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Fig 1: Hydrometer
Fig 1


Floating instrument which measures the relative density of a liquid.


Hydrometers are used in many industries to measure the relative density (known as the specific gravity) of liquids.

Usually consisting of a graduated glass tube above a bulbous weight, a hydrometer floats in a liquid at varying levels depending upon the density. The point at which the liquid level touches the graduated stem is recorded in grams per cubic centimetre (see Fig 1).

Hydrometers with different scales are available to test liquids of varying densities- these have different sized weights and/or more air in the enclosed tube.

The function of the hydrometer is based on an Archemedes principle that a solid suspended in a liquid will be buoyed up by a force equal in weight to the water displaced. Therefore the lower the density of the liquid, the lower the hydrometer will sink.

Hydrometers used for certain applications may be known as:

  • An alcoholometer is a hydrometer which is used for determining the alcoholic strength of liquids
  • A saccharometer is a hydrometer used for determining the amount of sugar in a solution
  • A thermohydrometer is a hydrometer that has a thermometer enclosed in the float section


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