Have a question regarding refractometers :
I use mine along with a hydrometer for OG measurements. I know that the alcohol present at FG makes it inaccurate, so I use the hydrometer for that. But I’m aware there are corrections that can be used for FG measurements. Are the corrections reliable enough to make it worthwhile? It seems that abv% would have a direct relationship on a correction. Thanks!
I looked at the calculations once and decided to just use my hydrometer for FG measurements. YMMV.
Dave
the calculations are very worthwhile - sean (a10t2) has put together a spreadsheet which makes it very easy. Refractometer FG Results « SeanTerrill.com
the refractometer calculator in BeerSmith2 has also been very good to me (i.e. same as hydro) once I got the correction factor dialed in.
I use Sean’s spreadsheet and have very good luck with it. I test it now and then by using a hydrometer to verify the calculations. It is also close enough to trust in my brewery .
Paul
He’s also got an online version, no spreadsheet necessary…
After deriving the equation, I solicited other brewers’ data. I don’t have the numbers on me, but six or seven people contributed 70 or so data points, and the standard deviation for those was 0.0013 - a little more than one “gravity point”. So statistically speaking, you could expect the correlation to be within one point 56% of the time, two points 88% of the time, and four points >99% of the time.
I will say that in my own experience the standard deviation is a bit smaller than the overall sample. So when using consistently and thoroughly calibrated instruments, the calculation is about as precise, or even a bit more so, than a cheap hydrometer.
Thank you Sean ! That’s more than reliable enough to start doing via your calculator. Appreciate it.
I’m going to give this a try. Thanks for posting!
Dave
i’ve tried several conversion methods and I never got an accurate reading for FG.
Yeah, I’ve never been able to get an accurate FG either, so I just use the hydrometer on the fermented side. The refractometer sure is handy as hell for runnings gravity, preboil gravity, and OG, though.
I use this calculator: Onebeer.net :: Homebrew :: Refractometer Calculator
The several times I checked it with a hydrometer it was very close. Close enough 
But even given the inaccuracy of the refractometer FG measurement, I still think it’s a great tool. After all, when I measure FG, I’m less concerned with the absolute accuracy of the FG reading, than I am in just monitoring fermentation completion. Unless I’ve completely screwed something up (pitch rate, oxygenation, ferm temp), I know it’s going to finish. So pragmatically, the FG reading is just a reading of “how done is it”, i.e. is it still dropping?? The refractometer is as good a monitor of this as is the hydrometer. If I get a reading of 8 brix for 3 times over a week, I know it’s done. Whether it’s 1.012, 1.011, or 1.010, doesn’t really matter.
My difference was always exceptional - like 8 points off or so. And I use the onebeer.net conversion as well. Not sure why it never worked for me.
Lately my refractometer has been giving me screwy reading on pre fermentation readings as well so I just use my hydrometer now for everything.
This seems obvious, but have you tried giving it a thorough cleaning?
It’s as clean as it can be.
Major- was the refractometer also off on the OG? I calibrate mine using distiller water, so I don’t know how one could calibrate it differently for FG to know it is spot on without using a hydrometer, like Sean did. I posted elsewhere that my lab grade FG hydrometer recently broke, which is a pain, because my cheaper ones are so hard to accurately read, that I was thinking of going to refractometer readings for both ends. Your post has me wondering…maybe I will do both for the next few batches and see if they correlate well enough to use the refractometer with Sean’s adjustment spreadsheet. I don’t brew a lot of high gravity beers, so it may not be significant differences for me…
Thanks for the insight.
Mine seems to just give screwy readings on the OG. If I take 3 readings sometimes all three will be different. I know some have attributed this to wort temperature (though mine is supposedly ATC) but even if I cool the sample first it will give me a different reading than my hydrometer. I’m on my second refractometer (dropped the first one) and both of them gave me problems like this, though the first seemed more accurate. The second one does not read the same as my hydrometer.
I have calibrated it with distilled water and or tap water (I don’t think the differences between the two calibration solutions are exceptional enough to throw a reading off as much as I am seeing).
Bottom, line is, I just feel more comfortable in the reading I get from my hydrometer.
Same here. I have 2 refractometers and they read different from each other and both are different than my hydrometer. I’ve gone back to hydrometer for everything.
Good info. Does calibrating have any affect?
Nope, not really.
I use my refractometer for measuring pre-boil/boil gravity only. I typically get pretty consistent readings (+/- 0.2° Plato). I still use a hydrometer to measure chilled wort (OG). The prism on the refractometer must be kept clean and dry for accurate readings. Calibration is also recommended every so often. Keep in mind that boiling wort sampling requires an agile/quick transfer time to avoid evaporation losses. In other words the wort sample must be transferred from the kettle to the prism “quickly” in an effort to avoid evaporation loss during the transfer process. Once the hot wort sample is on the prism and the daylight plate is closed, I allow about a minute for the temperature to stabilize then record the measurement.