I can understand the pressure increase below,but why did pressure decrease above as more bricks were added?

Image source: https://www.youtube.com/watch?v=4REdJIW6-Kg

Piston pressure 2

Piston pressure 1


1 Answer 1


Because as the piston was moved down by the mass of the bricks the volume above the piston was increased.

With the valve closed then the fluid expanded to still fill that increased volume and the pressure decreases.

  • $\begingroup$ Isn't oil incompressible, though? $\endgroup$ Commented Apr 3, 2023 at 16:58
  • $\begingroup$ oil is considered "nearly" incompressible in most applications. Small changes in volume can increase/decrease pressures dramatically in a closed system. $\endgroup$
    – user13416
    Commented Apr 3, 2023 at 22:03
  • $\begingroup$ "Isn't oil incompressible, though?" It might be incompressible but it is pressurisable (if that's a word). Think of water pressure increasing with depth in the ocean. It's not compressed but it is under increasing pressure with depth. $\endgroup$
    – Transistor
    Commented Apr 4, 2023 at 7:21
  • $\begingroup$ @OsvaldoSilva check out engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html then think about what might happen to a fluid subjected to reduced pressure... gas coming out of solution? $\endgroup$
    – Solar Mike
    Commented Apr 4, 2023 at 7:34

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