hydrometer | The Oxford Companion to Spirits & Cocktails

, which measures the densities of liquids, is an ancient invention that has been used to gauge the alcoholic strength of spirits since the eighteenth century. Like distillation itself, the hydrometer (or, to be precise, alcoholometer) relies on the difference in density between ethanol and water. Since ethanol is lighter than water, the lower a spirit’s density, the higher its proof and the deeper a weighted float will sink into it—the principle behind the device.

The first practical hydrometer for gauging the strength of spirits was invented and marketed by John Clarke, a London instrument maker, in the mid-1720s. It took the form of a hollow copper or brass globe with a long, flat graduated stem protruding from it and, opposite the stem, a spur onto which one of a set of calibrated weights was screwed, chosen according to the liquid’s temperature (which affects the density of the liquid) and range of expected proof. With the right weight, the surface level of the spirit falls in the (narrow) range marked on the stem and can be measured.

After decades of informal use, Clarke’s device was officially mandated in 1787. It was not entirely satisfactory, however, as it required at least thirty-four and (eventually) as many as three hundred different weights and used an archaic mode of calibration, displaying its results as the proportion of water one had to add to, or remove from, the spirit being measured to bring it to proof (50 percent alcohol, by weight). In 1817, Clarke’s hydrometer was replaced by a streamlined version developed by the master exciseman Bartholomew Sikes (1731–1803), which used only nine weights and was accompanied by an exhaustive book of tables that, accounting for temperature, enabled the gauger to precisely determine the percentage of alcohol by which a spirit was over or under proof. In Britain, Sikes’s hydrometer remained in use until the 1970s, its little wooden cases—each containing the brass hydrometer; its precisely cut, neatly arrayed brass weights; and a thermometer—becoming common fixtures wherever spirits were distilled, bottled, or wholesaled.While Britain relied on the variable-weight brass hydrometer and the system of measuring alcohol by weight, most other countries, including France, Germany, and the United States, preferred to use the “constant weight hydrometer.” This came in the form of a small set of glass hydrometers with much longer stems, allowing each to measure a much broader range of proofs than the short-stemmed British ones. Coupled with them were similar tables to Sikes’s, except these went one step further and converted the percentage of alcohol by weight to the percentage by volume according to the French Gay-Lussac or Cartier or German Tralles systems, which differ only in detail. These are still in use today, although they have been partly supplanted by electronic refractometers.

It should be noted that all buoyancy-based hydrometers will yield false readings if the spirit has been sweetened or had its density otherwise manipulated; this is known as “obscuration.” Such a spirit must be distilled down to its components to determine its precise proof.