What happens when you take a flexible air container (plastic bottle full of air) to 40 m deep?
All the divers and freedivers know what happens to your air spaces when you go underwater but there is big difference between both activities. While on freediving the air is compressed in your lungs as you go deeper and come back to normal when you reach the surface on scuba diving because the dive regulator were you breath from will give you air at the environment pressure that means your lungs will stay with the same volume all the time. Although you will still have to equalize your ears and mask as you go deeper as there is direct water pressure effect on both.
At 10 m deep/2 bar pressure the volume on the bottle is already reduced to 1/2 and air density inside of the bottle is 2x bigger!!! This is for me the danger zone between the surface and 10 m and again on the way up as you reduce the volume to half. Not only you need to equalize your air spaces especially the ears more often so you don't get any barotrauma as when you going up to the surface you should take 1 min from 10 m to reach the surface (on scuba diving) so you don't get bubbles forming in to your body and you don't get the risk of overexpansion your lungs.
At 20 m deep the volume inside of the bottle get´s 1/3 and the density of the air is now 3x more. So the variations of volume get less compared with the variations of the volume from the surface to 10 m deep which means less need of equalizing
At 30 m deep the volume inside is 1/4 as the pressure is already 4 bar and the density of the air is 4x compared to the surface. At this depth you will feel less need of equalizing your air spaces because the air volume variations are smaller and smaller but the effect on the wetsuit is getting bigger as the neoprene is a rubber with air spaces inside that means that it will be compressed too. A wetsuit with 7 mm can be reduce to 5 mm as you go deeper witch is a concern with body temperature. If you don't add air to your BCD as you go down you will fill that you are to heavy and you can't hold a proper neutral buoyancy and become to negative.
At 40 m deep the bottle get´s compressed with 5 bar of pressure that means the volume is reduce to 1/5 of its capacity and the density of the air that is still all their inside of the bottle but very much compressed get´s to 5x. At this depths it is very easy to see you pressure gauge going down as fast as you breath because the scuba regulator compensate the increase of the pressure and density by adding more air in to your lungs and keep them at the same normal size as in the surface. Also if you don´t have a compensated regulator it will get more difficult for you to breath with the air that get in your lungs being very dense (5x times more). This is the time you give value to the money you spend in a better regulator.
What you have to remind is that all the opposite factors occur as you go up to the surface. So the air in the bottle will expand until it get´s the same as in the surface. That means the nitrogen dissolved in your tissues will expand forming micro bubbles and if your not careful with the speed your going to the surface you can easily get a decompression disease. Also your lungs because they are always with a normal volume if you hold your breath on the way up (even more danger from 10 m to the surface) the air inside will expand and can easily get into a lung over expansion. Remember also that your wetsuit air bubbles will start expanding as the air inside of your BCD too so it is very important to release some air from your BCD as you go up to the surface even more frequently from the 10m.
PADI Master Instructor