Optimal Decompression Gas Mixes

Hello! Welcome back to your course on technical dive planning.

In our last lesson, we focused on the deep phase of the dive, learning how to calculate the 'best mix' of trimix to manage narcosis and oxygen toxicity at our target depth. Now, we'll address the other critical part of the dive profile: the ascent. Simply ascending from a deep, long dive isn't an option; we must perform a series of decompression stops, and the key to doing this safely and efficiently lies in the gases we breathe.

This lesson will teach you how to calculate optimal decompression gas mixes based on depth and oxygen exposure constraints. We'll explore the theory behind why we switch gases and then move to the practical calculations for selecting the right mixes for your ascent.

1. The Principle of Accelerated Decompression

During a dive, our body tissues absorb inert gases like nitrogen and helium. To ascend safely, we must eliminate this gas slowly to prevent it from forming bubbles, which cause decompression sickness (DCS). The goal of decompression is to make this elimination process (off-gassing) as efficient as possible.

The speed of off-gassing is driven by the pressure gradient between the inert gas in your tissues and the partial pressure of that same gas in your lungs. To maximize this gradient, we can do two things:

1. Decrease the ambient pressure (ascend to a shallower depth).
2. Decrease the partial pressure of the inert gas in our breathing mix.

We achieve the second point by breathing a gas with the highest possible fraction of oxygen. This creates what divers call the "oxygen window." By maximizing the oxygen partial pressure (PPO2), we minimize the partial pressures of the inert gases, creating a larger "window" for them to diffuse out of our tissues.

2. The Golden Rule: PPO2 and Maximum Operating Depth (MOD)

To maximize the oxygen window, we want to breathe the highest safe PPO2.

• For the bottom phase of a dive, where you are exerting yourself, the PPO2 limit is kept low (typically 1.2-1.4 ata).
• For the decompression phase, where you are resting, a higher limit is used: 1.6 ata.

This PPO2 of 1.6 ata is the fundamental constraint for any decompression gas. It dictates the Maximum Operating Depth (MOD) for that gas—the deepest you can safely breathe it.

The formula for MOD is a rearrangement of Dalton's Law. You learned this in the context of nitrox diving, and it applies perfectly here.

The formula for MOD in meters is:
$\text{MOD (meters)}=\left(\frac{\text{Target PPO2}}{\text{Fraction of Oxygen (FO2)}}-1\right)\times 10$

3. Standard Decompression Gases

Using the MOD formula with our 1.6 ata PPO2 limit, we can determine the MOD for a set of common, highly effective decompression gases.

100% Oxygen (O2)

This is the most efficient decompression gas because it contains no inert gas, maximizing the oxygen window.

• FO2: 1.0
• MOD: $\left(\frac{1.6}{1.0}-1\right)\times 10=(1.6-1)\times 10=6\text{ meters}$
• Usage: Used for the shallowest decompression stops (typically starting at 6m).

50% Nitrox (EAN50)

A very popular gas for intermediate decompression stops.

• FO2: 0.50
• MOD: $\left(\frac{1.6}{0.5}-1\right)\times 10=(3.2-1)\times 10=22\text{ meters}$
• Usage: Technical divers typically switch to EAN50 at their 21-meter stop to stay within standard 3-meter stop intervals.

These two gases form the cornerstone of most technical decompression strategies. For deeper or longer dives, other intermediate gases might be added.

4. Calculating the "Optimal" Mix for a Switch Depth

While using standard gases like EAN50 and O2 is common practice, you can also calculate the theoretically "optimal" oxygen percentage for any given switch depth. This is essentially the "best mix" calculation from our last lesson, but simplified for a decompression context.

Here, we aren't worried about narcosis (as we're ascending into shallower water). The only goal is to hit our PPO2 limit of 1.6 ata at the switch depth.

The formula is a simple rearrangement of Dalton's Law:
$\text{Optimal FO2}=\frac{1.6\text{ ata}}{\text{Absolute Pressure at Switch Depth (ata)}}$

Example:
Let's say you want to perform your first gas switch at 21 meters.

• Absolute Pressure at 21m = 3.1 ata
• Optimal FO2 = $\frac{1.6}{3.1}\approx 0.516$

This calculates to 51.6% oxygen. This is precisely why EAN50 is the standard gas for this depth—it's the closest practical mix to the theoretical optimum.

This calculation method is extremely useful for planning.

5. Strategy: How Many Deco Gases to Carry?

So, which gases should you plan for your dive? The answer depends on the dive's depth and bottom time, which together determine the total decompression obligation.

Here are some common strategies:

• Short/Shallow Technical Dives (e.g., 40m for 30 mins): A single decompression gas of EAN50 might be sufficient to accelerate the mid-depth stops, followed by breathing bottom gas in the shallows. However, carrying two gases (EAN50 and 100% O2) is far more efficient and provides redundancy.
• Standard Trimix Dives (e.g., 60m for 25 mins): Two decompression gases are standard. The plan would be to ascend on bottom gas to 21m, switch to EAN50, then ascend to 6m and switch to 100% O2.
• Long/Deep Trimix Dives (e.g., 70m for 40 mins): These dives may warrant three decompression gases. In addition to EAN50 and O2, a "deep deco" mix might be carried. This is often a lean trimix or "triox" (e.g., 35% O2, 25% He) used around the 30-40m range to begin accelerating decompression earlier.

The dekostop resource provides excellent, practical examples of how to select the number of decompression bottles based on your dive profile.

6. A Final Check: Isobaric Counter-Diffusion (ICD)

When you switch from a helium-rich bottom gas to a nitrogen-rich decompression gas (like EAN50), there's a theoretical risk. Helium may leave your tissues faster than nitrogen enters, but in some tissues, the rapid influx of nitrogen while helium is also present can increase the total inert gas pressure and potentially cause bubble formation. This is called Isobaric Counter-Diffusion (ICD).

While the full physiology is a topic for another day (Module 2), the practical takeaway for gas planning is simple: avoid large, sudden increases in the partial pressure of nitrogen (PN2) when switching gases.

A common rule of thumb is to ensure the PN2 of your new gas is not significantly higher than the PN2 of the gas you are switching from. For the standard switches we've discussed (e.g., Trimix -> EAN50 at 21m), this is generally not an issue. However, it's a critical safety check, especially if you were to consider switching to a high-nitrogen gas like Air at a deep depth.

Conclusion

You now have the tools to plan the ascent portion of your dive. By selecting appropriate high-oxygen mixes, you can significantly and safely shorten your decompression time.

Key Takeaways:

• Decompression is accelerated by breathing gases with a high PPO2, a principle known as the "oxygen window."
• The PPO2 limit for resting decompression is 1.6 ata.
• This limit defines the Maximum Operating Depth (MOD) of any decompression gas.
• The most common decompression gases are 100% O2 (MOD 6m) and EAN50 (MOD 21m).
• The number of decompression gases you carry (one, two, or more) is determined by your dive's depth and bottom time.
• You can calculate the "optimal" FO2 for any switch depth using the formula: FO2 = 1.6 / Pressure.

Preview of the Next Lesson:

We've planned our bottom gas and our decompression gases. The next logical question is: how much of each gas do we need? In our next lesson, we will apply rock bottom gas management principles to calculate the gas volumes required for a complete technical dive.