Mass flow meter
The mass flow meter does not measure the volume per unit time (e.g., cubic meters per second) passing through the device; it measures the mass per unit time (e.g., kilograms per second) flowing through the device. Volumetric flow rate is the mass flow rate divided by the fluid density. If the density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles.
Actual density can be determined due to dependency of sound velocity on the controlled liquid concentration.
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.