed for all of the various calculators is only there to correct for the budget Chinese made refractometers anomaly of (error in) calling Plato Brix. Though lastly, if the above formula incorporates this 1.040 factor it is likely to be correct (with respect to our needs and purposes).

 

[dmtaylor]

I believe the 1.04 means something completely different, nothing to do with Plato. But, I could be wrong.

 

[Big Monk]

I believe the 1.04 means something completely different, nothing to do with Plato. But, I could be wrong.

No you're right. The 1.04 (or similar number for each person's refrac) converts refractive index to Brix, not Brix to Plato.

 

[Silver_Is_Money]

Plato x 1.040 = Brix, no? Or is it the other way around, in which case, oops, ... never mind.

 

[Big Monk]

Plato x 1.040 = Brix, no? This has been my assumption ever since seeing a chart wherein 30 Plato = 31.2 Brix.

I don't think so. Refractometers read in RI. The default correction factor (this differs from brewer to brewer) converts RI to Brix.

However, if you happen to crunch numbers from Kai's table on Plato to Brix, you can see that for the most part, the Plato to Brix conversion factor does tend to be around 1.040 +/- 0.005.

This may just be by chance.

 

[Silver_Is_Money]

Plato to Brix, and Brix to Plato calculator (see weblink I've provided below). And it does say that 30 Plato = 31.2 Brix = 1.129 SG.

If this calculator, and calculators like it (along with charts derived from it) is in error, then therein lies all of my confusion.

On the scale of my refractometer, 30 Brix = 1.129 SG, so the Brix scale appears to actually be Plato.

http://www.brewblogger.net/index.php?page=tools&section=plato&action=entry

 

[Big Monk]

Plato to Brix, and Brix to Plato calculator (see weblink). And it does say that 30 Plato = 31.2 Brix.

If this calculator, and calculators like it (along with charts derived from it) is in error, then therein lies all of my confusion.

http://www.brewblogger.net/index.php?page=tools&section=plato&action=entry

It’s a pretty common misconception going all the way back to when ST did his calculator. He and Kai were fielding questions about it back then. It’s always worth remembering that the actual equations that govern these conversions (and other brewing related calcs) are typically simplified to yield quicker results.

Michael Hall’s “Brew by the Numbers” article from Zymurgy is great one to look at for multiple examples of how more rigorous (and more accurate) calculations can be simplified to be “close enough” for homebrewing.

1.040 may be a good number to use but the distinction should be made between that and the refractometer correction factor, which converts RI to °Bx and may vary widely between units.

 

[Silver_Is_Money]

It could actually be that the SG side of my refractometers scale is useless then, even for OG readings. If that is the case, I would likely have been better off saving a couple dollars and buying the model with Brix on both sides of the scale.

 

[dmtaylor]

The SG scale on a refractometer is notoriously very wrong and should never be used. The gauge does not measure SG; it only can measure Brix (or actually RI, as RPIScotty correctly points out).

 

[Silver_Is_Money]

The SG scale on a refractometer is notoriously very wrong and should never be used. The gauge does not measure SG; it only can measure Brix (or actually RI, as RPIScotty correctly points out).

It appears that the manufacturer of my refractometer made the exact same 1.040X math model over-simplification as did the weblink which I pointed to above. I will stop using the SG side of the scale altogether. Thank you dmtaylor and RPIScotty!!!

 

[dmtaylor]

I purposely bought a gauge that does NOT report SG because I didn't want to tempt myself with misinformation. Yes, seriously.

 

[Smellyglove]

I've had two refractometers before I ended up on the one I've used for over two years now. The two which I discarded had the SG-scale wrong comparing to the Brix-scale. I've seen this in a lot of refractometers.

I trust the one I have now, I always measure OG with a hydrometer, also so I can compare to the refractometer. The only times the refractometer is off by a point or two is when I do high grav with lot of hops. Low/normal grav with little or many hops are fine. Hydrometer is often within 0.5 pts comparing to hydrometer at reference temp.

Pre-boil they always show the same. It's the hop oils at higher gravity which skew the readings above 1.085 ish for me.

