# Refractometer Accuracy

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#### Brooothru

##### Supporting Member
HBT Supporter
So, I've been brewing beer and making wine for more than 25 years. Definitely broke a few hydrometers and wasted gallons of precious fluids during that time. That all led to eventually buying a refractometer about five years ago. I like the utility of using it and the accuracy of seeing a sharp line of demarcation rather than trying to read a meniscus and calculating temperature corrections (not to mention dumping samples down the drain) when using a hydrometer.

But I always wondered about how accurate my refractometer actually is. I did calibrate with distilled water @ 60F to confirm "0" refractive index and calculated my device's Wort Correction Index to be exactly the widely accepted value of 1.04, so I feel my Original Gravities are spot on. I always get my samples to "room temperature" before taking readings both before, during and after fermentition. The issue, however, are calculations made with the presence of alcohol in the sample.

The Brewer's Friend refractometer calculator does an excellent job of applying some advanced math into the task determining the non-linear values of alcohol on refractive indices. My question is, how accurate is the Wort Correction Index of 1.04 (corrected for maltose vs. glucose) after the majority of the sugars present in the original wort have been converted into CO2 and alcohol?

Shouldn't I be using a sliding exponential scale of WRI factor indices to determine gravity during fermentation and once FG has been reached? Sure, I could dust off the old hydrometer jar and take readings the old fashioned way, but that defeats the purpose of using a refractometer in the first place.

Before I get accused of "measuring with a micrometer and cutting with an axe", the scale errors become significant at gravities > 1.065, according to the literature. More and more of my brews are in this range of OG. At about 1.075 the ABV error is 0.5% ABV, so your 7.4% Strong Ale may actually only be a 6.9% India Pale Ale.

Any thoughts on this?

Brooo Brother

You should read the Brix side on refractometers. Especially above 1.040 the corresponding SG scale becomes more and more inaccurate.

Take an accurate reading on a calibrated hydrometer (degas it first!) of fermenting or finished beer. Then compare it with your corrected refractometer reading. Make a note of your new wort correction index for that sample to match the hydrometer reading. It may not be 1.040 at that point.

I usually only take one FG reading, so a sample is not too wasteful. Besides, I drink it. I don't bother with the refractometer after pitching the yeast. It is not worth it to me to even take the time to use a refractometer calculator.

Y'all take FG readings? I don't bother, just OG. LOL...

But yeah, when I do actually [rarely] take FG readings, I just use the Beersmith Refractometer Tool to convert the Brix value provided by the refractometer (having a known OG) into FG.

You should read the Brix side on refractometers. Especially above 1.040 the corresponding SG scale becomes more and more inaccurate.

Take an accurate reading on a calibrated hydrometer (degas it first!) of fermenting or finished beer. Then compare it with your corrected refractometer reading. Make a note of your new wort correction index for that sample to match the hydrometer reading. It may not be 1.040 at that point.

I purposely bought a refractometer with only a scale for refractive index and no SG scale because of the inherent inaccuracies, and I have done comparative tests of hydrometer vs. refractometer readings in the past. I established a personal rule of thumb that my equipment accuracy delta was +/- 2pts SG. But the scale divergence is exponential as SG increases. Since I am increasingly brewing higher gravity beers I'm concerned that the accuracy of my data are suffering not from the online refractometer calculator (which I think is very good), but rather from my observed Wort Correction Index factor of 1.04 once the sugars have been fermented out (at least by the percentage of attenuation).

So what I wonder is, does the WRI correction factor vary as the amount of maltose decreases with fermentation. After all, a brix refractometer is calibrated to the RI of glucose, thus the correction factor. Does that factor decrease as the amount of maltose decreases? Don't know.

If it does, to what extent does it affect accuracy? On the current beer I'm brewing (target ABV 7.0%) if I apply a WRI correction of 1.04 to OG and a 1.04 correction to FG, my ABV is 6.7%. If I apply 1.04 to OG but only 1.00 to FG the ABV is calculated to be 7.0%. I'll add that all other target values of this brew session were spot on, so I suspect there might be something missing in the calculations.

