A hydrometer is an instrument used to determine specific gravity. It operates based on the Archimedes principle that a solid body displaces its own weight within a liquid in which it floats. Hydrometers can be divided into two general classes: liquids heavier than water and liquids lighter than water. In the standard hydrometer scale, known as the specific gravity scale, distilled water equals 1.000, the initial point of measurement. Liquids lighter than water are scaled below 1.000 specific gravity, and liquids heavier than water are scaled above 1.000 specific gravity.

Function, Scale Types and Use

A hydrometer is made up of a thin glass or plastic tube sealed at both ends with a graduated or printed scale calibrated to a specific gravity. One end of the tube is bulb shaped and weighted with a ballast of either fine lead shot or steel shot. The ballast causes the instrument to float upright in a liquid like a fishing bobber. A second glass or plastic cylinder, commonly known as a hydrometer jar, is filled with the liquid being measured. The hydrometer is then placed in the hydrometer jar containing the sample liquid. The specific gravity of the sample liquid is indicated when the level of the sample liquid in the jar aligns with a point on the hydrometer scale. Depending on which scale is used, the number of times heavier or lighter than water the sample liquid weighs can now be recorded. In addition to reading specific gravity values, scales on a hydrometer can be calibrated to Baume, Brix, Alcohol, API (American Petroleum Institute Index) and others for specific chemicals.

Brix Scale A hydrometer calibrated to read in degrees of Brix or Balling, or percent of pure sucrose (sugar) by weight.

Baume Scale A hydrometer calibrated to read degrees of Baume, which is a pair of scales: one for liquids heavier than water and one for liquids lighter than water.

Alcohol Scale A standard specific gravity hydrometer used to measure specific gravity before and after a liquid ferments. The difference of the two specific gravity readings is referenced to an alcohol scale to determine percent alcohol by weight

API Scale The API scale is a measure of how light or heavy a petroleum-based liquid is compared to water. It was designed to allow a comparison between densities of petroleum liquids.

Method of Reading

Some hydrometers have one or more scales printed on the hydrometer. To use the hydrometer, fill the hydrometer jar with the sample liquid. Place the hydrometer in the jar and give it a quick twirl to dislodge any air bubbles. Once the hydrometer has settled, take the reading from the appropriate scale. In order for the measurement to be accurate, the sample liquid must be at 15.5°C. If the liquid is not at 15.5°C , the measurement should be adjusted.

When reading transparent liquids, the eye should be placed slightly below the plane of the surface of the liquid and then raised slowly until this surface, seen as an ellipse, appears as a straight line. The point at which the line sits on the hydrometer scale should be recorded as the reading of the hydrometer. When a liquid is not sufficiently clear and a reading cannot be made as described above, it will be necessary to read from above the surface and estimate as accurately as possible, the point to which the liquid rises on the hydrometer.

Accuracy

Accuracy depends on three main factors:

1. Cleanliness The hydrometer, hydrometer jar and the liquid in which the readings are taken should be cleaned properly. The surface of the hydrometer and the stem are especially important so that the liquid can rise uniformly, merging into an almost invisible film on the stem.

2. Temperature The hydrometer and liquid should be the same temperature of the surrounding atmosphere. This will prevent changes in density during the testing.

3. Proper Immersion A hydrometer jar should have an inside diameter of approximately 1 inch (25mm) greater than the outside diameter of the hydrometer.