Density and bulk modulus dependency

Density and bulk modulus dependency The density of the hydraulic fluid varies with pressure and with air content. The isothermal bulk modulus of the hydraulic fluid is defined to be:

Equation 2: Fluid bulk modulus as a function of the fluid density

where ρ is the density [kg/m3] of the fluid, P its pressure [Pa], B the bulk modulus [Pa] and T the current temperature [K] of the fluid. The subscript T and X mean that the bulk modulus is computed at constant temperature and constant composition.

If we integrate the Equation 2 from Patm to the current pressure P and for any temperature T, we obtain:

Equation 3: Density as a function of the bulk modulus

Once one of the quantities density or bulk modulus is defined, the other is also defined through Equation 2 or Equation 3. It is essential that values for the density and the bulk modulus are fully consistent with these formulae. The consequence of neglecting these relations is the loss of mass conservation. This can show up quite dramatically if there is cavitation.