# Force required to move a fluid out of piston at depth

I have a cylinder filled with water in it at the bottom of the ocean. Say 100 meters down. In this cylinder is a piston that moves up and down. Its job is to squish the water out the bottom of the cylinder to create an air space in the top half of the cylinder. There are lots of holes to let the water move freely out of the bottom of the piston. There is also plenty of air lines to the surface of the ocean that will let air freely into the top half of the cylinder.

Assuming we are at sea level and the density of sea water to be 1025 kg/m³, pressure increases by 1 atm with each 10 m of depth.

So the cylinder is under about 10.9204 atm of pressure from the surrounding water at 100 meters down.

It is my understanding to get the piston to move down and squish the water out of the cylinder you need to overcome the 10.9204 atm of surrounding water pressure.

My question is this. If you were to push on the ocean bottom piston with another piston of the same displacement located at the surface of the ocean would that not make the 10.9204 atm of pressure irrelevant? (the pistons would we connected with hydraulic lines)

My thinking is the Elevation head due to the fluid's weight, the gravitational force acting on a column of fluid in the hydraulic line from the piston above is already exerting 10.9204 atm of pressure.

Is this correct or am I missing something?

• Yes, if you use the head of seawater to help drive the piston, you get the benefit of the weight of that column of water. On the other hand, you get a lot of friction and the expense of all that tubing... May 28, 2015 at 12:59