First of all, if it no longer involves prizms, it will no longer be a refractometer IMO. It will then be a conductivity meter.
Randy Holmes-Farley has many articles explaining that the standard refractometer is only accurate for NaCl, or, when calibrated with a calibration fluid, only accurate at that specific range.
It's amazing how many people with a standard refractometer swear up and down that their refractometer is accurate with using water to calibrate as well as using calibration fluid. The only way this is possible is when using a seawater refractometer.
Randy on Refractometers and Salinity
This is his Archived Article on using Conductivity to Measure Salinity
Randy on Home Made Calibration Standards Including Conductivity
All that being said, I still use my SeaTest swing arm hydrometers that I started with over 18 years ago. I check them about once a year against my certified calibrated hydrometer from Fisher Scientific and it is always on.
I rinse after every use and soak overnight in vinegar once a week.