 The corresponding "potential density anomaly" is denoted by formula_5 kg/m3.Because the compressibility of seawater varies with salinity and temperature, the reference pressure must chosen to be near the actual pressure to keep the definition of potential density dynamically meaningful.Reference pressures are often chosen as a whole multiple of 100 bar; for water near a pressure of 400 bar (40 MPa), say, the reference pressure 400 bar would be used, and the potential density anomaly symbol would be written formula_6.