Decompression Profiles for Normoxic Trimix Dives

Hello! Welcome back to your course on Normoxic Trimix Operations.

In our last lesson, we explored the critical trade-offs involved in selecting a normoxic trimix. We saw how choosing a mix like Trimix 18/45 for a 60-meter dive prioritizes mental clarity (low END) at the expense of higher cost and thermal loss, while a mix like Trimix 20/30 does the opposite.

Now that we understand how to select a bottom gas, we must analyze the consequences of that choice. Every minute spent at depth incurs a "debt" that must be repaid with time during the ascent. This lesson is about understanding that debt.

By the end of this 60-minute lesson, you will be able to:

• Analyze decompression obligation profiles for typical normoxic trimix dives.

We will break down what a decompression profile is, why it's structured the way it is, and how you can estimate and interpret the decompression required for a given dive.

1. The Fundamental Principle: On-gassing and Off-gassing

Before we can analyze a profile, we must be certain about why it's necessary. At depth, the increased ambient pressure causes the inert gases in your breathing mix—nitrogen and helium—to dissolve into your body's tissues. This is called on-gassing. The deeper you go and the longer you stay, the more gas dissolves.

To avoid decompression sickness (DCS), this absorbed gas must be released slowly and in a controlled manner during your ascent. This is called off-gassing. A decompression profile is simply a structured plan for your ascent that manages the rate of off-gassing to prevent bubbles from forming in your tissues and bloodstream.

2. Decompression Models: The "Brains" Behind the Profile

How do we calculate the exact stops needed? This is the job of a decompression model. These are mathematical algorithms that simulate how different "tissue compartments" in your body absorb and release inert gases. You've likely heard of some of them:

• Haldanian Models (e.g., Bühlmann): These are dissolved-gas models. They track the theoretical gas pressure in various tissue compartments and ensure it never exceeds a maximum permissible value (an M-value) at any given depth.
• Bubble Models (e.g., VPM, RGBM): These models go a step further and account for the physics of bubble formation and growth, often resulting in deeper initial stops to control microbubbles early.

You will explore these models in greater depth in Module 3 of the course. For now, it's important to know that they are the engines that generate the decompression profiles we use.

3. The Anatomy of a Decompression Profile

A decompression profile is more than just a list of stops. It has a distinct structure dictated by the physics and physiology of off-gassing.

3.1. The Time Penalty: Bottom Time vs. Decompression Time

The first thing to understand is that the relationship between your time at depth (bottom time) and your required decompression time is not linear. A small increase in bottom time can cause a large jump in your decompression obligation.

This exponential penalty is a core reason why technical dive planning must be so precise.

3.2. The Ascent Structure: Stops and Gas Switches

A typical trimix decompression profile involves a series of stops at progressively shallower depths. These stops are not all the same; they serve different purposes and often involve switching to different gas mixtures to maximize the efficiency of off-gassing.

This example shows the key phases:

1. Deep Stops: Initial, short stops in the deeper part of the ascent to manage fast-off-gassing tissues (like blood and organs) and control bubble formation.
2. Intermediate Stops & Gas Switches: As you ascend, you switch to gases with higher oxygen fractions (e.g., Nitrox 50). This creates a large "oxygen window," significantly increasing the gradient for inert gases to diffuse out of your tissues.
3. Shallow Stops: The final, longest stops (e.g., at 6 meters on 100% O2) are designed to allow slow-off-gassing tissues (like joints and bones) to release their gas load before you surface.

3.3. Controlling Conservatism: Gradient Factors

When using a Bühlmann-based algorithm, you can adjust the conservatism of your profile using Gradient Factors (GF). They are expressed as two numbers, like GF 30/85.

• GF Low (e.g., 30): This controls the depth of your first stop. It dictates how much supersaturation you'll tolerate deep in the dive. A lower number forces a deeper first stop.
• GF High (e.g., 85): This controls how much supersaturation is left when you surface. A lower number means you will be "cleaner" (less residual inert gas) upon surfacing, which requires longer shallow stops.

4. Analyzing a Profile with Ratio Deco

While dive computers and planning software calculate the exact profile, a powerful way to analyze and understand the structure of a decompression obligation is to use a well-established heuristic called Ratio Decompression (Ratio Deco).

Ratio Deco provides a set of simple rules to estimate the decompression time and structure for standard trimix dives. It's an excellent mental model for planning and for verifying what your computer tells you.

Let's Analyze a Normoxic Dive

Let's use the Ratio Deco rules to analyze a typical normoxic trimix dive: 55 meters for 30 minutes.

1. Find the Ratio (from Section 3.2):

   • The table gives a ratio of 0.5 for 45m and 1.0 for 60m.
   • For 55m, we can interpolate a ratio of roughly 0.8.
   • O2 Time = Bottom Time × Ratio = 30 min × 0.8 = 24 minutes.

2. Structure the Ascent (from Section 3.3):

   • Interval 1 (6m on O2): Total time = [O2 time] = 24 minutes.
   • Interval 2 (21m to 9m on Nitrox 50): Total time = [O2 time] = 24 minutes.
   • Interval 3 (36m to 24m on Bottom Gas): Total time = [O2 time] / 2 = 24 / 2 = 12 minutes.

Analysis of the Profile:
For a 30-minute bottom time at 55 meters, our decompression obligation is approximately:

• 12 minutes of stops between 36m and 24m.
• 24 minutes of stops between 21m and 9m.
• 24 minutes of stops at 6m.
• Total Decompression Time: 12 + 24 + 24 = 60 minutes.
• Total Run Time: 30 min (bottom) + 60 min (deco) + ascent time ≈ 95-100 minutes.

This quick analysis reveals the significant time commitment and shows how the obligation is distributed across different depth ranges and gases.

To see a fully worked-out example, let's look at one from the document.

Conclusion

Today, we've dissected the decompression obligation that follows a normoxic trimix dive. You now have the tools to look at a dive plan and understand not just the 'what' but the 'why'.

Key Takeaways:

• Decompression is the controlled release of inert gases (N2, He) absorbed during a dive.
• Decompression profiles are calculated by mathematical models (like Bühlmann or VPM) and can be adjusted for conservatism using tools like Gradient Factors.
• A typical trimix profile consists of progressively longer stops at shallower depths, using high-oxygen gas switches (e.g., Nitrox 50, O2) to accelerate off-gassing.
• The time penalty for staying at depth is non-linear; decompression obligations increase rapidly with small additions to bottom time.
• Heuristics like Ratio Deco provide a powerful mental model for analyzing the structure and duration of a decompression profile.

Preview of the next lesson:
We have now covered how to choose a bottom gas (Lesson 2) and how to analyze the resulting decompression profile (this lesson). The next logical step is to put everything together. In our next session, we will develop a complete theoretical dive plan for a normoxic trimix profile, including gas selection, volume calculations, and contingency planning. This will involve calculating exactly how much of each gas you need to carry to safely execute the dive.