Hydrometer Definition in Science

What is a hydrometer and what is it used for?

This is an example of a common type of hydrometer used to measure specific gravity of a density column.
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A hydrometer or hydroscope is a device that measures the relative densities of two liquids. They are typically calibrated to measure the specific gravity of a liquid. In addition to specific gravity, other scales may be used, such as API gravity for petroleum, Plato scale for brewing, Baume scale for chemistry, and Brix scale for wineries and fruit juices. The invention of the instrument is credited to Hypatia of Alexandria in the latter part of the 4th century or early 5th century.

Key Takeaways: Hydrometer Definition

  • A hydrometer is an instrument used to measure liquid relative density based on buoyancy.
  • Usually, a hydrometer consists of a sealed tube that is wider at the bottom than the top and contains a heavy ballast. When placed in a liquid, the hydrometer floats. Markings on the tube's stem correlate to the liquid's relative density.
  • The function of a hydrometer is based on Archimede's principle. An object suspended in a fluid experiences a buoyant force equal to the weight displaced by the submerged portion of the object.

Hydrometer Composition and Use

There are several different types of hydrometers, but the most common version is a closed glass tube with a weighted bulb at one end and a scale going up the side. Mercury used to be used to weight the bulb, but newer versions may use lead shot instead, which is much less hazardous in case the instrument breaks.

A sample of liquid to be tested is poured into a sufficiently tall container. The hydrometer is lowered into the liquid until it floats and the point where the liquid touches the scale on the stem is noted. Hydrometers are calibrated for various uses, so they tend to be specific for the application (e.g., measuring fat content of milk or proof of alcoholic spirits).

How a Hydrometer Works

Hydrometers function based on Archimedes' principle or the principle of flotation, which states a solid suspended in a fluid will be buoyed up by a force equal to that of the weight of the fluid that is displaced. So, a hydrometer sinks further into a liquid of low density than into one of high density.

Examples of Uses

Saltwater aquarium enthusiasts use hydrometers to monitor the salinity or salt content of their aquariums. While the glass instrument may be used, plastic devices are safer alternatives. The plastic hydrometer is filled with aquarium water, causing a tethered float to rise according to salinity. Specific gravity can be read on the scale.

Saccharometer - A saccharometer is a type of hydrometer used to measure the concentration of sugar in a solution. This instrument is of particular use to brewers and winemakers.

Urinometer - A urinometer is a medical hydrometer used to indicate patient hydration by measuring the specific gravity of urine.

Alcoholmeter - Also known as a proof hydrometer or Tralles hydrometer, this device simply measures liquid density but isn't used to directly measure proof of alcohol, since dissolved sugars also affect the reading. In order estimate alcoholic content, measurements are taken both before and after fermentation. The calculation is made after subtracting the initial reading from the final reading.

Antifreeze Tester - This simple device is used to determine the ratio of antifreeze to water used for engine cooling. The desired value depends on the season of use, hence the term "winterizing" when it's important the coolant doesn't freeze.


  • Assaad, F.A.; LaMoreaux, P.E.; Hughes, T.H. (ed.) (2004). Field Methods for Geologists and Hydrogeologists. Springer Science & Business Media. ISBN:3540408827.
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Helmenstine, Anne Marie, Ph.D. "Hydrometer Definition in Science." ThoughtCo, Feb. 16, 2021, thoughtco.com/definition-of-hydrometer-605226. Helmenstine, Anne Marie, Ph.D. (2021, February 16). Hydrometer Definition in Science. Retrieved from https://www.thoughtco.com/definition-of-hydrometer-605226 Helmenstine, Anne Marie, Ph.D. "Hydrometer Definition in Science." ThoughtCo. https://www.thoughtco.com/definition-of-hydrometer-605226 (accessed February 1, 2023).