Hydrometers

A hydrometer is an instrument used to measure the relative density, or specific gravity, of liquids. It operates on Archimedes’ principle: a floating body displaces a weight of fluid equal to its own weight, so the depth to which the device sinks depends on the liquid’s density. The stem is graduated so the point where the liquid’s surface intersects the stem corresponds to a density value. Temperature affects density, so readings are usually corrected to a standard reference temperature.

Construction and scales: Most hydrometers consist of a weighted bulb and a long, narrow stem, typically made of glass or plastic. They are calibrated to read specific gravity or density, and may carry related scales such as Baumé or Brix. Some models are designed for concentrated solutions, viscous liquids, or immiscible mixtures.

Usage: Clean the container and the hydrometer, fill a tall, clear cylinder with the test liquid, and gently lower the hydrometer until it floats freely. Avoid air bubbles clinging to the stem. Let the instrument settle and read at the bottom of the meniscus, at eye level. Readings are specified at a stated temperature, and a temperature correction may be applied if necessary.

Applications: Used in brewing and winemaking to estimate sugar content or potential alcohol, in battery maintenance to check electrolyte density, and in various industries to assess concentration of liquids. They are also employed in environmental testing and soil science for density-related measurements.

Limitations and history: Readings can be influenced by temperature, viscosity, surface tension, or foaming liquids. Hydrometers require sufficient sample volume and a suitable container. The term hydrometer comes from Greek hydōr (water) and metron (measure); the concept dates back to antiquity, with modern instruments developed in the 17th–19th centuries.