Kgf/cm2 help (subsea pressure)

[cps.13]

Hi,

I was wondering (in a subsea context) how to interpret 10kgf/cm2. If for example I have a box with a surface area of 400cm2, at 100m depth you have roughly 10kgf/cm2 acting upon it.

Would this work out to be 10/400 = 0.25kgf average force against the whole box?
OR
Would this work out to be 10*400 = 4000kgf acting upon the whole box (seems doubtful!)?
OR
Am I completely wrong?

thanks,

 

[jbriggs444]

It would be 4000 kilograms force if applied to a flat 400 square cm surface. As applied to a box, you would have a force on each of the six faces.

Are you interested in the force that is squeezing the box (i.e. how strong the sides need to be) or the net upward force of buoyancy supporting the box?

 

[cps.13]

Oh right, that's much more force than I expected.

I am interested in the force squeezing the box.

 

[jbriggs444]

If you are interested in the vertical squeezing force then you would care about the area of the top and bottom of the box only. The force on those two sides would be would be [nearly] equal and opposite. The vertical squeezing force would be the force from one of those faces, not the sum of both.

 

[cps.13]

Thanks - I will do some calculations!

does the same apply for a cylinder or is there a different rule?

thanks

 

[jbriggs444]

The total force on an object (flat, curved or irregular -- it does not matter) in a given direction is equal to the cross section of the object in the plane perpendicular to that direction multiplied by the pressure of the fluid.

Imagine that you divided a cylinder in half, cutting it in two lengthwise. The total force of the water on the one half will be equal to the cross-section of that half times the pressure of the water. The total force of the water on the other half will be [nearly] equal and opposite. The squeezing force at the dividing line will be equal to the force on either of the two halves. It doesn't matter which since the two forces are equal.

 

[cps.13]

Thanks.

So if I had a cylinder 200mm long and with a diameter of 75mm it would be a cross-sectional area of 15cm2.
At a depth generating 10kgf/cm2 it would be a force of (10*15) 150kg
This acting on both halves of the cylinder would produce 300kg crushing force acting upon the cylinder

Am I correct?

 

[jbriggs444]

20 cm by 7.5 cm makes 150 cm²

 

[cps.13]

Ok - but apart from that arithmetic error is the rest of it correct?

 

[jbriggs444]

Ok - but apart from that arithmetic error is the rest of it correct?

Twice above I mentioned that you should only count the pressure from one half, not both.

 

[SteamKing]

If you are interested in the vertical squeezing force then you would care about the area of the top and bottom of the box only. The force on those two sides would be would be [nearly] equal and opposite. The vertical squeezing force would be the force from one of those faces, not the sum of both.

A fully submerged box will have pressure applied to all six faces, not just the top and bottom surfaces.

The difference in force due to the hydrostatic pressure applied to the top and bottom surfaces should equal the buoyant force acting on the box.

There is a pressure gradient applied to the sides which also must be taken into account if one is interested in seeing if the box can be crushed horizontally by hydrostatic forces. See the following graphic: