Hydrometer

A hydrometer is an instrument for measuring the density or specific gravity of liquids. It operates on Archimedes' principle: a sealed float displaces a volume of liquid proportional to its density, and the weight of the float is balanced by buoyant force. The depth to which the hydrometer sinks in a sample determines its density, and this depth is read from a calibrated scale on the instrument's stem.

Most hydrometers are glass with a weighted bulb and a long narrow stem holding one or more density scales. They are calibrated to a reference temperature, often 20 or 25 degrees Celsius, because liquid density changes with temperature. To take a measurement, the hydrometer is immersed in a sample and allowed to settle; the sample must be free of foam and must reach rest before reading. In some designs a mirror or indented line is used to aid reading.

Common scales include specific gravity, density, and saccharometer or Baume scales used in sugar solutions, as well as API gravity or other industry-specific scales. Hydrometers are widely used in winemaking and brewing to estimate sugar content and potential alcohol, in the oil and petroleum industry to gauge crude density, in battery maintenance to check electrolyte density, and in medicine for urine specific gravity.

Limitations include temperature sensitivity, sample cleanliness, and potential errors from air bubbles or non-homogeneous samples. Proper calibration, temperature compensation, and appropriate materials for corrosive liquids extend accuracy and service life.