formula for calculating pressure per sq. inch

"Does anyone have a formula for calculating the pressure exerted on a sheet of steel at various depths in salt water?"

Pressure, Barometers, and Barometric Pressure

That should help. Density of sea water wont change pressure signifigantlys.

Thank you.

netmaker

1 psi pressure for every 2.31 feet of depth.

Density of sea water wont change pressure significantly.

At any elevation, the pressure in salt water will be higher than the pressure in normal water at the same elevation, due to the heavier density of salt water. Therefore, when you calculate the pressure exerted by a certain column of liquid, you have to consider the weight of that column of liquid by multiplying its volume by its density.

Or calculate the pressure of normal water and multiply that pressure by specific gravity of liquid under consideration. I recommend to take sea water density into consideration.

Sp. Gr. = Density of liquid / Density of normal water.

OK........

Given:

a piece of hardware 42" x 21" rectangle

1500' of water

42x21= 882 sq.in

882/34' =25.94

25.94 x 14.7 = 381 pounds of pressure per square inch?

Is this correct?

netmaker

"Is this correct?" In a word, NO!

At the air to water surface the pressure is 14.7 psig

At a depth of 1500 ft. the pressure is:[1500ft.]/[2.31 ft./1 psi] = 649 psi.

If you are asking what pressure this exerts on the 42" x 21" sheet then the area of the sheet is 43" x 21" = 462 sq. in. one side.

Total pressure then is 462 sq.in x 649 psi = 299,838 lbs. or 149.9 TONS. for both sides it is twice that or 299.8 TONS.

These are close approximations as no account is taken of the fine points of salinity of sea water, compression with depth, etc. (BUT IT IS Good enough for government work!)

WHY THE INQUIRY AS TO PRESSURE ON A PLATE OR SHEET AT SUCH A DEPTH?

At a depth of 1500 ft. the pressure is:[1500ft.]/[2.31 ft./1 psi] = 649 psi.

That is good Stirling.

But if we take sea water with specific gravity 1.2, the pressure shall be : (649 psi) x 1.2 = 778.8 psi

Abdel - SG of sea water is nowhere near as high as 1.2. In the oceans, total dissolved solids varies from roughly 3 % w/w at the poles to 4 % at the equator. So maximum SG is about 1.04.

Cheers...Codey

"(BUT IT IS Good enough for government work!)"

Didn't realize there was about a 20% difference between 649 and 778.8.

However is makes little difference as far as the question of concern.

Thanks, SS

Hang on . . . The pressure will be around 780 psi, but that won't have an effect on SS sheet if both sides are subjected to that pressure! The yield strength of SS is much, much higher than 780 psi!

If the SS sheet is subjected to 780 psi on one side and 15 psi on the other, then the FORCE on the sheet will be 765 psi X 462 sq in = 353,430 lb_f.

I am looking into a plastic type repalcement on our deepwater otter doors.

The manufacturer was concerned about 'crush depths' of his material, which is as yet unseen or untried by me.

42x21 +/- is a standard size for the 4 meter sampling nets.

Thank you for the reply.

netmaker

Hello Netmaker - if you're talking about direct crushing of the material, as opposed to stresses in a structure, eg a sheet with 1500' on one side and no pressure on the other - these sorts of stresses are no problem. Common plastics, uPVC, PP, PE can be used with a design tensile stress (much more onerous condition) over 2 x what we're seeing here.

Codey

Thank you.

We will be trying the first set this August......... SW of the Cortez Banks. The material is a plastic wood....... a potential substitute for steel.

Thank you again.

netmaker

* These are traditional wooden and steel doors. We would be substituting the pvc plastic wood for the pine wood shown here.

Just a question, but .........

What would happen to any air pockets trapped inside the "plastic" wood when it is molded?

.....at the 1500' depth.

"What would happen to any air pockets trapped inside the "plastic" wood when it is molded? .....at the 1500' depth."

I would expect some caving in or local deformation in the areas of the air pocket(s). Otherwise should work as well as the wood.

1 foot of water heigth exerts .433 psi.

For a rough general purpose calculation, simply divide depth, in feet by 2 to get PSIG. Add 14.7 for PSIA, which can normally be ignored for your purposes.

Example:100 feet of water = 50 psi.This number is or course a little high,(about 15%) but good enough for general purposes, but this can be considered a built -in safety factor

If you have the same depth on both sides of the sheet, the strain across the sheet is zero.In a sluice type gate, the pressure will vary along the depth of the gate.

After seeing your phote, another thought comes to mind:

If you are talking about replacing the wood with the plastic-wood type decking material, you might want to check with the manufacturer of the material to see if it is rated for full submersible service.It may just disentigrate over long exposure to water under pressure, even though it may have a lifetime weather warranty.

Hope this helps.

I am at present not sure of what exactly this new material is.

The samples have not arrived at our shop.

The Otter doors will be subjected to a rather fast drop to the depths of 900' and then towed at 3 knots into 1200'-1500' of water where they will function for 15 minutes picking up a benthic sample.

The doors and net will then be hauled back onboard .

The doors will be subjected to 900' to 1500' feet of pressure for no more than 35 minutes tops and then retrieved (hauled back).

So......... with all of these calculations, what will be the total pressure, on the entire 42"x21" surface at 1500' in the Pacific ocean?

Thank all of you.

netmaker