Description
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: those scaled for liquids heavier than water those 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.
A hydrometer is 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. The hydrometer is then placed in a 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.
Improving Accuracy
Accuracy depends on three main factors:
- 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.
- Temperature: The hydrometer and liquid should be the same temperature of the surrounding atmosphere. This will prevent changes in density during the testing.
- Proper immersion: A hydrometer jar should have an inside diameter of approximately one inch (25 mm) greater than the outside diameter of the hydrometer. The jar should also be at least one inch taller than the hydrometer.