We are looking at what is happening to you when you go scuba diving and the laws of physics, which we are subject to while diving.
A volume of gas in an air space is inversely proportional to the pressure, so the deeper down you go the smaller the volume, think back to that balloon we all learned about, at 10m the volume has halved even though we have not taken any gas from it. Boyles law explains why we have to equalise our ears, equalise our mask, adjust our BCD as we change depth and can not hold our breath and ascend.
Gay Lussac’s 2nd Law
Which is about pressure and temperature, this is why we wait for our cylinders to cool down when filling them up. If you have a hot cylinder at 200bar on the surface because it has been sat in the sun for a while and then jump into the cool water then your tank pressure will go from 200 down to about 180 just from the change in temperature.
This law basically explains pressure gradients and ultimately why we get narked, can not breathe too much oxygen at depth and decompression illness. As the pressure of a gas increases outside of your body, the more is absorbed into your tissues, the higher pressure is sort of pushing more gas into your tissues.
The usual analogy is the fizzy drinks bottle; kept at relatively high pressure the carbon dioxide is kept in solution inside the drink but when you release the pressure the gas expands out of solution. At depth, if you open that same bottle not much will happen because the pressure has not changed very much and the gas will stay in solution. Send it up to the surface and those bubbles will start showing up, if you follow the bottle up too fast then you will have some bubbles of your own, because of decompression sickness.
The composition of gases and partial pressures as a representation of a gases total pressure. Air at sea level is about 20.9% Oxygen and about 78% Nitrogen.
As you dive down the ratio does not change but thanks to Henry’s Law, the increase in pressure changes the amount we absorb. Partial pressures make it easy for divers to understand the concentration or amount of a certain gas, such as oxygen, in a breathing mix at a given depth so they know if it is breathable. At sea level, Dalton’s total pressure is one atm. 21% of that, 0.21, is the partial pressure of Oxygen at sea level. At 10m the pressure is double so that 1.0 becomes a 2.0 and that 0.21 becomes 0.42, if that 0.21 gets much higher than 1.6 then bad things can happen.
The difference between weight and buoyancy. This is why we need weights to sink and how our BCDs adjust our buoyancy. It is all about displacing the weight of water that you weigh, balancing the equation by displacing more or less water. Imagine the space that you and your gear takes up in the water and imagine what that amount of water weighs.
That weight of water is the buoyant force pushing you up and you need to equal that force with gear and weights. As you descend and the volume of the airspace in your BCD decreases you are displacing less water so you descent, that is why it is important to use the correct weight as possible and therefore as little air in your BCD as possible. A lot of gear is made to be as neutrally buoyant as possible to make your weight calculations as easy as possible.