Understanding Enriched Air Nitrox

Another one of the conditions of diving Nitrox is it limits the depth which you may safely dive. If diving on Nitrox provides extended bottom time, it also limits/restricts the depth which you safely may go.

The higher the percentage of Oxygen in your cylinder is, the longer your bottom time yet the shallower you must dive.

Another factor to consider, monitor, and manage is the time exposed to higher PPO.

You will be monitoring Oxygen Exposure Time. Sometimes referred to as Oxygen Toxicity Load. Just as you have been monitoring your Nitrogen exposure, you will now do the same for Oxygen exposure. Now that you will be diving Nitrox you will be monitoring both Nitrogen and Oxygen load. Your Nitrox dive computer may report your Oxygen Exposure as CNS.

The image above shows a computer set to 32% Nitrox. The MOD for a 32% mix is 111fsw. Apparently the diver has not performed a dive yet, and their CNS shows 0%. After performing the dive, the CNS value will be a positive integer representing a percentage of O² exposure on the body. Oxygen Exposure will be covered in more detail later.

The table below shows that with a fixed gas percentage (%), the partial pressures (pp) will change in open circuit diving.

A VERY important concept. As the Ambient pressure increases so does the percent equivalent of a given gas. As the ambient pressure decreases, so does the equivalent percentage. As you rise and fall along the water column, so does the percent equivalent of a gas. This is one concept you must know and understand. Your life depends on it.

Where is the danger in that? Fair enough question…

As Oxygen is placed under increased pressure, the higher the percentage the body receives on a given breath. A scuba cylinder filled with 32% Oxygen will on the surface have you breathing 32% but take that same cylinder down to 2 ATA and you are breathing the equivalent of 64%. At 100 feet you are breathing the equivalent of 128% Oxygen. The danger is the higher PPO and longer the exposure the more danger you subject yourself to.

Question: How can we increase the PPO we are breathing, while at sea level without increasing the percentage in the cylinder?

The answer may be found in the article above. It may not be spelled out in black and white, but it is there… Got to think.

Answer: Pressure. Increasing or decreasing pressure will increase or decrease the PPO. Consider how a ‘recompression’ chamber works.

You just learned Pressure has a direct effect on a gasses. The deeper you go, the more pressure is exerted, the higher the equivalent percentage goes.

At this point you have been subjected to the term, Equivalent Percentage. What is meant by Equivalent Percentage? It is the percentage in your cylinder multiplied by the ATA. Since the percentage in the cylinder remains constant, the factor that changes the percentage is pressure. When you apply pressure to a gas, the equivalent percentage increases. At the surface a Nitrox cylinder may be filled with 32% Oxygen, you are breathing 32% Oxygen. Take that same cylinder down to 33fsw and you are now breathing the equivalent of 64% Oxygen. Your PPO increased by a factor of 2. This subject will be covered in more detail later and why it is important to grasp the concept.

Question: How does breathing higher concentrations of Oxygen affect the body?

Everyone is physiologically different. Everyone is affected differently and to different degrees. No one denies that breathing higher percentage of Oxygen does not adversely affect the body. The longer one is exposed to Partial Pressure of Oxygen, PPO greater than that found at sea level, the more severe the complications.

Delving into the chemistry of exposing the body to higher PPO over extended periods of time is beyond the scope of this article. For the purpose of knowing, exposure to higher percentages over an extended period of time causes stresses on the body’s tissues and Central Nervous System. Something we must monitor and minimize at all times.

This is why it is important to minimize the time spent as well as the percentage over a period of time? Divers want to spend as much time as they can diving and do so as safely as possible. There must be a balance where the scales tip more towards safely than time. This can not be more true when it comes to diving Nitrox.

Above are some of the warning and dangers of diving on higher PPO. The more repetitive dives on Nitrox you make, the more physiological irritation your body must endure. Just like Nitrogen accumulations over repeated dives, the physiological effects of Oxygen on the body also accumulate. You must give your body rest from this irritation by not constantly subjecting your body to the irritant. This may be done by reducing the PPO or eliminate it by taking a break from diving Nitrox or diving for a day. (go do a land tour, sit around the pool)

This author has been Nitrox certified for almost 20 years. Though I have experienced physiological anomalies due to Nitrox, lucky enough I am still here to write about it. Some of my fiends are not. Let me add a disclaimer on what I just wrote. Diver Error was the cause for all my friends mishaps. Perhaps it was their comfort that came with diving over a long period of time and that comfort brought complacency. Do not let yourself become complacent. Do not challenge the dangers of Nitrox diving. On land that’s one thing, underwater that could lead to injury and/or drowning.

The unknown variable in Nitrox diving is the diver. No one can predict how any one diver will perform while on Nitrox. The above is not intended to frighten you away from Nitrox, just to make you more informed.

You just learned as the dive depth increases, so does the pressure. This increase in pressure affects the equivalent percentage of any given gas. As the percentage increases, so does the risk and the primary benefit is increased bottom time. You were introduced to PPO and how pressure affects the PPO as well as the pros and cons of that increase. You also learned what Equivalent Pressure is and how it factors in with the PPO.

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