# Eli5. Why does the compression change in breathing compressed air at different depths? Surely the ‘oxy-mix’ metal container doesn’t physically ‘compress’ any further than it has in 1atm (unless at near Hadal Depths)

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Im obviously confused somewhere. But let me explain further.

You have a metal container, carrying your breathable gas mix (depending on your depth). If this container doesn’t actually physically compress further due to the rigidity of the metal at say 800ft, then why would breathing a 80-20 mix of Ni and O in a compressed container cause any effects?

I understand that free diving, or diving in mailable suits will obviously cause your body to compress under the weight of the water around you. But saturation divers are very different. Are they not safe from the ‘physical’ pressure of the water, yet unsafe from the compression of air? What is the difference between our bodies not compressing at this depth, compared to the oxygen in our lungs compressing?

Edit: another question. If the oxygen inside the container is already at an extreme pressure, then why would releasing it cause any change to us physically from breathing it at sea level, compared to 1mile under the sea?

In: 13

It’s not the fact that the air tank doesn’t compress, it’s that your body *does.* Your body is not rigid metal so it compresses as water pressure increase. The result of this is that the air tank has to deliver air to your body at the ambient water pressure to keep the balance between your body and the water pressure. In other words, when you’re at a depth of 10m (33 feet), where the water pressure is 2 atmospheres (2x sea level pressure) the air you’re breathing from the tank is *also* at 2 atmospheres – it has to be.

Since both nitrogen and oxygen become toxic at higher pressures, once you start getting to depths at which those gases become toxic at the ambient pressure, you have to start using different breathing gas combinations. Typically this means removing some or all of the nitrogen and replacing it with another gas like helium and also adjusting the ratio of oxygen.

The air in a SCUBA tank is at extremely high pressure, typically around 200 atmospheres (3000psi). The diver breathes through a regulator, which regulates the air pressure down to match the pressure of the surrounding water.

It is necessary for the pressures to be equal – if the gas was regulated down to surface pressure, the diver would be unable to inhale because of the pressure difference. So the deeper a diver goes, the higher the pressure of the gas they’re inhaling.

However, at elevated pressures, gases can have different effects on the body. Below a depth of about 30m, the nitrogen that makes up most of ordinary air can cause [nitrogen narcosis](https://en.wikipedia.org/wiki/Nitrogen_narcosis), which has effects somewhat similar to drunkeness. Divers going deeper than this will often breath alternative gas mixtures, such as helium and oxygen.

The container is usually at a very high pressure, when full at least. Normal SCUBA cylinders operate around 200 bar (200 times atmospheric pressure. You can’t breathe that directly, I hope I don’t need to explain why…

So, SCUBA tanks have a “regulator” at the mouthpiece. This controls the pressure which is delivered from the tank to the mouthpiece. At the surface, the air pressure coming out of the mouthpiece is more or less the same as the surrounding atmospheric pressure. Let’s say the tank holds enough air for you to breathe for 1 hour on the surface. If you descend 10 meters, the surrounding water pressure is now double what it was at the surface. The regulator delivers air to the mouthpiece at this same local pressure, i.e. 2 bar. You now only have enough air for 1/2 an hour, if you stay at 10 meters.

I’m not sure if I have really answered your question, but I hope I have at least made a start.

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