Calculating Underwater Pressure
Most recent answer: 10/22/2007
- dee wood
atlanta, georgia - usa
Well, let’s see if we can work this one out. First, we have to understand how pressure is measured. If we were to take a one inch column of air all the way up through the atmosphere, it would weigh 14.7 pounds. For this reason, we like to say that the standard air pressure (at sea level) is 14.7 pounds per square inch (psi). Water weighs more than this, though. Each one inch column that’s one foot deep will weigh 0.445 pounds.
So if you’re right at sea level, the pressure will be 14.7 psi. And for every foot you go underwater, you add another 0.445 psi. So at one foot deep, the pressure would be 14.7 psi + 0.445 psi = 15.145 psi. And at two feet deep it would be 14.7 psi + 2*(0.445 psi) = 15.59 psi, etc. In order to get to 2 atmospheres worth of air pressure, you would need to get to the point where there’s 29.4 psi (2 times 14.7 psi). To get to 29.4 psi, it turns out that you would need to be 33 feet deep. (Since 14.7 psi + 33*0.445 psi = 29.4 psi.)
(published on 10/22/2007)
Follow-Up #1: density of water
- Brian Sargent (age 62)
(published on 11/03/2007)
Follow-Up #2: density of sea water
(published on 01/14/2008)
Follow-Up #3: SI pressure
- Scott Norton (age 18)
Trumbull, CT, 06611
(published on 09/17/2009)
Follow-Up #4: Water pressure under an ice cream cone and a test tube
- Jackie (age 16)
Pressure is how much force you feel per unit area. Every force that acts on a surface corresponds to a pressure. For example, when you press a button, you are putting pressure on it. In water (or air), the weight of the water (or air) above you exerts a force on you, so you feel water pressure (or atmospheric pressure). The presence of water pressure does not require air, so we can measure pressure directly under water. There are many ways to measure pressure. For example, we can measure how much force is exerted by water and divide it by the area of the detector.
You mentioned an important point that water pressure is the same at the same depth, regardless of the shape of the container. This may be a little counter-intuitive because there is more water in a giant ice cream cone than a giant test tube. But remember that the cone is also supporting some weight of the water! The surface of the cone is facing upward; while the wall of the test tube is vertical and cannot support any water. As a result, you would feel the same water pressure if you were in these containers of the same height.
(published on 01/20/2011)
Follow-Up #5: pressure under seawater
- Bob La Bonte (age 73)
Victoria, TX USA
Hi Mr. La Bonte,
I unfortunately could not locate the exact information on the website that you are referring to. But as I understand, the issue is about the pressure at sea level. So as you dive deeper down the sea, the total measured pressure will be sum of the atmospheric pressure exerted on the water surface + liquid pressure due to a water column between you and the surface. So 14.7 corresponds to the former value, whereas your 0.445 psi/ft correspond the latter contribution. So it all depends on what one refers to when talking about the "pressure".
Yes, the discrepancy between 0.443-0.445 depends on the salt level, since pressure under some height of a liquid is density dependent, which in turn depends on the salt concentration. So such numbers may slightly vary between the Pacific Ocean and the Mediterrenean Sea. Freshwater would differ by more, since it's 2 or 3 % les dense than seawater.
(published on 12/12/2015)