Selecting a Hydrometer on Amazon.com

A hydrometer measures the specific gravity of a liquid, indicating whether it is more or less dense than an equal volume of water. Many hydrometers, similar in appearance to a thermometer, are made of glass and have a weighted bulb at the bottom and a cylindrical stem with a scale. The bulb end is placed into a cylinder containing the sample being tested, the hydrometer is allowed to float, and the scale is read where the surface of the liquid touches the stem. A test cylinder is included with certain hydrometers. A digital hydrometer has a readout, usually an LCD, that displays readings. Some digital models also measure temperature, have data logging capability, and have an output for transferring data to a computer. Hydrometers are used in applications such as manufacturing, scientific research, water quality testing, brewing, food processing, and automotive maintenance.

Among the factors to consider when selecting a hydrometer are its testing range, accuracy, and graduations. Hydrometers are most often calibrated for a specific material such as alcohol, saline or brine, sugar, milk, or soil. An alcohol hydrometer, also known as a proof and Tralle hydrometer, generally has a triple scale to determine not only specific gravity, but also sugar content and potential alcohol to determine the proof of alcohol. A battery hydrometer measures the specific gravity of battery acid to determine the state of charge.

In addition to being calibrated for a specific material, a hydrometer may be calibrated for a specific reference and sample temperature, or temperature range, since density varies with temperature. For optimum results, the liquid should be measured at the specified sample temperature. Certain digital hydrometers offer temperature compensation to account for variations in sample temperature. Some instruments meet standards such as National Institute of Standards and Technology (NIST) and ASTM International.