The physics of scuba diving

The physics of scuba diving

Another unit is the 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 around 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 above you pushing down. If so, send me a picture.) In terms of force and area, it equals 100,000 newtons per square meter.

Water is also made up of tiny moving molecules that act like balls, and these molecules collide with underwater objects (like people), producing pressure. The water has many more molecules than the same volume of air, which means there are more collisions to produce greater pressure. But just like going to the top of Mount Everest decreases air pressure, sink deeper into the water increase pressure, as gravity pulls down on water molecules. For every 10 meters of depth, the pressure increases by 1 bar, or 14.5 psi. This means that when diving 20 meters (about 60 feet) below sea level, there would be a water pressure of 43.5 psi, three times the air pressure at the surface of the earth.

(The fact that the pressure increases with depth prevents all the water in the ocean from collapsing into an infinitely thin layer. Since the pressure is greater the deeper you go, the water in below it rises more than the water above it pushes down. This difference compensates for the downward gravitational force, so the water level remains constant.)

It may seem like 43.5 psi is too much for one person to handle, but it’s actually not that bad. Human bodies are very adaptable to changes in pressure. If you’ve been to the bottom of a pool, you already know the answer to that pressure problem: your ears. If the water pressure outside your eardrum is greater than the air pressure inside your inner ear, the membrane will stretch and it can really hurt. But there is a good trick to solve this problem: if you push air into your middle ear cavity by pinching your nose while trying to expel air from 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 airspace you need to equalize while diving: the inside of your scuba mask. Remember to add air to it as you go or this thing will awkwardly smash your face.

There is another physics mistake a diver could make. It is possible to create a closed 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 inside your lungs will remain the same during this ascent because you have the same lung volume and they hold the same amount of air. However, the water pressure outside them will decrease. The reduced external pressure on your lungs makes them feel overinflated. It can cause tears in lung tissue, or even force air into the bloodstream, which is officially a bad thing.


There’s another problem to deal with when you’re underwater: floating and sinking. If you want to stay underwater, it helps to sink instead of float, up to a point. I don’t think anyone wants to sink to such depths that they never come back. Plus, it’s nice to be able to float when you’re on the surface. Fortunately, divers can modify their “buoyancy” 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 float neutrally and neither rise nor fall. It’s like hovering, but in the water, and that’s basically what you want to do in scuba diving.

#physics #scuba #diving

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