Compression volume ΔVK

Volume which must also be compressed into a space due to the compressibility of a fluid in order to generate a specific pressure there:

where V o = volume under pressure,

Δ p = pressure increase in the space.

In the typical pressure ranges up to 400 bar, V K becomes approximately 0.7 % of V o for each 100 bar. This means, for example, that if V o = 300 dm3 and p = 300 bar, some 6 l of oil are required to reach the pressure level.

This calculation only covers the compressibility of oil. The strain volume associated with the container wall stills needs to be added. It is often possible to disregard this with steel, although not with pressure hoses. This gives for the oil and container together the equivalent bulk modulus.

If the increase in pressure should take place within a specific time, this will determine the size of pump. Example: an increase time of 5 s in the above case would require a pump with 12 dm3/s.