Understanding Enriched Air Nitrox

Question: What is the Percent Equivalent of oxygen at a PPO of 1.4?

Answer: Convert the 1.4 to a percentage. Take the decimal, move it two places to the right and replace it with a % character. At 95 feet with an FO of 36% you are breathing the equivalent of 140% Oxygen…

Question: If contingency is up to 1.6 PPO, what is the depth, on a cylinder with an FO of 32% you would need to reach for a PPO of 1.6?

Answer: Contingency Depth = ((( PPO / FO ) – 1 ) * 33 ) It is the exact same formula for MOD, just with a different PPO, in this case 1.6. You should have come out with approx 132 feet.

Contingency Depth = ((( 1.6 / .32 ) – 1 ) * 33 )

Do not plan dives to contingency depths. Plan dives no deeper than MOD. MOD simply tells you the depth to the Nitrox floor. You do not want to go through the floor and if you must, try and keep it to no more than the contingency and as short a period as possible.

Calculate the following on your own.

You know the formula to calculate MOD, here is the formula to calculate PPO:

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

Presume a MAX PPO of 1.4 for the following questions. Answers will follow after the last question. Try not to peak before answering.

Question 1: What is the maximum depth for a 40% Nitrox mix?

MOD=

Question 2: What is the PPO at 90fsw on a 40% Nitrox mix?

PPO=

Question 3: Is it safe to plan a dive to a depth of 90fsw on a mix of 40%

( Yes / No ) and Why?

Question 4: What is the Oxygen Equivalent on a tank filled with 30% Oxygen at a depth of 72 feet?

PPO=

ANSWERS:

1. 82.5fsw. You could round up or down.
2. For a 40% Nitrox mix at 90fsw the PPO is 1.49
3. Your answer should be No. The MOD for 40% Nitrox is 82.5fsw at a PPO of 1.4. 90fsw exceeds the 1.4 PPO
4. PPO=95 The formula is: PPO=(ATA * FO) ; ((Depth / 33 ) + 1) * FO ); hence the formula is ((72 / 33) + 1 ) * .30 )

The image above depicts three separate blends. 21%, 32% and 36%. A diver on a cylinder filled with air will have a NDL of 55 minutes at 60 feet. The diver on EAN32 will have 90 minutes, while a diver on EAN36 will have an NDL of approx. 121 minutes depicted by the column on the left.

The MOD for each of the three gasses are depicted by the depth of the plain for the respective gas. EAN36 has an MOD of approx. 95fsw, EAN32 111fsw and air is off the charts (approx 187fsw).

Calculate out the MOD for the three gasses (.21, .32, and .36)

MOD=(((PPO / FO) – 1) * 33)
MOD=
MOD=
MOD=

The chart shows the higher the EAN level, the more time you gain yet the shallower the dive must be.

If the three gasses above represented three divers which will dive together, all three would begin their ascent before the diver on air reached their NDL. You dive as a team. You do not let one diver go alone to the surface because their Bottom time has run low. This was poor planning. Divers on a team should be diving similar mixes amongst other things.. This is a separate subject all together. The point above being the one on air will have to surface before the other two.

I would like to note that we have been discussing MOD. MOD does does not mean you must dive the MOD. MOD is for you to know where the Nitrox floor sits. In fact, you should not be diving your max PPO on every dive. I would encourage you plan dives at a lower PPO.

This bring up the next question: What mix should you be requesting for your dives?

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