# Bulk Modulus

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics Bulk modulus, abbreviated as B or K, also called bulk modulus of elasticity or modulus of volume expansion, is the elastic relationship between an applied pressure that acts to change the volume of the substance and the ability of a substance to withstand changes in volume when under compression.

Most liquids like oil or water are considered incompressible.  In practice, all materials are compressible to a certain extent.  The bulk modulus of a liquid is the measure of compression with an increase in pressure.  A material with high bulk modulus will undergo lesser compression than a material with a low bulk modulus.  Therefore the hydraulic fluid must have high bulk modulus for proper working.

## Bulk modulus formula

$$\large{ K = -V_i \; \frac { \Delta p }{ \Delta V } }$$
Units Metric Metric
$$\large{ K }$$ = bulk modulus $$\large{\frac{lbm}{in^2}}$$  $$\large{ Pa }$$
$$\large{ \Delta p }$$ = pressure change $$\large{\frac{lbf}{in^2}}$$ $$\large{ Pa }$$
$$\large{ \Delta V }$$ = volume change $$\large{ in^3 }$$ $$\large{ mm^3 }$$
$$\large{ V_i }$$ = initial volume $$\large{ in^3 }$$ $$\large{ mm^3 }$$ 