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Can you use a refractometer to test final gravity?

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3 Answers 3

As far as I know, you can determine the final gravity solely with a refractometer only if you know the original gravity (either with a hydrometer or the refractometer). I am a small-batch brewer, and exclusively use a refractometer for gravity measurements. A few tips:

  1. You have to calibrate your refractometer to read zero when using distilled water as your test sample.
  2. If your refractometer has a specific gravity scale (e.g., 1.050), don't use it. Many refractometers have an incorrect scale because they have used a formula for the cubic equation to convert from degrees Brix to specific gravity that was incorrectly published on byo.com. Take the Brix reading and use a calculator to convert it instead. Source 1; Source 2. Here is the incorrect byo.com online article.
  3. The refractometer is designed to read samples of sugar water and fruit juice, not wort. You can determine your refractometer's wort correction factor. To do so, you need to take concurrent hydrometer readings. The wort correction factor is usually around 4%. I decided that it is impractical to take hydrometer readings when brewing small batches, and that I will just assume that adjusting by 4% is close enough for my purposes.
  4. This calculator by Sean Terrill is probably the most accurate to determine final gravity.
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Actually, as asked by me personally a few weeks ago, you can actually guestimate the ABV by doing a Brix-reading (refractormeter) and a FG-reading (hydrometer). homebrew.stackexchange.com/questions/10823/… –  Sander Nov 12 '13 at 6:38
    
@Sander - that is true, but whether you can estimate ABV was not the OP's question. OP asked if you can determine FG w/ a refractometer. Maybe you are right in assuming that is what the Op really wants to know. –  Chino Brews Nov 12 '13 at 20:56
    
Can you cite #2 in your answer? I'm curious to read about this, as I've used my dual-scale refractometer for the SG on many batches, and would like to know if I've been off in my readings. –  Scott Nov 12 '13 at 21:06
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I updated #2 in my above answer to include the cites. The polynomial formula in byo.com was printed as "SG = 1.000019 + [0.003865613(Brix) + 0.00001296425(Brix) + 0.00000005701128(Brix)]" but should have been "SG = 1.000019 + [0.003865613(Brix) + 0.00001296425(Brix)^2 + 0.00000005701128(Brix)^3]", which throws the formula off slightly but materially per Sean Terrill. –  Chino Brews Nov 12 '13 at 22:52
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Yes, but you have to factor in the alcohol so you'll need a refractometer adjustment calculator. Northern Brewer has one on their website and there are other online calculators as well. But no two calculators seem to agree perfectly.

Personally, if I'm in the ballpark I'm happy. But if you want to be certain of getting the most accurate measurement possible you could do a side-by-side comparison of hydrometer measurements vs adjusted refractometer measurements using various calculators to see which comes closest. Then just use that going forward.

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Perhaps, but I have tried this with a couple of online calculators and with brewtarget and each time, the calculated final gravity is significantly lower than my actual hydrometer reading. I finally gave up. I actually find the hydrometer a bit simpler for final gravity anyway. The refractometer with ATC comes in handy with hot first and second runnings and pre and post boil gravity. I can also use it without wrecking my plastic at high temps. Final gravity is always room temp (or lager temp) and I like to taste my sample at that point as well.

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I agree hydrometers can be a reliable way to measure specfic gravity, but the sample size is commonly too big to work with small batches (unless you are with OK dumping the sample back in). I assume you have calibrated your hydrometer, and you adjust your hydrometer readings for temperature - most hydrometers are calibrated to read 1.000 on distilled water at 60°F. –  Chino Brews Nov 13 '13 at 17:49
    
Most of the newer (and cheaper) ones are calibrated to 68F. Dumping the sample back is not normally an issue. You have to sanitize your thief to get the sample; just sanitize the hyrdometer and put it in your sanitized thief. Heck, you can even leave it in the neck of carboy so you don't spill. –  Wyrmwood Nov 14 '13 at 19:20
    
Note that ATC in a refractometer only corrects for changes to the prism with temperature - it doesn't know about or account for gravity changes to the wort. So, if your room is at 75F and the refractometer calibrated at 65F then you'll need to use calibration tables to account for the 10F difference. –  mdma Nov 15 '13 at 16:30
    
@mdma, sorry, but I don't comprehend your last comment. As I understood it, after I calibrate my ATC refractometer at room temperature it should be accurate for Brix readings if the ambient temperature is between 68-86°F (20-30°C), and refractometer is at same temp. I just need to give the 1-2 drops of wort about 15 seconds to adjust to temp of the prism. Am I wrong on that? –  Chino Brews Nov 16 '13 at 0:23
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