The physics of diving

Another unit is the bar, where 1 bar equals 14.5 psi. The value of 1 bar is very close to the atmospheric pressure 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 mean to judge you. You may be 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 is , 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 small moving molecules that act like balls, and those molecules collide with underwater objects (such as people), creating pressure. Water has a lot more molecules than the same volume of air, meaning more collisions to produce greater pressure. But just like going to the top of Mount Everest decreases 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. That means that during a dive 20 meters (about 60 feet) below sea level, there would be a water pressure of 43.5 psi, three times higher than the air pressure at the Earth’s surface.

(The fact that pressure increases with depth prevents all of the ocean’s water from collapsing into an infinitely thin layer. Since the pressure is higher the deeper you go, the water below pushes up more than the water above pushes down. This difference compensates for the downward gravity, so that the water level remains constant.)

It may sound like 43.5 psi is too much for a person, but it’s actually not that bad. Human bodies are highly adaptable to changes in pressure. If you’ve been to the bottom of a 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 of the air in your inner ear, the membrane will stretch and that can really hurt. But there’s a neat trick to fix this: If you push air into your middle ear cavity by pinching your nose while trying to blow air out, air will be forced into this cavity. With more air in the inner ear, the pressure on both sides of the diaphragm will be equal and you will feel normal. This is called ‘equalizing’, hopefully for obvious reasons.

There is actually another air space that you need to equalize when diving: the inside of your diving mask. Don’t forget to add air to it as you go deeper or else that thing will clumsily squash your face.

There is another physics error that a diver can 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 in your lungs remains 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 will decrease. The reduced external pressure on your lungs makes them appear to be overinflated. This can cause tears in the lung tissue or even force air into the bloodstream, which is officially bad stuff.

Buoyancy

There’s another problem you’ll face when you’re underwater: floating and sinking. If you want to stay underwater, it’s helpful to sink instead of float – to a point. I don’t think anyone wants to sink so low that they never return. It’s also nice to be able to float when you’re on the surface. Fortunately, divers can adjust their “buoyancy” to different situations. This is called buoyancy control.

Things sink when gravity pulling downwards is greater than buoyancy pushing upwards. If these two forces are equal, the object will be neutrally buoyant and neither rise nor fall. It’s like floating, but in the water, and it’s essentially what you want to do when you go diving.

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