I guess the only way to determine is draw and degas a hydrometer sample.

Brooo Brother

I usually only take one FG reading, so a sample is not too wasteful. Besides, I drink it. I don't bother with the refractometer after pitching the yeast. It is not worth it to me to even take the time to use a refractometer calculator.

I'd do the same thing except that I spund when the SG is ~5 points of FG, so I need to take several samples during the attenuative phase of fermentation. 'Primary' takes 4-7 days, dump trub, put on spund. Pressure builds during this 'secondary' to ~15 psig and only takes 7-8 days. Beer comes out fully cleared and carbed 10 to 15 days after pitch. It also speeds up "conditioning" of the finished beer. Don't know how, but the proof is in the tasting.

The only way I could turn it this fast is by spunding and 'secondary' under pressure, and unfortunately that requires almost daily monitoring of SG.

Brooo Brother

Y'all take FG readings? I don't bother, just OG. LOL...

But yeah, when I do actually [rarely] take FG readings, I just use the Beersmith Refractometer Tool to convert the Brix value provided by the refractometer (having a known OG) into FG.

I really like Beersmith, EXCEPT the refractometer calculator and the water chemistry adjustment tools. The Brewer's Friend online calculators are easier to use and more accurate IMO.

Brooo Brother

I am just getting into the world of refractomers corrections, calculators, etc. so this might not be relevant, or even the right approach for addressing your issue, but...

When you took sample readings to determine your WRI, what gravities were you using? Higher gravities like you're concerned about, lower gravities, a variety across the range?

I'm wondering if you took say 30 different samples in the higher OG range to determine your WRI if that would be different from taking 30 samples in the lower OG range. If so, and you took your samples across the range (high OG, low OG, and everything in between), then wouldn't the value you determined sort of be an average of the two?

Again, I want to reiterate, I don't really know what I'm talking about here... Just guessing. Determining the WRI of my refractometer is on my list of things to do, but I'm not super concerned about it. The beer is going to finish where it finishes regardless of how precise and accurate my refractometer is. And I don't really care about a few tenths of a percent.

Don't blindly take 1.04. It varies by recipe too.

I've had amazingly accurate results when using clean, degassed samples (run it through a coffee filter at least once), taking an accurate OG with a precision hydrometer, and using the novotny correction equation for final gravity. Depends how accurate you want.

Don't blindly take 1.04. It varies by recipe too.

I've had amazingly accurate results when using clean, degassed samples (run it through a coffee filter at least once), taking an accurate OG with a precision hydrometer, and using the novotny correction equation for final gravity. Depends how accurate you want.

I've got a Hanna digital refractometer for beer and have confirmed by precision Plato saccharometer that there is no correction factor, 1.04 or otherwise. I also use a coffee filter to clear and degas samples, and also use the Novotný equation for correction. Gets me an accurate enough picture to know when to spund, I've been refractometer-only for quite some time.

The Brewer's Friend refractometer calculator does an excellent job of applying some advanced math into the task determining the non-linear values of alcohol on refractive indices. My question is, how accurate is the Wort Correction Index of 1.04 (corrected for maltose vs. glucose) after the majority of the sugars present in the original wort have been converted into CO2 and alcohol?

Shouldn't I be using a sliding exponential scale of WRI factor indices to determine gravity during fermentation and once FG has been reached?

Excellent questions. I believe the Novotny equation used by Brewer's Friend assumes a constant WRI before and after fermentation for "normal" OG beers (probably less than 1.065), which is part of the reason the higher OG beers will start to go off the curve. I've found the same results with my high OG beers. I haven't developed a correction for these but I think it's doable with more data.

Great discussion topic. Cheers!

EDIT: For the one batch I've made recently where OG was 1.098, it looks like the WRI swung way up to about 1.13 if it were to be used to compare with the actual hydro FG of 1.023. (My usual WRI for my refractometer for most other beers has been 0.99 on the nose.)

