To my understanding, after fermentation has begun, you cannot really get an accurate reading on the refractometer and that the hydrometer is much more accurate. It has something to do with the actual fermentation process.
That's what I was told, so I use the refractometer up until crush, then I switch over. Always possible that what I've been told is wrong, but just passing along what I've heard.
Kathy
You need the pre-ferment refractometer readings. Then, you can use some estimations/calculations down to the end. Not as accurate as a hydrometer, IMO but many people use it.
Using A Reflectometer During Fermentation. Is There A Formula?
in pdmq's Blog
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You need the pre-ferment refractometer readings. Then, you can use some estimations/calculations down to the end. Not as accurate as a hydrometer, IMO but many people use it.