Hydrometer

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A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water.

A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. The liquid is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which 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.

The operation of the hydrometer is based on the Archimedes principle that a solid suspended in a liquid will be buoyed up by a force equal to the weight of the liquid displaced. Thus, the lower the density of the substance, the lower the hydrometer will sink.

In light liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in heavy liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments, one for heavy liquids, on which the mark 1.000 for water is near the top of the stem, and one for light liquids, on which the mark 1.000 is near the bottom. In many industries a set of hydrometers is used — covering specific gravity ranges of 1.0–0.95, 0.95–0.9 etc — to provide more precise measurements.

[edit] Commercial uses

Image:Checking the specific gravity.jpg

A homebrewer using a hydrometer

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A modern hydrometer in a sugar solution

Because the commercial value of many liquids, including alcohol beverages such as beer and wine, depends directly on the specific gravity, hydrometers are used extensively.

An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids. It is also known as a proof & traille hydrometer.

A saccharometer is a hydrometer used for determining the amount of sugar in a solution. It is primarily used by brewers and winemakers.

A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section.

For measuring the density of petroleum products, like fuel oils, the specimen is usually heated in a temperature jacket with a thermometer placed behind it since density is dependent on temperature. Light oils are placed in cooling jackets, typically at 15^oC. Very light oils with many volatile components are measured in a variable volume container using a floating piston sampling device to minimize light end losses.