Welcome back to my "Becoming a PADI Instructor" Blog Series yet again...are you getting sick of me yet? Let's talk about Gases!
Dalton's Law
Dalton's Law states: the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases. What the heck does that even mean? For the sake of simplicity and examples throughout this series we'll consider air 21% oxygen and 79% nitrogen (Ignoring traces of Neon, Krypton, Hydrogen, Xenon Radon, Carbon Monoxide, Argon, and Carbon Dioxide).
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Think of 1 bar / atmosphere as 100% of a gas mixture. In air you have 21% Oxygen + 79% Nitrogen = 100% of Gas Mixture. In terms of partial pressures it is the same: .21 bar/ata (Partial Pressure of Oxygen) + .79 bar/ata (Partial Pressure of Nitrogen) = 1 bar/ata. As you get deeper the proportion of those mixtures stays the same, meaning at 2 bar/ata the partial pressure of Oxygen is .42 (.21 x 2 bar/ata) and the partial pressure of Nitrogen is 1.58 (.79 x 2 bar/ata). .42 PO2 + 1.58 NO2 = 2 bar/ata.
A large part of dive computer algorithms use the partial pressures of gas mixtures. It's important to know the partial pressures of these mixtures as you plan your dives. Gases that are not deadly at surface can become deadly at depth, like Carbon Dioxide and even Oxygen (exceeds 1.4 bar).
Dalton's law is most applicable when diving with EANx (Nitrox) because when diving under normal air within recreational limits you won't exceed 1.4 bar/ata for Oxygen. Now if you take an EANx36 mixture you can exceed 1.4 bar by 90ft.
You can figure out what the best Nitrox mix is for your planned depth you can do that by dividing 1.4 by atmospheres at your desired depth. If you plan to dive to 100 ft salt water you'll first determine the atm at 100 ft (100 ft / 33 ft per atm + 1) = ~4. Then you divide 1.4 (maximum partial pressure of Oxygen) by your desired atmospheres which is 4. 1.4/4 = 0.35 hence you should get EANx35.
Henry's Law
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Henry's Law is the basis for decompression modeling. All dive tables are based off of how gases dissolve into and out of a liquid or body tissues. Henry's Law states: At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Now in regular people's terms: if pressure increases the more gas that is dissolved in to the liquid, if the pressure decreases the gas will be released from the liquid. A prime example that we deal with from time to time is opening up a can of Soda, Pop, Coke. CO2 is dissolved in to the liquid. If the can is shaken the pressure inside the can increases and the C02 is dissolved in to the liquid until the C02 gas tension is equal to that of the partial pressure of the soda in the can. When you open the can the pressure lessons and quickly releases the C02 forcing it to rush from the can... and all over you. This is the reason for safety stops, ascending too fast would be the equivalent to your body being the can and nitrogen being the C02 that is exploding all over the place. If you could slowly release the pressure from your soda can you could control the release of the C02 and perform a safe ascent/release of gas.
Gas tension is the pressure exerted within the liquid by the gas. Gases molecules always exert some sort of pressure and continue to exert that same amount when they are inside of a liquid. Gases like to be free roaming, when they are inside or around other gases or elements they want to escape.
Dive tables rely on what is called the Pressure Gradient. The pressure gradient is the difference between the pressure of the gases in contact with a liquid and the gas tension within the liquid (your body tissues).
When the gas tension is equal to the partial pressure of the gas in contact with the liquid the state is known as saturation. This means that no more gas can be dissolved in to the liquid. The amount of gas that can be dissolved increases as you go deeper and the partial pressures increase (more room as gas compresses). When the partial pressure is less than the gas tension this is a state of supersaturation. As long as the partial pressure is decreased slowly the gas will be released without causing bubbles because the pressure gradient is low.
Now lets put all of that up there in to how it would go during a typical dive.
As you dive you start breathing compressed air, the oxygen from the compressed air is mostly consumed by your body, but the nitrogen is absorbed in to your tissues. Some tissues absorb the nitrogen faster than others (fast and slow tissues). Fast tissues are muscles that have more blood flow and accumulate nitrogen faster. Slow tissues accumulate nitrogen slowly (typically fats) but dissolved in higher concentrations as nitrogen is 4.5 times more soluble in fat than it is in water and fats have much more room for nitrogen to accumulate. On ascent, the reverse is true as the fast tissues release nitrogen more quickly and the slow tissues release it slower requiring more time at a certain partial pressure in order for the nitrogen to off gas. As you go deeper the nitrogen compresses more and even more is absorbed in to your tissues. If you reach the maximum bottom time or greater allowed by dive tables your tissues become saturated by nitrogen. When this happens decompression stops are mandatory. When you start your ascent nitrogen reaches a state of supersaturation (off-gassing). This is why it is very important to keep your ascent rate around or below 18 metres/60 feet per minute. Slower is acceptable and encouraged (PADI) . If you ascend too fast, like the C02 in the soda can the nitrogen gas can be released too fast and form bubbles causing decompression illness. In a state of saturation from diving over time limits you must stop at certain depths in order to reach the next level of saturation until the pressure gradient is low enough to allow for a normal ascent.
I touched on solubility slightly above. Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the physical and chemical properties of the used solute and solvent as well as on temperature, pressure and the pH of the solution. The extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begin to precipitate the excess amount of solute.
The combination of Henry's Law and the solubility of water and body tissue is the basis for your dive tables. Below is a more in depth explanation on a chemical level of how solubility of substances works.
As always, I welcome your input and questions/corrections and any discussions you'd like to have on these topics as I write about them. Thanks for reading!
Disclaimer: I am not a PADI Instructor and am writing this up as a learning aid for myself to better understand and explain the theories as I learn them. You should NOT be using these as your sole resource for learning how to dive or dive education. I suggest you sign up for a PADI or a respective company course to continue your diving education. If you find any errors in these posts, please feel free to contact me. Thank you.