Refractometer for seawater and marine fishkeeping aquarium 0-100 ppt This optical refractometer (salinity meter) has scales in the range of 0 - 100 ppt(0 - 10 %). The reading resolution is 1 ppt. It is designed for the measurement of seawater (marine aquaria) or very low concentrated salt solutions. The second scale is for reading of the salt solution density in range 1.000 to 1.070 g/cm3 (resolution 0.001 g/cm3).