Velocity of the transmission of pressure in a fluid (speed of sound). It is dependent on the bulk modulus K of the medium and its density ρ:
In air in the normal state it is 333 m/s. In mineral oil it is dependent on its air content. In pure oil it is approx. 1 500 m/s and drops rapidly with undissolved air in the oil. For this reason an average value of 1 300 m/s is used in the majority of cases.
In hose lines the velocity of sound pressure waves drops again significantly due to the flexibility of the wall.