Salinity Refractometers are used to measure the salinity, salt content, of sea water or salt water. Sea water contains different ions other than just sodium chloride, therefore the refractive index results will be different.
MASTER-S/Millα Handheld Seawater Salinity Refractometer, Salinity: 0 - 100‰ (ppm), ±2‰, Specific Gravity: 1.000 - 1.070, ±0.001, ATC
MASTER-S/MillM Handheld Seawater Salinity Refractometer, Seawater Salinity 0 to 100‰ (ppm), Specific Gravity: 1.000 to 1.070
PAL-03S (Sodium Chloride in water, w/w) Handheld Digital Sodium Chloride Refractometer 0.0 to 28.0% (g/100g)