Can a refractometer tell me if all sugar has been fremented

Dex-Pistol

Howdy. I use a hydrometer to calculate abv in my cider, but I really don't care about abv. All I care about is that all of the fermentable sugars have been fermented, i.e. all I care about is the number .990
Can a refractometer tell me if I have reached the equivalent .990?
Thank you kindly

HoochinHank

Probably!
But you have to take your STARTING brix reading from refractometer and record it on a sheet of paper. Then, when you think it's probably done fermenting, take another brix reading, and use this calculator (skip to "Part II", plug in your original brix, then the latest brix, and it should come up with a reasonably close estimate of final gravity and abv%).
If you didn't record your starting brix reading, you should at least be able to compare subsequent brix readings, and when they stop changing from one day to the next, that's a very good indication that ferment has finished (or so I've been told).

sibelman

I'm imagining that, as with beer, cider fermentations may not all attenuate equally due to various factors (of course not including mash temperature

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). Is 0.990 believed to be the predicted final gravity for cider generally, or is it a rough target? (sorry if this is a bit of a tangent from OP's measurement question)

hotbeer

It'll still just be an assumption, IMO. Especially considering all the corrections and mistakes others seem to make with their refractometers till they get experienced with them after being shown by others the mistakes in their understanding of what some corrections are for.
Even a hydrometer taking a SG reading is a assumption about the completeness of sugar being fermented out. But they are less expensive and less prone to mistakes of ones knowledge.

Andre3000

Refractometer calculators that predict ABV become wildly inaccurate when when the final gravity gets that low.
I experienced this when I was brewing a Brut IPA and the refractometer calculator (Sean Terrill's, which is quite good) kept telling me I had 4 or 5 points to go to 1.000. I measured with a hydrometer and turns out I was already at .998 or so.
However, it sounds like you for your purposes you could determine a Brix reading that would correspond with .990. IE if that number is 3 Brix, don't rely on the calculator and just wait until you hit 3 Brix; that would be reliable. But you'd have to test to see what that number is with your hydrometer for a given recipe. If your recipe changes (OG, fermentibilty, yeast, etc.), then that number would not be reliable anymore.
If you want to be safe, using the hydrometer is the best route.