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I've got a Hanna digital refractometer for beer and have confirmed by precision Plato saccharometer that there is no correction factor, 1.04 or otherwise. I also use a coffee filter to clear and degas samples, and also use the Novotný equation for correction. Gets me an accurate enough picture to know when to spund, I've been refractometer-only for quite some time.

I surmise the difference between 1.00 and 1.04 has to do with one's specific instrument and exactly what it's calibrated against.

I have the MW887 (Brix) and my correction factor against the precision hydrometer is around 1.017. I sometimes will be a little lower or a little higher but for the same recipe it doesn't change.

I still always take the precision OG measurement with a plato hydrometer because i am generally spunding and i've found when using the novotny equation it matches my real life measurements better when i used the actual corrective value for the batch rather than some average. But at that point you're talking about a single gravity point.

I surmise the difference between 1.00 and 1.04 has to do with one's specific instrument and exactly what it's calibrated against.

I have the MW887 (Brix) and my correction factor against the precision hydrometer is around 1.017. I sometimes will be a little lower or a little higher but for the same recipe it doesn't change.

I still always take the precision OG measurement with a plato hydrometer because i am generally spunding and i've found when using the novotny equation it matches my real life measurements better when i used the actual corrective value for the batch rather than some average. But at that point you're talking about a single gravity point.

And I take Novotný as close enough and let the spunding valve cover me if I transfer a bit too soon. Since I also use refractometer + Novotný for my FFT, I can't go too far astray. As you suggest, it's down to your personal tolerance.

I've got a Hanna digital refractometer for beer and have confirmed by precision Plato saccharometer that there is no correction factor, 1.04 or otherwise. I also use a coffee filter to clear and degas samples, and also use the Novotný equation for correction. Gets me an accurate enough picture to know when to spund, I've been refractometer-only for quite some time.

I'm not sure I understand. Are you saying that the Hanna digital does not require a correction factor, or that no refractive correction is ever needed for any refractometer. If it's the former, that's a really cool feature and wish I could justify the price. If it's the latter, then I'll want to read up on that since different liquids will refract light at different angles through a prism dependent on what is dissolved or suspended in that liquid. That's why you calibrate with distilled water instead of tap or even RO.

Do I have the physics right? Not trying to be snarky, but just looking to get my head around the topic to be more accurate in my measurements.

Cheers,

Brooo Brother

I don't know. I didn't find a significant correction factor in my previous optical unit either. But I never had a good feeling that I was reading it correctly. That led me to the idea that eliminating subjectivity in favor of the instrument's own objective self interpretation would be preferable. I think (innocent) user error is a major factor in difficulties with handheld refractometers.

I have always been suspicious of the accepted notion that one must generate an instrument-specific correction factor by observing a series of wort samples. I think this is more likely to reflect an apparent bias that is in fact user error. Accepting the idea that each optical instrument would require a specific adjustment would seem to me to discredit the very validity of the science of optics, or at least the trustworthiness of the manufacturer.

I have always been suspicious of the accepted notion that one must generate an instrument-specific correction factor by observing a series of wort samples. I think this is more likely to reflect an apparent bias that is in fact user error. Accepting the idea that each optical instrument would require a specific adjustment would seem to me to discredit the very validity of the science of optics, or at least the trustworthiness of the manufacturer.

Excellent insight.

Perhaps your personal user-error correction factor is 1.000. If true, lucky you. Mine is still 0.99 right on the nose. Or should I say, 0.990.

I think the difference in correction comes down to how the instrument scales refraction to a particular unit.

Remember that a refractometer only measures refraction. The readout is typically given in Brix, or SG, or Plato, not index of refraction.

Plato and specific gravity are measures of density. Brix is a measurement of sucrose concentration, which closely related to density. But brewers have very little sucrose in solution so there’s a small, but measurable difference when dealing with wort.

