First time using a refractometer throughout fermentation. Prior to pitching yeast my Zinfandel was at 24.5 brix. 5 days into fermentation it shows a reading of 6.5. If I use the brix calculator form MoreWine to account for the alcohol, that would suggest my brix is -5.6, which isn't possible. I tested SG and came up with a reading of 0.998, which would be 9.65 brix. Why is my brix off by so much? (My refractometer has ATC and reads a perfect 0 brix with a drop of water)

Bad maths methinks. Calculator spitting out wrong figures. Trust the SG. Once fermentation is started I discard the refractometer anyway. Walking 10mi. to get 1. Starting 24.5% Brix° .998 SG finish. Cut and print And check out FermCalc. The gold standard of wine calcs. You name it-it does it. http://www.fermcalc.com/FermCalcJS.html

The morewine spread sheet indicates your actual specific gravity is .979 based on the refractometer numbers you provided. Your looking at the wrong column.

The MoreWine spreadsheet appears to find the actual specific gravity by converting the observed brix and accounting for alcohol content, then it simply converts the SG number back to brix to find actual. So 0.979 is the same as -5.6 brix. Either way, 0.979 isn't even possible, is it? and it certainly doesn't match the 0.998 reading I got when I used a hydrometer. I guess my real question is whether or not people find the conversion chart reasonably accurate when using their refractometer after fermentation has started. I'm only really using it as a daily check to see if sugar is still being depleted.

There are all kinds of calculators on the net, you just need to find a good one that works for you, many of them have correction factors that need to be applied or even tweaked. This one indicates .991 using the default 1.04 correction factor. https://www.petedrinks.com/abv-calculator-refractometer-hydrometer/ Personally I use a hydrometer, but during fermentation even with a hydrometer you need to take the reading quickly or bubbles collect on it and give a false high reading. I just use the hydrometer as a quick indicator of what's going on, no precise calculations going on here.

Regarding fermcalc, i can't say enough good things about it. Put in must SG, and it tells you how much sugar to add for a specific SG final. It is always spot on!

the refractometer correction spreadsheet may have errors once the end of fermentation is reached. it is a good tool for monitoring fermentation progress as it only takes one drop to measure status. once the fermentation is complete I think it gets a little erroneous. use a hydrometer to measure final sg.

Yea that's what I did. Refractometer spreadsheet wasn't accurate at all. Tested all musts with a hydrometer and got 0.994-0.996 for all of them. Pressed last night.