Understanding Enriched Air Nitrox

Determining the percentage of Oxygen in the cylinder?

You request the dive shop your desire to have your cylinder filled to 32%. When you return to pick up your tank, you will analyze the Oxygen content. You do this through the use of an analyzer. You do this so you know what percentage of Oxygen is in your cylinder. When you know the percentage of oxygen is in your cylinder, you know the PPO and the PPO is used to calculate Maximum Operating Depth, MOD for that blend. Sound like rocket science? Really, it’s not.

Below are just a few pictures of Oxygen analyzers you may come across. The details of how an analyzer works is beyond the scope of this article. In simplest terms let’s say it counts the number of oxygen molecules passing over its membrane and provides a numeric readout representing percentage.

Here are a few analyzers so you may get an idea what one may look like. As you may see on some of the displays on the analyzer the number 20.9. This represents the percentage of Oxygen in the air.

This is your starting point. You must calibrate the analyzer to start at 20.9% or 21% before you begin analyzing the percentage of Oxygen in the cylinder. One way to do this is to calibrate it a know gas, an AIR Cylinder. You then hold the opening of the analyzer up to the opening of the cylinders valve and open the valve ever so slightly, so that just enough gas is able to escape the cylinder and into the intake of the analyzer.

You will see the numbers of the display rise and eventually stop at or near your requested percent filled. Once the readout has stabilized that number should be documented on the tank along with the MOD. Using a piece of painters tape will do.

Below is an example of marking the tank. You see the percent is 36.2 (PPO=.36), the pressure is 3600psi, the MOD of 95′ and the date filled.

Mentioned earlier to gain time, we loose depth. Once you know the percentage of Oxygen in the cylinder, you can figure out your Maximum Depth, what we’ve been calling MOD, your Maximum Operating Depth. Above on the marking is written 95′ for MOD. How did they come up with the MOD?

Question: How do you calculate MOD once you know the percentage of Oxygen in the cylinder?

MOD is based off of the highest PPO you should not exceed, that PPO being 1.4, with a contingency of 1.6.

Studies have show that exposing your body to high levels of Oxygen over extended and repeated periods of time is unfavorable no matter how slight. Exposure to high levels of PPO regardless of time could be severely detrimental. The dive industry dictates we keep our PPO below 1.4.

You have all the numbers needed to determine MOD. How is it calculated?

One way is to use a chart. Charts similar to the one seen here are usually found near the analyzer. On the chart below the far left column is PPO. The top row is FO, the percent O² in your cylinder.

Click on the image to enlarge it

To locate the MOD for the mix you analyzed, locate the intersecting row and column, (PPO and FO) to come to the MOD for that mix of gas. Since you are looking for the MAX depth, you search down the left column, the PPO column for the number that comes closes to 1.4 (without exceeding 1.4), scan to the right for the box where the PPO and FO meet. The number in the intersecting box represents your MOD.

Question: Using the chart above, what depth is the MOD for a cylinder analyzed with 32% O²?

Your answer should be 110.7fsw (or 111fsw)

If you do not have a chart readily available, you may use the tried and true.. The Calculation for MOD is:

MOD = (( PPO / FO ) – 1 ) * 33
MOD = (( 1.4 / .32 ) – 1 ) * 33
MOD = (4.375 – 1 ) * 33
MOD = 3.375 * 33
MOD = 111 fsw

Imagine being on a boat and someone asks, What is the MOD for a nitrox mix of 36%?

You pull out your pad and paper, whip through the formula to tell them the MOD at 1.4 PPO on 36% Oxygen is 95 feet.

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