If pressure increases, gasses and fluids become compressed. The relation between volume (V) and pressure (p) for gasses and fluids under the idealization of elasticity1 is expressed, using the compressibility modulus K, as follows:
    dV/dp = - V/K
And, using the fact that mass is conserved (pV = constant), this becomes:
It is apparent from the above equation that K has the same dimension as a stress; the dimension according to the SI is the Pascal (1 [Pa] = 1 [N/m2] = 1 [kg m-1 s-2])

The value of K increases with increasing pressure. However, for a large range of pressures the value of K for water (without gas bubbles!) is practically constant, namely equal to roughly 2.2 x 109 [Pa].

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1 In reality this is not the case, but the idealization holds up in most cases

July 8, 2001

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