ELASTICITY

BULK MODULUS, B

• The Bulk Modulus Elasticity is a material property characterizing the compressibility of a fluid - how easy a unit of the fluid volume can be changed when changing the pressure working upon it.

• The Bulk Modulus Elasticity can be expressed as

E = - dp / (dV / V) (1)

where

E = bulk modulus elasticity

dp = differential change in pressure on the object

dV = differential change in volume of the object

V = initial volume of the object

• The Bulk Modulus Elasticity can be alternatively expressed as

E = dp / (dρ / ρ) (2)

where

dρ = differential change in density of the object

ρ = initial density of the object

• An increase in the pressure will decrease the volume (1). A decrease in the volume will increase the density (2).

• The SI unit of the bulk modulus elasticity is N/m² (Pa)
• The imperial (BG) unit is lb_f/in² (psi)
• 1 lb_f/in² (psi) = 6.894 10³ N/m² (Pa)

• A large Bulk Modulus indicate a relative incompressible fluid.
• Bulk Modulus for some common fluids can be found in the table below:

Bulk Modulus - E	Imperial Units - BG (105 psi, lbf/in2)	SI Units (109 Pa, N/m2)
Acetone	1.34	0.92
Carbon Tetrachloride	1.91	1.32
Ethyl Alcohol	1.54	1.06
Gasoline	1.9	1.3
Glycerin	6.31	4.35
Mercury	41.4	28.5
Paraffin Oil	2.41	1.66
Petrol	1.55 - 2.16	1.07 - 1.49
SAE 30 Oil	2.2	1.5
Seawater	3.39	2.34
Water	3.12	2.15