The Other Limit: Understanding PPO2, Nitrox, and Oxygen Toxicity

In the world of diving, we spend a lot of time talking about nitrogen. We manage our NDL (No-Decompression Limit) and our Gradient Factors to avoid Decompression Sickness. But there’s another gas we’re breathing that has its own, equally serious limit: Oxygen.

There’s a common myth that Enriched Air Nitrox is “safer” than air. This isn’t quite true.

Nitrox is a trade-off. By increasing the oxygen (e.g., to 32%) and decreasing the nitrogen, you get a longer NDL. But in exchange, you must now manage a new and different risk: Oxygen Toxicity.

This is why your advanced dive computer has a completely separate “brain” dedicated to tracking oxygen.

Chapter 1: The Problem: Oxygen Can Be Toxic

At the surface, oxygen is our friend. But as we descend, the Partial Pressure of oxygen (its concentration in a given volume) increases. If the partial pressure of oxygen (PPO2) gets too high, it becomes toxic to our Central Nervous System (CNS), with the primary risk being an un-warned underwater convulsion.

This is the “other limit.” While our “nitrogen brain” (the Bühlmann algorithm) is tracking our NDL, our “oxygen brain” is tracking our CNS exposure.

Chapter 2: What is PPO2?

Let’s keep it simple. Your dive computer is constantly doing a simple math problem:

[Ambient Pressure (ATA)] x [O2 Percentage (%)] = PPO2

• On Air (21% O2) at 132 ft (5 ATA): 5 ATA x 0.21 = 1.05 PPO2
• On Nitrox (32% O2) at 132 ft (5 ATA): 5 ATA x 0.32 = 1.60 PPO2

Dive training agencies have set the standard “working limit” for PPO2 at 1.4. As you can see from our example, at 132 feet, 32% Nitrox is over that limit.

This is why, when you set up your Shearwater Teric for a Nitrox dive, it doesn’t just ask for the percentage (32%). It also asks for your Max PPO2 (e.g., 1.4).

With this information, the computer will:
1. Calculate your maximum depth for that dive (MOD).
2. Warn you if you approach or exceed that depth.
3. Track your cumulative “oxygen dose” using the “CNS Clock.”

Chapter 3: The “CNS Clock”

Your computer tracks your oxygen exposure as a percentage, often called the “CNS Clock.” You can think of it as a separate NDL timer, but for oxygen.

It’s tracking your “dose” of high-PPO2 exposure. If you get close to 100% on this clock, your computer will warn you that you have reached your oxygen limit for the day, even if your nitrogen (NDL) is perfectly fine.

A computer with a clear display, like the Teric, can show you both your NGL (Nitrogen) and your CNS (Oxygen) limits, so you have a complete picture of your decompression status.

Chapter 4: A Lesson from the User Reviews: Altitude & PPO2

In the Amazon reviews for the Teric, a user mentioned being confused at high altitude until customer service told them to “adjust the low PPO2 setting.”

Why? This is a brilliant example of this science in action.

That “Ambient Pressure” (ATA) number in our equation is the key. * At sea level, the ambient pressure is 1.0 ATA. * At a high-altitude dive site (e.g., 8,000 ft), the ambient pressure might only be 0.7 ATA.

A smart computer like the Teric knows this (or you tell it). It knows its “starting point” is 0.7, not 1.0. This means it can calculate PPO2 perfectly from that lower-pressure environment. A simpler computer might get confused, assuming you’re always starting at 1.0 ATA. This is what the user was experiencing—a mismatch between what the computer thought the PPO2 was and what it actually was.

Conclusion: Managing Two Clocks

Being a modern diver means managing two clocks simultaneously:
1. The Nitrogen Clock (NDL): How long until you need a mandatory stop.
2. The Oxygen Clock (CNS): How much “oxygen dose” you’ve absorbed.

Nitrox is an amazing tool. It gives you more time underwater by slowing the Nitrogen Clock. But it speeds up the Oxygen Clock.

Understanding this trade-off, and trusting the tool (your computer) that tracks both, is what makes you a safe, informed, and capable modern diver.