Question

If the pressure at the surface of a tank of water is 0. What is the water pressure 20 ft below the surface in units of psi (pounds per square inch)? Note the density of water is 1.94 slugs/ft³.

Show all unit conversions

Answer 1

pressure at any depth can be found as

P = gh

where is density, g is gravity and h is depth below surface

slug can be written as lb.sec²/ft

Here are some unit conversions

so,

1.94 slugs/ft³ = 1.94  lb.sec²/ft⁴ = 9.355e-5 lb.sec² / in⁴  

g value = 386.09 in / sec²

1 ft = 12 inch

so,

P = 9.355e-5 lb.sec² / in⁴  * 386.09 in / sec² * 240 in

P = 8.67 lb / in²

or

P = 8.67 psi