The easiest thing to do is just check OG against a precision hydrometer on a few batches and you’ll know how your refractive device needs to be compensated.

I don't know. I didn't find a significant correction factor in my previous optical unit either. But I never had a good feeling that I was reading it correctly. That led me to the idea that eliminating subjectivity in favor of the instrument's own objective self interpretation would be preferable. I think (innocent) user error is a major factor in difficulties with handheld refractometers.

I have always been suspicious of the accepted notion that one must generate an instrument-specific correction factor by observing a series of wort samples. I think this is more likely to reflect an apparent bias that is in fact user error. Accepting the idea that each optical instrument would require a specific adjustment would seem to me to discredit the very validity of the science of optics, or at least the trustworthiness of the manufacturer.

Totally agree that many errors are indeed human bias errors. I'm curious if your Hanna is designed specifically for beer (therefore maltose). If so, it may have pre-programmed reflective index corrections built in to the circuitry or software. As you know, the manual refractometer we use are basic brix ("sugar") instruments that measure the most common sugar which is glucose.

Different sugars refract light at different angles. When I measure SG in wine there is generally no brix reflective index correction because fructose and sucrose found in wine have nearly identical refractive angles to glucose. The angular difference in maltose (the predominant sugar in beer making) however is greater.

For accuracy a correction factor can be applied to measure SG throughout the fermentation. Since the concentration of maltose decreases as fermentation occurs my question is whether that decrease is significant in the calculations for FG, and thus ABV. We already know that the presence of alcohol causes refractive errors regardless of the sugar involved and have equations to correct for these errors. But does the absence (more correctly: decreasing 'presence') of maltose also impact the readings?

I wouldn't be concerned except that the divergence is non-linear, and anecdotally I have seen as much as 0.5% ABV variance with and without a corrective factor applied. So there is a difference. What's unclear to me is which methodology yields the most accurate data. Time to dig out that hydrometer, I guess.

BTW, maximum envy on the Hanna!

Brooo Brother

A friend of mine is a retired professor from Cornell. He wrote up this calculator to deal with the compensation factors for post-fermentation wort. He has all the stoichiometry derived and documented

http://ithacoin.com/brewing/ABV_calc_frontpage.htm

He tested his calculated result vs. measured result (by boiling down a sample) and estimated a mean error of 0.1% ABV and std deviation error of 0.4%, which is good enough for me

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A friend of mine is a retired professor from Cornell. He wrote up this calculator to deal with the compensation factors for post-fermentation wort. He has all the stoichiometry derived and documented

http://ithacoin.com/brewing/ABV_calc_frontpage.htm

He tested his calculated result vs. measured result (by boiling down a sample) and estimated a mean error of 0.1% ABV and std deviation error of 0.4%, which is good enough for me

Perfect. This is exactly what I was looking for, and his spreadsheet/calculator accounts for corrections without having to enter any values for maltose refractory differences.

It's interesting (or maybe just coincidence) that his standard deviation value of 0.04 equates to my calculated 1.04 correction factor (~4%). It appears at first glance that his numbers are much tighter at about 1%, so I look forward to comparing my normal methodologies against his calculator and verifying with narrow range hydrometers.

Good thing I haven't yet pitched the yeast on the Winter Warmer I brewed today. My refractometer indicated brix of 16.8 with a calculated SG of 1.066, using a 1.04 wort reflective index correction factor.

I was bummed out because my target was 1.074, though all the specialty grains were two or more years old so missing on the low side was not unexpected. It would be nice to find out that OG actually is closer to target than what I calculated.

Thanks for the link. I owe you a beer.

Brooo Brother

Perfect. This is exactly what I was looking for, and his spreadsheet/calculator accounts for corrections without having to enter any values for maltose refractory differences.

It's interesting (or maybe just coincidence) that his standard deviation value of 0.04 equates to my calculated 1.04 correction factor (~4%). It appears at first glance that his numbers are much tighter at about 1%, so I look forward to comparing my normal methodologies against his calculator and verifying with narrow range hydrometers.

