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.)
(republished on 08/02/06)
(published on 11/12/07)
(published on 01/21/08)
(published on 09/17/09)
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 03/29/12)