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Ah! Oxygen - the gas of life! Without it for a few seconds and the world disappears from view. Too much of it can be a bad thing as well.
Visiting a person receiving common oxygen therapy in a hospital refutes the notion that the oxygen is pure. A heart attack victim may be breathing oxygen through a nasal cannular. Air is also entering the nose at the same time the oxygen is. Even "oxygen tents" were not totally closed. Breathing pure oxygen over a period of time may lead to serious lung irritation and even pneumonia. Breathing pure oxygen at pressures greater than 1 atmosphere can lead to fatal consequences.
Years ago there was a scuba introduced that allowed the diver to breathe pure oxygen. Because of the design, the small bottle of oxygen, about the size of today's pony bottles, allowed the diver to remain underwater for up to 4 hours! The unit had the appearance similar to an accordion. The air was extracted from it and then it was filled with oxygen from the bottle. As one breathed the bag would get smaller. As one breathed out the exhaled air would enter the bag and it would get bigger again. However, the amount of oxygen returning would be less due to the body's needs, so more oxygen would have to be added to the bag. To remove the poisonous CO2 in the exhaled air there was a canister that contained caustic Barium Hydroxide (similar to Drano). As the exhaled air passed through the canister the chemical would combine with the CO2 removing it from the breathing oxygen. So, the oxygen molecules were breathed over and over again until they were converted to CO2. The unit was called a rebreather.
Conventional scuba is wasteful. A breath is taken, 21% oxygen goes into the lungs, 18% comes out. Little CO2 goes in and about 4% comes out. Even though there is plenty of oxygen left in the exhaled air, it is discharged into the water. Just to get rid of the CO2 we waste the oxygen. Conventional scuba is known as an "open-circuit" breathing apparatus. Oxygen rebreathers are called, "closed-circuit." Rebreathers emit no bubbles unless the diver is ascending and the bag gets too full. U.S. Navy Seals use rebreathers to avoid detection!
If a person breathes pure oxygen at a pressure above 1 atmosphere there is a possibility of it becoming toxic. Above 2 atmospheres almost insures it. Using a closed-circuit rebreather at 33' would allow the diver to breathe 2 atmospheres of oxygen. The National Oceanographic and Atmospheric Administration (NOAA) prohibits their divers from exceeding 1.6 atmospheres of oxygen. The higher the pressure, the faster the poisoning. High-pressure oxygen toxicity starts with the twitching of smaller muscles that are in use such as the ones around the mouth holding the mouthpiece. Then the twitching can lead to full-blown convulsions. Needless to say, convulsions and diving do not really go together! If fact, if you are not rescued by another diver you will probably drown. Tech divers may switch to Nitrox or pure oxygen during their dives. To protect them from drowning due to a "mistake" they wear full face masks.
The U.S. Navy Seals are a hearty lot. They have a rather rigorous training program. They must be oxygen tolerant. One of the parts to that training consists of weeding out those that convulse easily when breathing pure oxygen. Potential Navy Seals are required to sit in an air atmosphere breathing pure oxygen from a hand-held face mask at a depth equivalent to 60' of seawater for 30 minutes. If convulsions do occur they drop the mask and are quickly returned to breathing air so the convulsions will stop. If they fail the test they are booted out of the program.
Don't think you are off the hook if you dive breathing air. Air at depth can cause oxygen poisoning. At the surface air supplies 21% oxygen. Since the pressure at the surface is one atmosphere, the pressure of the oxygen alone (partial pressure) is 0.21 atmospheres. The absolute pressure at 33 feet of seawater (fsw) is 2 atmospheres, therefore the oxygen partial pressure would be 21% of 2 or 0.42 atmospheres. If you project the partial pressure of oxygen at 132 fsw it would be 21% of 5 atmospheres or 1.05. That would be the same as breathing pure oxygen at a depth of over 33'! The absolute pressure at 297' is 10 atmospheres, so the partial pressure of the oxygen would be 2.1 atmospheres and it could very well be toxic! Toxic air at 297' is a no-no!!
Since we metabolize oxygen and nitrogen causes decompression sickness, it would stand to reason that if the gas a diver breathes has a higher percentage of oxygen and less nitrogen we might be able to stay down longer and/or reduce the risk of DCS. Enriched Air Nitrox is just that. Popular blends for "Nitrox" are 32% and 36% oxygen. On EAN-36 a diver at 100' would be just like a diver on air at 75'. The US Navy Decompression Table (air) sets a 25 minute No Decompression Limit (NDL) for 100'. PADI's EAN-36 NDL is 35 minutes. Nitrox has become a popular breathing gas so divers may extend their dives at the same time reducing the chance of getting DCS. The downside is that the maximum operating depth (MOD) for nitrox is less than air because the elevated oxygen may cause oxygen poisoning. Breathing EAN-36 at 130 feet would put the diver at 1.78 atmospheres of oxygen, well over the PADI/NAUI limit of 1.4. At Deep-Six we periodically run one day courses training divers to become certified to dive with EAN.
So, what's the deal on Trimix Diving? Trimix divers have invested a lot of time in training, and a lot of money for equipment. They are able to dive to 300 or more feet to explore areas way out of the conventional scuba diver limits. How is this done and why so much training? The Technical Diver may be wearing 4 tanks. Two tanks on the back may contain trimix which might be a mixture of 3 gases such as 15% oxygen, 45% helium, and 40% nitrogen. The reduced oxygen is to prevent oxygen poisoning at depth, the nitrogen reduces the chance of helium tremors, and the helium reduces the narcosis caused by the other gases. On dives of less that 400' the nitrogen is not needed so the mixture might be heliox (only helium and oxygen). One of the 4 tanks willl have nitrox-36, and the 4th tank will have 100% oxygen. The EAN is a travel mix and the pure oxygen is for rapid decompression.
The Technical diver will start on the EAN-36 travel mix and descend to 90'. Then the diver switches to the heliox or trimix and descends to the bottom, wreck, or whatever the dive goal might be. At the end of the bottom time ascent is made to 90' and the travel mix is used to get the diver to 20' There the pure oxygen is breathed resulting in a rapid elimination of the helium and nitrogen from the tissues (Graham's Law). Because the depth of the dive was extreme the heliox or trimix supply is quickly consumed. Some Technical Divers use very sophisticated rebreathers that meter in the oxygen according to what is consumed and the percentage changes, but not the oxygen partial pressure, according to the depth. Chi-Chang!
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