Red Sea Hydrometer-Thermometer is a saltwater aquarium hydrometer with digital thermometer for continuous (when submerged) or periodic reading of specific gravity in saltwater aquariumsUnit is calibrated for use at 25 deg C (77 deg F)Hydrometer : floating needle type with readable scale from 1.010 to 1.030 in 0.001 graduationsAll swing-arm plastic hydrometers for aquarium use are only accurate when used within the normal range of operating temperatures for marine aquariums (approx 25 deg C / 77 deg F). A digital thermometer to ensure that the water being tested is at the correct temperature.