My red sea refractometer says to use ro/di water to calibrate. While using 35ppt solution seems better, do the solutions on the market actually have the calcium, magnesium, etc in them that seawater does? Are they adjusted to read 35ppt on a seawater refractometer if not? 35ppt seawater and 35ppt NaCl will not give the same reading. Just as a normal refractometer will give different results than a seawater refractometer on the same solution. Every solution I find says pure or high quality na/cl at 35ppt, but will this solution be 35ppt on a seawater refractometer or will it read 35ppt on a brine refractometer? Any experts out there? I am loving my salinity for hypo and small differences mater. @Randy Holmes-Farley