0 ppt means no salt in the water, at least in the case of our SW
refractos. A refractometer measures how light bends when passed thru saltwater. If you do not calibrate it at a known reference point, all you know is when there is no salt in the water, the light doesn't bend and you see 0 ppt. You really have no idea if the refracto is calibrated properly when there is sw
on the lense and the light bends.
FWIW, I can calibrate at 0 ppt, but when I use calibration fluid it only shows about 34 ppt so may refracto is about 1 ppt off at that range. When I calibrate at 35 ppt and use RO
water, it's slightly over 0 ppt.