The refractometer is a precision optical instrument used to determine the salt concentration in saltwater aquariums. Its exact value can be determined in seconds and can be read either as absolute salt concentration (°/°°, ppt) or as specific density (kg/l). When a beam of light enters the refractometer it is bent or "refracted" by the water in the test panel. This refraction is proportional to the salinity of the sample and can be directly read by viewing the internal scale. As salinity varies in inverse proportion to temperature refractometer is fully temperature compensated. The strong plastic carrying case contains the refractometer, a pipette for the water sample, a screw driver for calibration and full instructions.