Good thing I haven't yet pitched the yeast on the Winter Warmer I brewed today. My refractometer indicated brix of 16.8 with a calculated SG of 1.066, using a 1.04 wort reflective index correction factor.

I was bummed out because my target was 1.074, though all the specialty grains were two or more years old so missing on the low side was not unexpected. It would be nice to find out that OG actually is closer to target than what I calculated.

Thanks for the link. I owe you a beer.

Brooo Brother

glad to help. Send Jim a note via email if you get a chance; he'll get a kick out of knowing others are using his calculator

glad to help. Send Jim a note via email if you get a chance; he'll get a kick out of knowing others are using his calculator

Will do.

Small update: I bit the bullet and filled a sample jar for two different narrow range hydrometers, both calibrated at standard temperature. Wort temp was 59F. Both hydros registered 1.068, so at least I picked up two points in gravity.

I did another refractometer reading as well but did not apply a WRI correction, which came out 1.069. So I'm bracketing my original reading, all of them within 0.04 standard deviation. I guess they're all "close enough" and I'm obsessing on something too small to worry about.

But I did learn some things and got linked to a nifty spreadsheet that is more accurate than what I've been using. So there's that!

Thanks to all.

Brooo Brother

Thanks to @twd000 for providing the link to the lesser known Gossett calculator page. Being a numbers nerd, I've spent a couple of hours incorporating the equations into my own custom spreadsheet, and used it to compare against 5 other refractometer conversion calculators. Of the 6 total calculators, I find this site to be the 2nd most accurate of all, averaging within 0.0008 of actual FG. Very nice. I haven't run numbers on ABV as that's always a swag, rather I've only compared actual FG vs. calculated FG conversions.

I'm still at the point where I think a properly used & calibrated refractometer can be every bit as accurate as a trusty old hydrometer, and maybe even more accurate than a hydro -- for instance, final Brix can be measured to two significant digits (e.g., 5.6, 6.7, etc.), whereas FG is often only good to one sig fig (e.g., 1.001 thru 1.009 -- the 1.00 at beginning is essentially meaningless to brewers and only the final digit means anything to us).

Cheers all -- I've stayed up too late nerding out -- it's 1:00 AM. Fortunately I don't have to work in the morning.

A while back I did a bunch of DME solutions and checked my refractometer vs my hydrometer.

Bottom line, for typical wort SGs, my refractometer was within 2 points of my hydrometer. Of course, worts can be different. I wouldn’t launch a beer rocket with a refractometer number. But I’ve been using a refractometer when brewing under 1.060 and calling it good enough. YMMV.

Of the 6 total calculators, I find this site to be the 2nd most accurate of all

And the winner and still champion is...?

And the winner and still champion is...?

Brewer's Friend / Petr Novotny (Linear) takes the #1 prize overall for calculations involving any wort of any gravity at any time.

However, interestingly, following discussions I've had with Sean Terrill which I continue to analyze, I further rank all the equations based on whether FG ends up greater or less than 1.014, as follows:

FG < 1.014 Sean Terrill (New Cubic)

FG > 1.014 L.K. Bonham / Jim Gossett (Old Cubic)

The Terrill equations become wildly inaccurate for FG greater than 1.014; whereas, all other equations are great at all ranges. Overall, whenever in doubt, it's never wrong to use Brewer's Friend / Novotny, and fortunately they also have perhaps the most popular AND best managed web page out there.

Good to know, thanks for all the work. Novotný linear is what's in my own spreadsheet.

thanks for the comparison, @dmtaylor. Jim Gossett has been brewing mostly high ABV Belgian beers so it's quite possible his method is tuned to match the high end of the range

thanks for the comparison, @dmtaylor. Jim Gossett has been brewing mostly high ABV Belgian beers so it's quite possible his method is tuned to match the high end of the range

Hmm... I'll look into that...