 

[sbsmann]

Man, this is all very good stuff & I appreciate all the quality feedback. I will have to look at my refractometer tonight to see if the OG & Brix values are conveniently 4% different.

It does seem I will be best served in buying another hydrometer just to insure the most accurate FG readings (good thing they are cheap). After poking around on several different sites, I came up with 4 different calculated ABV's from my batch of beer using the refractometer estimatated FG's.

My current FG from the refractometer was 1.029 and if I convert that to BRIX it gives me 7.3 (which may or may not be accurate). My OG reading from the refractometer on brew day was 1.054 (which may or may not be accurate). Using those values, I came up with estimated FG's of 1.0142 to 1.015 with ABV in the range of 5.1-5.4% from the various online estimation tools. Close enough of an approximation I suppose. Perhaps after work today I will stop by my LBS, get a cheap hydrometer and test my beer to see how the real FG compares to these values:

BrewersFriend - 5.29 (1.015)
Onebeer - 5.38 (1.013 est FG)
SeanTerrill - 4.7 (1.0142 est FG)
Beersmith App - 5.4 (1.013)

 

[BIAB-Tim]

I just got a new Brew Tapper Dual scale refracomenter with ATC ! Any feedback on its reliability on its conversion scale Thanks for your reply!

 

[dmtaylor]

The conversion scale is wrong. Use only the Brix side.

 

[Silver_Is_Money]

I was wrong about my refractometer. 30 BRIX on my refractometers scale actually corresponds to 1.117 on the SG scale across from it, and not to 1.129 as I had originally written above. This is even stranger than I thought.

 

[thekraken]

I purposely bought a gauge that does NOT report SG because I didn't want to tempt myself with misinformation. Yes, seriously.

The homebrewing community needs to band together and convert to brix/plato. Plato 2017!

... and the metric system.

 

[doug293cz]

The Plato and Brix scales are both defined as weight percent sucrose in a water solution. Any differences are due to measurement errors by the teams measuring solutions to make the definition tables. Differences between the scales are out in the fourth decimal place, e.g. SG of 1.040 is 9.99325 °Bx or 9.99359 °P. Refractometer correction factors are needed because wort is not a simple sucrose solution, and this affects index of refraction, which is what a refractometer actually measures.

Brew on

 

[Silver_Is_Money]

The Plato and Brix scales are both defined as weight percent sucrose in a water solution. Any differences are due to measurement errors by the teams measuring solutions to make the definition tables. Differences between the scales are out in the fourth decimal place, e.g. SG of 1.040 is 9.99325 °Bx or 9.99359 °P. Refractometer correction factors are needed because wort is not a simple sucrose solution, and this affects index of refraction, which is what a refractometer actually measures.

Brew on

Wow, on my refractometers scale, as best as I can judge it, 1.040 SG sits directly across from about 10.25 BRIX.

 

[Nosybear]

One must tread with caution when using any Brix to SG conversion formula, as the Brix scale in our refractometers is actually the Plato scale. If the formula does not take this into consideration (and if the formula is correctly converting Brix and the refractometers are in error by using Plato, why on first glance should it?) it will be way off.

The 1.040 correction factor witnessed for all of the various calculators is only there to correct for the budget Chinese made refractometers anomaly of (error in) calling Plato Brix. Though lastly, if the above formula incorporates this 1.040 factor it is likely to be correct (with respect to our needs and purposes).

Brix and Plato are the same to three decimal places. My hydrometer is difficult to read with that great an accuracy. I've cross-checked the corrected value from the refractometer with my finishing hydrometer and get the same result to three decimal places, or one gravity point. Close enough for me.

 

[doug293cz]

Wow, on my refractometers scale, as best as I can judge it, 1.040 SG sits directly across from about 10.25 BRIX.

You might want to make up some calibration solutions for your refractometer. A 10 Brix/Plato solution is just 10 grams of sucrose (table sugar) in 100 grams of solution, 20 Bx/P is 20 grams in 100 grams. Just measure out the sucrose on your water salts scale, and then add pure water to get 100 g (do not add 100 g of water.) Make sure everything is dissolved, and then check to see what your refract reads with these two solutions (make sure your refract reads 0 on pure water first.)

Brew on