Another unit is bar, where 1 bar equals 14.5 psi. The value of 1 bar is very close to the air pressure on earth. The atmospheric pressure of the air surrounding you right now is probably 14.5 psi. (Yes, I said “probably” because I don’t want to judge you. Maybe you’re reading this from the top of Mount Everest where the pressure is only 4.9 psi because there’s less air pushing down above you. If so, send me a picture.) In terms of force and area, this equates to 100,000 Newtons per square meter.
Water is also made up of tiny moving molecules that act like spheres, and these molecules collide with underwater objects (like people) and create pressure. has water many more molecules than the same volume of air, meaning there are more collisions to create greater pressure. But just like climbing Mount Everest sinks the air pressure going deeper into the water increases the pressure, because gravity pulls the water molecules down. For every 10 meters of depth, the pressure increases by 1 bar or 14.5 psi. This means that if you dive 20 meters (about 60 feet) below sea level, the water pressure would be 43.5 psi, three times higher than the air pressure at the surface.
(The fact that pressure increases with depth prevents all the water in the ocean from collapsing into an infinitely thin layer. Because the pressure increases the deeper you go, the water below pushes up more than the water above pushes up bottom. This difference balances the downward force of gravity, so the water level remains constant.)
It might sound like 43.5 psi is too much for one person, but it’s actually not that bad. The human body is very adaptable to changes in pressure. If you’ve been to the bottom of a swimming pool, you already know the answer to this pressure problem – your ears. When the water pressure on the outside of your eardrum is greater than the pressure of the air in your inner ear, the membrane stretches and it can be really painful. But there’s a neat trick to fix this: If you’re forcing air into your middle ear cavity by pinching your nose while trying to blow air out, you’re forcing air into that cavity. With more air in the inner ear, the pressure on both sides of the membrane is equal and you feel normal. This is called “Equalization” for hopefully obvious reasons.
There is actually one more airspace that you need to equalize when diving – the inside of your scuba mask. Don’t forget to add air as you go deeper or the thing will awkwardly crush your face.
There is one more physical mistake a diver might make. It is possible to create a sealed air space in your lungs by holding your breath. Suppose you hold your breath at a depth of 20 meters and then move to a depth of 10 meters. The pressure in your lungs stays the same during this ascent because you have the same lung volume and they contain the same amount of air. However, the water pressure outside of them decreases. The reduced external pressure on your lungs makes them appear as if they are over-inflated. This can cause ruptures in the lung tissue or even force air into the bloodstream, which is officially bad stuff.
There is one more problem to contend with underwater: swimming and sinking. If you want to stay underwater, it’s useful to sink rather than swim – up to a point. I don’t think anyone wants to sink so low that they never come back. It’s also nice when you can float on the surface. Fortunately, scuba divers can change their “float ability” for different situations. This is called buoyancy control.
Things sink when the gravitational force pulling down is greater than the buoyant force pushing up. If these two forces are equal, then the object will have neutral buoyancy and will neither rise nor sink. It’s like floating but in water, and it’s essentially what you want to do when you dive.