Nautical Hydrometers

Hydrometer is an instrument used to determine specific gravity, especially a sealed, graduated tube, weighted at one end that sinks in a fluid to a depth used as a measure of the fluid's specific gravity. It is usually made of glass with a graduated stem and indicates the specific gravity of a liquid by the depth to which it sinks in it, the zero of the scale marking the depth to which it sinks in pure water. Extra weights are sometimes used to adapt the scale to liquids of different densities.

A good quality nautical hydrometer is important to maintain optimal salinity. Hydro meters work on Archimedes principal, that is, the upthrust on the body immersed in a fluid equals the weight of the fluid displaced. The point where the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity can be read directly in grams per cubic centimeter. In light liquids like kerosene, gasoline, and alcohol, the hydrometer must sink deeper to displace its weight of liquid than in heavy liquids like brine, milk, and acids.