Understanding Enriched Air Nitrox

Let’s review all the things mentioned so far.

We learn that Nitrox is the addition of Oxygen and a substitution of Nitrogen in your scuba cylinder. We also learned the reason for using Nitrox which is to extend bottom time. Also when is it the right time to acquire a Nitrox certification. We also learned that the Partial pressure of a given gas rises and falls as we descend and ascend the water column and that exposing our bodies to an increase in PPO for an extended period of time is an irritant on the bodies tissues.

Lets Dive Deeper on the subject.

Knowing the primary reason for Nitrox is extending bottom time, How about depth?

How deep may you go on Nitrox?

To say to gain one thing we have to give something else up. There must be balance and the thing you give up once you begin diving Nitrox is Depth. The higher the percentage of Oxygen in the cylinder, the shallower you must dive.

The industry rule teaches us to remain below a PPO of 1.4 with contingency for a PPO of 1.6. To get contingency out of the way it means do not plan for a dive with a PPO greater than 1.4. Should you ‘need’ to break past this floor of 1.4, try not to exceed a PPO of 1.4 for any extended period of time. For example, if the sandy bottom is say 10 feet below you and you are at a PPO of 1.39, you drop your spear, reel, knife, etc.. descending down 10 additional feet to a PPO of 1.52 to recover the item should be done long enough to recover the item then ascend to a safer PPO level.

Let us look at the subject of depth and how the PPO plays a role. We need to understand this for later you will use it to identify your Maximum Operating Depth, MOD.

Earlier you were introduced to PPO. You learned how to calculate ATA and apply that value to determine your PPO at a given depth. Let us now dive deeper into how PPO is used to determine your MOD while diving Nitrox.

Partial Pressure of Oxygen, PPO at depth is the percentage of Oxygen multiplied by the amount of pressure exerted upon it. In the case of diving as we descend, pressure from the column of water presses down on us. It compress gasses forming tighter and tighter groups. For example, at 1ata (sea level) let say there are 1 million molecules of Oxygen per square inch. Head to 2ata and that same square inch now has approx twice the number of molecules occupying it, ~ 2 million. This means on every inhalation, at 2ata you intake twice the number of molecules of Oxygen as you did at 1ata.. If you were at 3ata, the count would be 3 times the cylinders percentage of Oxygen.

As pressure is exerted on a given gas the gas form tighter groups. The density will increase while the volume decreases.

To help put a picture to how pressure affects a gas, the image below depicts a gas bubble and it size when pressure is increased and decreased.

The the image above, you find a representation of a bubble. To the left of the vertical line is the bubble under increasing pressure, where on the right is a bubble under decreasing pressure. To help explain and clarify the depiction, the bubble from the surface taken down to 2ata is 1/2 its original size. The formula is 1/ATA. A bubble formed at 2ata taken to the surface has doubled its size and is depicted as ATA/1

Another example. Let’s say you went down to 99fsw for an extended time. The gas that was captured would begin to expand on your ascent. Upon reaching the surface, that bubble would have grown approx 400% in size. That is one reason for ascending slowly, say 30’/min vs the older mythology of 60’/min. Add to that ascent the newer mythology of half stops. Stopping 1/2 the distance from your deepest point for no less than 1 minute, then resuming your ascent towards the surface. For example, if the deepest point of your dive was 100′, you would stop when you reached 50′ for no less than 1 minute to help facilitate off gassing.

Question: Assuming no decompression obligation, using the new ascent mythologies how long, at a minimum will it take you to break the surface from a depth of 90 feet?

Your Answer: (My answer may be found at the end of this article)

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