The Extech RF20 is a portable analog refractometer designed for precise measurement of salinity in the range of 0 to 100 parts per thousand (ppt), with an integrated Automatic Temperature Compensation (ATC) system that operates effectively between 10°C and 30°C (50°F to 86°F). It also displays refractive index from 1.000 to 1.070. Ideal for aquarium maintenance, marine biology, aquaculture, and environmental testing, the RF20 delivers accurate, repeatable results with just 2–3 drops of sample. The unit includes a protective carrying case, calibration screwdriver, and calibration solution.