The Hikari Accuprobe Hydrometer "Dry Hands" design avoids contamination of aquarium or water sample Takes a more "true" water sample at a 6-inch depth avoiding inaccurate surface sampling. Accurate to 0.001 specific gravity & salinity (PPT) measurements. Auto-temperature correcting design. Durable construction for extended life. Designed to give a temperature-corrected reading in warm water aquariums - 68F to 85F (20C to 30C).