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Understanding Enriched Air Nitrox

Another Benefit: Reduced Nitrogen Load

When some percentage of Nitrogen from the cylinder has been removed, you do not accumulate as much Nitrogen compared to someone diving on air. The advantages here are increased bottom time, reduced surface interval times and longer repetitive dive times. You gain these benefits because your body has absorbed and stored less nitrogen over time.

The reduction of Nitrogen loading is a reason why a new diver would consider a Nitrox Certification.

At this point, you learned What Is Nitrox, Why Use Nitrox and When is Nitrox an option for you. You were introduced to terms and acronyms such as Partial Pressure of Oxygen (PPO), as well as Fraction of Oxygen (FO). Let’s now look at Managing dive profiles for planning purposes.

Wait! Computers are being pushed today. We can stop here and you can just go out, buy a Nitrox compatible dive computer and trust your computer after getting your certification. You may close this window now OR you may continue to read and learn the physics and physiology on the use of Nitrox as well as formulas in hopes your computer doesn’t malfunction (3 times for this author) and never have to hold a Nitrox conversation with anyone.

DANGERS of using Nitrox

We can all agree we need Oxygen to survive. We also agree the percentage of Oxygen in the air is 21%.

Before we dive into the depths on the dangers, let’s get a level set on our base leading up to the dangers. Having the knowledge of how and what under belt will help with the why.

Question: At what depth are you when you reach 2 atmospheres absolute in sea water?

Your answer should be 33 feet.

Question: What is an Atmosphere?

An atmosphere is the total pressure exerted by a column of air between sea level and space above or a specific depth of water, in our case. That column has weight.

Question: Now much does one atmospheres of air weight?

One atmosphere of air weights approx 14.7psi. That is 14.7 pounds per square inch, psi.

Question: How many feet of sea water, fsw makes up one atmosphere?

1 column of salt water, 1″x1″x12″ weights approx. 0.445lbs.

If 14.7psi is the weight of one atmosphere we can divide 14.7 by 0.445 and we will get 33ft.

Question: How many atmospheres is 44.1psi?

44.1 / 14.7 = 3atm

Every 33fsw is 1atm and every 1atm is 14.7psi then every 33fsw is 14.7psi

Question: How much pressure is exerted on us at 33fsw?

Your answer should be: 29.4psi the amount of pressure exerted on us at 33fsw.

Every 33fsw exerts 14.7psi. The amount of pressure exerted by the column of water at 33fsw is 14.7psi. Add to that pressure of water the pressure from 1atm of air and you end up with 2ata. At a depth of 33fsw, you have (2×14.7) 29.4psi bearing down on you. 1atm of air and 1atm of water for a total of 2ata.

Question: What is Atmosphere Absolute?

Atmosphere Absolute is the pressure exerted upon us, in our case by the column of water above plus the atmospheric pressure of air upon that. It is denoted by the characters, ata (atmosphere absolute). In the previous question ata was used at the end of the last sentence.

Question: How much does one atmosphere of water weight?

One atmosphere of water weights 14.7psi
Where the depth for one atmosphere of fresh water is 34 feet, and salt water is 33 feet.

Question: How do you calculate Atmospheres Absolute, ATA?

The formula to get your current ATA in salt water is:

ATA = ( Depth / 33 ) + 1
ATA= ( 33 / 33 ) + 1
2 = 1 + 1

The quotient from the two numbers found between the () is the number of atmospheres of water above you. The +1 is the atmosphere of air. The sum of the two is the total atmospheres exerting pressure above you. Your ATA.

Question: What is your ATA at a depth of 85fsw?

At 85 feet you are at approximately 3.57 atmospheres absolute. That is 2.57atm of water + 1atm of air.

What is this Absolute? Putting it simply it is the addition of the atmosphere of air at sea level added to the atmosphere in water.

At 66 feet you are under 2 atmospheres of water plus 1 atmosphere of air for a total of 3 atmospheres absolute. If you do not include the 1 atmosphere of air, then you would denote atmospheres with (atm). Include the 1 atmosphere of air it then becomes (ata).

Question: How many psi is exerted at 3 ATA?

( 3 x 14.7 ) = 44.1psi

you could write the equation as (( Depth / 33 ) + 1 ) * 14.7

Perform the following calculation. Do not look at the answers following them. When done calculating, look at the answers to see if yours are the same.

Question: a) How many atmospheres absolute are you at a depth of 99 feet sea water and b) how many psi of pressure is exerted at this depth?

Answers: a) (99 / 33 ) + 1 = 4ata b) ( 4 * 14.7 ) = 58.8psi -or- ((( 99 / 33 ) + 1) * 14.7)

Equivalent Percentage. What is it and what does it mean? Equivalent is resembling something without being that something. In the context of Oxygen Equivalence it is to take the FO of Oxygen in the cylinder and multiply it by the ATA to determine the equivalent percent oxygen you are breathing at depth. This becomes the PPO at depth.

Examining the contents of a Nitrox tank you determine the FO to be .32. When you are at 2ata, the PPO you inhale is .64 or 64%. We say you are breathing the equivalent of 64% Oxygen at 2ata. We say this because the percentage of oxygen in the tank remains at 32% but the Oxygen you breath at 2ata is 64%.

The formula to calculate PPO is:

PPO=( ATA * FO )

PPO=((( Depth / 33 ) + 1) * FO)

Question: What is the equivalent percentage of Oxygen breathed at 3 atmospheres?

Your answer should be 3 x %age (3 times percentage) or thrice that on the surface. If you were at 4ATA, then the answer would be 4 times that on the surface.

To Calculate your Equivalent Oxygen at depth simply take the PPO and convert it to percent.

For example if your is PPO=.64 your Equivalent Oxygen is 64%.

Question: Why do we need to know the equivalent percentage of Oxygen?

The short and direct answer, You are strongly advised not to breath more than a 1.4 PPO. a PPO of 1.4 equates to 140%. When you know the FO, you can determine the maximum depth you may dive on that mix. This Maximum depth becomes known as Maximum Operating Depth, or MOD.

Question: Why is knowing the equivalent percentage of Oxygen important?

It is important because the use of nitrox accelerates and compounds the physiological effects as a result of Oxygen being at higher PPO on the body. The higher and longer your body is exposed to increased PPO the more change occurs within the body. The more change, the greater the dangers.

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