### Question

How far down can an object go in the ocean before it will reach 10,000 PSI?

Asked by: S. Poirier

### Answer

You can estimate this yourself using the fact that seawater has a density of about 64 lb.
per cubic foot. Since 10,000 psi means that a column of water above one square inch would
weigh 10,000 lbs., ignore any compression or temperature effects and calculate how tall that
column would have to be.

Because a 12 x 12 x 12 inch cube of water weighs 64 lbs. each of the 144 (12 x 12) square
inches of the cube's base supports 64/144 = .444 lbs. of water. That means that pressure
increases by .444 psi for every foot of depth below the surface. Dividing 10,000 psi by
444 tells you that you would need a column of water about 22,500 feet high to get a weight
of 10,000 lbs. over every square inch. That means you would need to descend to a depth of
22,500 ft/5,280 ft per mile or approximately 4.25 miles.

Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor