Another unit is the bar, where 1 bar is equal to 14.5 psi. The value of 1 bar is very close to the pressure of air on Earth. The atmospheric pressure of the air that surrounds you right now is probably 14.5 psi. (Yes, I said “probably” because I don’t want to judge you. Maybe you are reading this from the top of Mount Everest, where the pressure is just 4.9 psi, because there is less air above you pushing down. If so, send me a picture.) In terms of force and area, it is equal to 100,000 newtons per square meter.
Water is also made of tiny moving molecules that act like balls, and those molecules collide with underwater objects (like people), producing pressure. Water has many more molecules than the same volume of air, which means there are more collisions to produce a greater pressure. But just like going to the top of Mount Everest decreases the air pressure, going deeper in water increases the pressure, because gravity pulls downward on the molecules of water. For every 10 meters of depth, the pressure increases by 1 bar, or 14.5 psi. That means that on a dive 20 meters (around 60 feet) below sea level, there would be a water pressure of 43.5 psi, three times greater than the air pressure at Earth’s surface.
(The fact that pressure increases with depth prevents all the ocean’s water from collapsing into an infinitely thin layer. Since the pressure is greater the deeper you go, the water underneath pushes up more than the water above it pushes down. This difference compensates for the downward gravitational force, so the water level stays constant.)
It might sound like 43.5 psi is too much for a person to handle, but it’s actually not that bad. Human bodies are very adaptable to changes in pressure. If you have been to the bottom of a swimming pool, you already know the answer to this pressure problem—your ears. If the water pressure on the outside of your eardrum is greater than the pressure from the air inside your inner ear, the membrane will stretch, and it can really hurt. But there is a nice trick to fix this: If you push air into your middle ear cavity by pinching your nose closed while attempting to blow air out of it, air will be forced into this cavity. With more air in the inner ear, the pressure on both sides of the membrane will be equal and you will feel normal. This is called “equalization,” for hopefully obvious reasons.
There’s actually another air space that you need to equalize while diving—the inside of your scuba mask. Don’t forget to add air to it as you go deeper, or that thing will awkwardly squish your face.
There is one other physics mistake a diver could make. It’s possible to create an enclosed air space in your lungs by holding your breath. Suppose you hold your breath at a depth of 20 meters and then move up to a depth of 10 meters. The pressure inside your lungs will stay 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 will decrease. The reduced external pressure on your lungs makes it as though they are overinflated. This can cause tears in lung tissue, or even force air into the bloodstream, which is officially bad stuff.
Buoyancy
There’s another problem to deal with when you are underwater: floating and sinking. If you want to stay underwater, it’s useful to sink instead of float—to a point. I don’t think anyone wants to sink to such depths that they never return. Also, it’s nice to be able to float when you’re at the surface. Luckily, scuba divers can change their “floatiness” for different situations. This is called buoyancy control.
Things sink when the downward-pulling gravitational force is greater than the upward-pushing buoyancy force. If these two forces are equal, then the object will be neutrally buoyant and neither rise nor sink. It’s like hovering, but in water, and it is essentially what you want to do when scuba diving.