Hydrometers are instruments that measure the density of liquids. Although hydrometers actually measure density, measurements are commonly expressed in terms of specific gravity; which is the ratio of the density of the substance being measured to the density of water at the same temperature. In precise measurements, the reference may be to pure (double-distilled) water at 4°C (39.2°F). In engineering practice, the reference frequently is to pure water at 15.6°C (60°F); the value of unity established for water is 1.000. Thus liquids with a specific gravity less than 1.000 are lighter than water and those greater than 1.000 are heavier than water.
The specific gravity for a given gas is the ratio of the density of the gas to the density of air. Since the density of air varies markedly with both temperature and pressure, precise measurements should reflect both conditions. Common reference conditions are 0°C at 1 atmosphere.
Hydrometers work on Archimedes principal, that is, the upthrust on the body immersed in a fluid (either liquid or gas) equals the weight of the fluid displaced. According to this principal, the hydrometer will sink until it displaces its own mass. The volume of liquid displaced is indicated by the level of the scale and the density is equal to the mass divided by the volume. Since fluid density
fluctuates with temperature, proper temperature corrections must be made. A separate thermometer or a combined hydrometer containing its own thermometer should be used to determine the temperature of the sample.
