Milwaukee MA887 Digital Refractometer. The MR series refractometers are precision optical instruments used for measuring aquarium water salinity. They work using principle of light refraction through liquid. Specifications: * Range: PSU: 0 - 50 PPT: 0 - 150 S.G.: 1.000 - 1.114 * Resolution: PSU: 1 PPT: 1 S.G.: 0.001 Temp: 0.1 C/F * Accuracy: PSU: ±2 PPT: ±2 S.G.: ±0.002 Temp: ±0.3C / ±0.5F Temp: 0 - 80C / 32 - 176F * Calibration: automatic