Hydrometer is an instrument used for measurement of density (weight per unit volume) or specific gravity (relative density, the ratio of the density of the liquid to the density of water) of liquids. Hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits. In low-density liquids such as kerosene, gasoline, and alcohol, the hydrometer will sink deeper, and in high-density liquids such as brine, milk, and acids it will not sink so far. In fact, it is usual to have two separate instruments to provide more precise measurements.

Hydrometer Design Construction and Operation

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 to be tested is poured into a tall container, often a graduated cylinder, 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 scale inside the stem, so that the specific gravity can be read directly. A variety of scales exist, and are used depending on the context. Operation of the hydrometer is based on Archimedes’ principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced. The Gravity/Density Measurement Scales used in Hydrometer are:

  • API gravity, universally used worldwide by the petroleum industry.
  • Baumé scale, used in industrial chemistry and pharmacology
  • Brix scale, primarily used in fruit juice, wine making and the sugar industry
  • Oechsle scale, used for measuring the density of grape must
  • Plato scale, primarily used in brewing
  • Twaddell scale, used in the bleaching and dyeing industries

Hydrometer Types and Applications

  • Lactometer (or Galactometer) is a milk quality testing hydrometer.
  • Alcoholometer is a hydrometer used for determining the alcoholic strength of liquids. It is also known as a proof and traille hydrometer.
  • Saccharometer is a hydrometer used for determining the amount of sugar in a solution. It is used primarily by winemakers and brewers, and it can also be used in making sorbets and ice-creams.
  • 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.
  • Urinometer is a medical hydrometer designed for urinalysis. As urine’s specific gravity is dictated by its ratio of solutes (wastes) to water, a urinometer makes it possible to quickly assess a patient’s overall level of hydration.
  • Barkometer is calibrated to test the strength of tanning liquors used in tanning leather.
  • Battery Hydrometer – The state of charge of a lead-acid battery can be estimated from the density of the sulphuric acid solution used as electrolyte. A hydrometer calibrated to read specific gravity relative to water at 60 degrees Fahrenheit is a standard tool for servicing automobile batteries.
  • Antifreeze Tester Hydrometer – Another automotive use of hydrometers is testing the quality of the antifreeze solution used for engine cooling. The degree of freeze protection can be related to the density (and so concentration) of the antifreeze.
  • Acidometer (Acidimeter) is a hydrometer used to measure the specific gravity of an acid.
  • Soil Analysis Hydrometer – Hydrometer soil analysis is the process by which fine-grained soils, silts and clays, are graded. Hydrometer analysis is performed if the grain sizes are too small for sieve analysis.