Fluid Compressibility of with density $\begin{array}{l}\rho_f\end{array}$ and molar volume $\begin{array}{l}V_{mf}\end{array}$ :

(1)	$\begin{array}{l}\displaystyle c_f = \frac{1}{\rho_f} \frac{\partial \rho_f}{\partial p} = -\frac{1}{V_{mf}} \frac{\partial V_{mf}}{\partial p}\end{array}$

The multi-phase compressibility is a linear sum of compressibilities of its phases.

Approximations

Incompressible fluid

(2)	$\begin{array}{l}\displaystyle c_f(p) = 0\end{array}$

(3)	$\begin{array}{l}\displaystyle c_f(p) = \rm const\end{array}$

(4)	$\begin{array}{l}\displaystyle c_f(p) = \frac{1}{p}\end{array}$

(5)	$\begin{array}{l}\displaystyle c_f(p) = \frac{c_0}{1+c_0 \cdot p}\end{array}$