Article about using refractometer during initial fermentation

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Aug 29, 2019
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I’m working on my first 6 gal. batch of muscadine wine. As a newbie I still have a lot to learn. I bought a spectrometer and realized it can be used before the fermentation process but will not be accurate as the alcohol develops.

I ran across an article about a fellow who believed that there should be a formula that would compensate for the presence and fluctuations of alcohol during fermentation. Anyone ever heard of this sort of calculation or tried it? It would seem an easier testing process if it is true. Here is a section of the article I read;

A refractometer with ATC (Auto Thermal Compensation) is unaffected by temperature, fermentation gases and only takes one to two drops to measure the sugar content of your must. However, it is effected by the different refractive index of ethanol. As the amount of alcohol increases the refractive index changes algorithmically. Unlike a Hydrometer the reading in a refractometer will never go to zero.

With a Refractometer you:

1.) Fill an eye dropper with a couple drops of juice

2.) Drip a drop or two on the refractometer lens

3.) Read the refractive index through the refractometer

In theory the ethanol effect is predictable and a formula should be able to compensate for it's refractive change.

To test the theory we fermented a Zinfandel kit and took measurements with both a professional refractometer and precision hydrometer using a formula from the CRC Handbook of Chemistry and Physics ("Concentrative Properties of Aqueous Solutions: Conversion Tables", 69th Edition Table 88 Sucrose). We then evaluated the correlation between the formula compensated refractometer readings against those from a precision hydrometer from start to finish of fermentation.

The formula tracked like rocket science. Over the 10 day fermentation we averaged less than 1% difference and that falls into the realm of user error.

Check our test results and download the file for your own use!

Our experiment results ( in .pdf)

Spread sheet to auto compensate for ethanol effect on a refractometer during fermentation (.xls Requires MSN Excel).

Formula for compensation of ethanol effect on refractometer: 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)

SG = Specific Gravity, OB = Original Brix, AB = Actual Brix (Brix Readings During Fermentation)

Formula to convert from SG to Brix (for those who prefer Brix measurements):

Brix (Plato) = -676.67 + 1286.4*SG - 800.47*(SG^2) + 190.74*(SG^3)

Temperature compensation formula for hydrometers calibrated at 600F:

Correction = 1.313454 - 0.132674*F + 0.002057793*(F^2) - 0.000002627634*(F^3)

SG corrected = SG + (correction * 0.001)

Bubbles during fermentation attach to the hydrometer and skew your reading. Temperature adjustments are also necessary as most hydrometers are calibrated at 60 0F.


During our experiment we pulled 5750mL (1.5 Gallons) for the twice daily hydrometer readings. Compare to the 10ml of juice at right for a refractometer. Note that the small samples for the refractometer are not reintroduced back into the fermenter. Greatly reducing risk of contamination.


©2002-2006 The ValleyVintner. All Rights Reserved
You can certainly use a refractometer after you have pitched the yeast and there is alcohol in the solution that you are testing but as you note, because there is alcohol in solution and because a refractometer is calibrated with water and not alcohol you need to use a calculator to correct for the difference in refraction (the angle at which the light is bent) as it travels through a solution composed of different percentages of alcohol compared to water. You also need to know the starting gravity as that is part of the formula needed to compensate for the increasing quantity of alcohol.
Ahh. Down the road I might try it out. I didn’t get my S.G. on this first go round. I honestly didn’t know enough when I put the recipe together as to how important it was. In my naive state I figured if one had a good recipe, good results “just happened.”
At the risk of sounding argumentative... I have a hard time understanding the need to use or attempt to use a refractometer for taking readings during a fermentation.
In the OP posted comments, I assume from someone who is pushing for the use of a refractometer INSTEAD of a hydrometer, I see several concerns.

> First is the idea that errors are easy to make when trying to read a hydrometer - While that is partially true in some conditions, generally speaking such reading are NOT so critical that an error of .001 to .004 even would cause any serious issues. In fact when it matters most, at the conclusion of a ferment, the wine must is generally quite clear and free of anything but CO2 bubbles. Perhaps a little more time learning to read a hydrometer correctly would help folk. I've found that with only a little effort, I can take a decent reading. Or course there are some folks who simply leave their hydrometer in their fermentation bucket during the ferment. Whatever, that works too. Give it quick twirl and check it out. My hydrometer is color coded so If I'm doing a number of readings over a few days time, Just a as soon as I drop it in the test tube I have an Idea if the news is Good, meh, or Hmm what's happening. I don't have to wait to do any calculations.

> Secondly the claim about temperature correction. Hmmmm Really - because when I use "Brewers Friend" Hydrometer temperature adjustment calculation again I see very very small error or adjustment that needs to be made. As shown in the attached photo a hydrometer calibrated for 60 degrees used in a must at a temperature 75 degrees would be off .0001 for a must and would give a reading of 1.020 when it should in fact be 1.021. Again during fermentation that 'in process' measurement is just a snapshot of where the wine must is at this time. I have a wine must that has dropped from 1.018 yesterday to 1.005 today in less than 24 hours. If my measurement was off .002 or even .004 that's not critical - I can tell the fermentation IS happening.

> As for risk of contamination... If your sanitization processes are good then you have nothing to worry about. Even an Eyedropper used to pull a sample could 'contaminate a batch if not properly sanitized.

BUT I will agree - if you just love running calculations and playing with formulas... that refractometer will sure give you an opportunity to do that.

Again I know this sounds argumentative but for so many of us an error of .001 to .004 even just isn't critical during the time when the measurements are often rapidly changing anyway. Using a refractometer before fermentation - Sure if you've got one and like them - by all means do so.

Side question - how reliable is a refractometer with a very cloudy, fiber laden must like a thick peach must? Does the presence of those solids interfere with the reading?

Last edited: I have been using this for the past 15 years. works like a charm. I had one editor for Winemaker Magazine argue it was inaccurate at 1.00sg. which it probably is but for monitoring the fermentation it works great. at the end use a hydrometer. after multiple use experience will tell you the refractometer reading that corresponds to finished fermentation. best use is when step feeding a wine to achieve higher abv for a port development. for nominal fermentation a couple of reading will do. it does not make a mess as using a hydrometer might do.