Hydrometer is an apparatus used to measure the density or specific gravity of worts and beers. See specific gravity. It relies on buoyancy and has the great value of allowing the brewer to measure how concentrated their worts are, and thus their potential for alcohol production. Hydrometers used in brewing are termed “saccharometers” to reflect their application to sugar solutions.

In construction a saccharometer is a sealed glass tube weighed down with an amount of lead suitable to allow it to float within the density band of the relevant liquid. The tube has a narrow neck that protrudes from the liquid and allows a scale to be read at the meniscus of the liquid. This scale is calibrated to give the density in terms of percentage of sugar, degrees Plato, or specific gravity.

The saccharometer was first applied to brewing in the late 18th century and despite initial resistance soon became indispensible in determining the strength and consistency of beers. Prior to its application the strength of worts and beers was determined by their taste or stickiness. Part of its promotion was by customs officers seeking a reliable determination of wort strength for taxation purposes.

Set of hand-blown mercury hydrometers with ivory measurement rulers. English, mid-19th century. charles finkel

As wort ferments, the specific gravity falls and the difference between the initial and final levels can be used to calculate the percentage of alcohol using a simple equation. The measurement of specific gravity depends critically on temperature and corrections are required if measurements are taken at a different temperature than that specified. More advanced saccharometers will contain a temperature correction scale inside the glass housing, simplifying calculation for the brewer.