Howdy,
I've been trying to stick with using a refractometer, but have bought hydrometers too to double-check numbers.
I'm brewing a belgian at the moment and it's measuring 1.007 on my hydrometer, but 1.027 on my refractometer. It's high even after compensation calculations.
My understanding is that the sugars and alcohol content post-fermentation throw off the refractometer. Given this is a 7%+ Belgian, I'm assuming it's even harder for the refractometer to be right and this is a bit of a death nail for using them, right?
Don't blame the tool :P
For SG on fermented/fermenting beer you need the original & current brix.
Here's a calculator:
https://www.northernbrewer.com/pages/refractometer-calculator
I don't think it's a death nail. You just need to know it's strengths and limitations.
I really only use mine to decide when to cut the sparge. But even then, a PH meter is more appropriate.
If we could just ignore that I forgot to tick "fermented wort" and that when I did the numbers were bang on that'd be great. Thanks ;)