Density
A number of techniques as well as standards exist for the measurement of density of materials. Such techniques include the use of a hydrometer (a buoyancy method for liquids), Hydrostatic balance (a buoyancy method for liquids and solids), immersed body method (a buoyancy method for liquids), pycnometer (liquids and solids), air comparison pycnometer (solids), oscillating densitometer (liquids), as well as pour and tap (solids). However, each individual method or technique measures different types of density (e.g. bulk density, skeletal density, etc.), and therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question.
Saltwater intrusion
The figure shows the Ghyben–Herzberg relation. In the equation, formula_1 the thickness of the freshwater zone above sea level is represented as formula_2 and that below sea level is represented as formula_3. The two thicknesses formula_2 and formula_3, are related by formula_6 and formula_7 where formula_6 is the density of freshwater and formula_9 is the density of saltwater. Freshwater has a density of about 1.000 grams per cubic centimeter (g/cm) at 20 °C, whereas that of seawater is about 1.025 g/cm. The equation can be simplified to formula_10.