The Red Sea is supposed to be Sea Water refractometer. That's why you calibrate to 0, many other refractometers sold for the hobby are actually salt water (brine) refractometers.
It's not a bad idea to calibrate to 35ppt regardless since calibrating to 0 assumes a perfect refractometer.
FWIW, everything I've read on the Red Sea refractometer talks about how quickly it goes out of calibration, that it needs calibration every. Single. Time.
Veegee is supposed to make a good one, if you are looking to replace the red sea.
It's not a bad idea to calibrate to 35ppt regardless since calibrating to 0 assumes a perfect refractometer.
FWIW, everything I've read on the Red Sea refractometer talks about how quickly it goes out of calibration, that it needs calibration every. Single. Time.
Veegee is supposed to make a good one, if you are looking to replace the red sea.
