The MA887 Milwaukee Instruments Seawater (Salinity) Refractometer is an optical instrument that employs the measurement of the refractive index to determine the salinity of natural and artificial seawater, ocean water or brackish intermediates. The digital refractometer eliminates the uncertainty associated with mechanical refractometers. To use, after calibrating, you just put a sample of your water into the prism tray of the Milwaukee Seawater Refractometer. Within seconds the refractive index and temperature are measured and converted into one of three popular measurement units; Practical Salinity Units, Salinity in parts per thousand or Specific Gravity. The measurement is displayed digitally on the LCD screen.