Parameter for the volume elasticity of a fluid, comparable to the modulus of elasticity of solid substances.
Unlike the modulus of elasticity, however, the (genuine) bulk modulus is not a constant value, but is dependent on changes in pressure and volume:
This genuine bulk modulus determines the stiffness of an oil column.
In order to calculate the reduction in volume in advance, i.e. the compression volume V k = V o – V(Figure K 26), all that is required is the "average" bulk modulus:
where V o = volume under pressure,
Δ V k = compression volume.
The size of the bulk modulus for air-free oil at 50°C is around 1.5 · 105 N/cm2. Air in the form of bubbles reduces the bulk modulus significantly (at least for pressures under 200 bar). In practice therefore, the bulk modulus is often 1.0 to 1.2 x 105 N/cm2 (see also equivalent bulk modulus).
Figure K 26: Definition of the average or real bulk modulus