The Problem With Hydrometers by Robert Frease and Brian Christman Hydrometry is a popular method of salinity analysis among estuarine water quality monitoring programs. However, many groups are not aware of a problem concerning the hydrometric method as published in Standard Methods for the Examination of Water and Wastewater through 1985. In fact, most hydrometers manufactured today are based on a different scale than that used to generate the conversion tables published in Standard Methods . Measuring Salinity by Using a Hydrometer First let's review the basic principles involved in using a hydrometer to measure salinity, starting with the definition of density.
Density is dependent upon temperature.
If a sample of water is warmed, the water will expand and occupy a larger volume. However, the mass remains constant, so density decreases. Conversely, as the water is cooled, it contracts and the density increases. If the sample is diluted, the density decreases. Specific gravity is the ratio of the densities of two substances.