Normoxic Trimix for 45-60m Dives

Hello! Welcome to the first lesson in the "Normoxic Trimix Operations Theory" module.

So far in this course, we've built a strong foundation in the physics and physiology of diving. You've learned about how pressure affects gases (Dalton's Law), and the physiological challenges of deep diving, such as nitrogen narcosis and oxygen toxicity.

Today, we'll start putting that theory into practice. We're moving from the "why" to the "how." This lesson will focus on Trimix, the gas blend that allows technical divers to safely explore depths beyond the reach of traditional recreational diving.

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

• Define normoxic trimix.
• Calculate appropriate mixes for dives in the 45-60 meter range.

This is your first step into the practical world of technical dive planning. Let's get started.

1. What is Trimix?

At its simplest, Trimix is a breathing gas consisting of three components: oxygen, helium, and nitrogen. But why add a third gas? On a deep dive using air (which is roughly 21% oxygen, 79% nitrogen), two major problems arise:

1. Oxygen Toxicity: The partial pressure of oxygen becomes dangerously high.
2. Nitrogen Narcosis: The high partial pressure of nitrogen causes significant cognitive impairment, often compared to being intoxicated.

Trimix solves this by replacing a portion of the nitrogen with helium and, if necessary, reducing the oxygen content.

To get a solid overview of what Trimix is and why it's used, please read the following short sections from the Wikipedia article "Trimix (breathing gas)" and watch the introductory segment of the "What is TRIMIX?" video.

In summary, helium is used because:

• It is not narcotic, reducing the impairment divers experience at depth.
• It is less dense than nitrogen, which reduces the effort (work of breathing) required to breathe the gas at depth, where it becomes thick.

2. Defining "Normoxic" Trimix

The term "Trimix" alone isn't specific enough. We classify Trimix blends based on their oxygen content, which determines whether they can be safely breathed at the surface.

• Hypoxic Trimix: Has an oxygen percentage so low (<18%) that it cannot be safely breathed at the surface. It is only used as a "bottom gas" at depth.
• Normoxic Trimix: Has an oxygen percentage high enough (typically 18% to 21%) to be safely breathed at all phases of the dive, including at the surface.
• Hyperoxic Trimix: Has more oxygen than air (>21%). These are often used at shallower technical depths to accelerate decompression.

Our focus today is Normoxic Trimix. The key takeaway is that a normoxic mix is breathable from the surface all the way to the maximum depth of the dive and back.

The following resources provide a clear definition.

As you've seen, the consensus is that a normoxic mix must contain at least 18% oxygen. This ensures the partial pressure of oxygen (PO2) at the surface (1 atmosphere) is at least 0.18 ata, which is sufficient to support consciousness.

3. Calculating an Appropriate Normoxic Mix

Now for the main event: how do we select the right blend for a specific dive? This isn't a single calculation but a balancing act between three key constraints. For a dive in the 45 to 60-meter (150 to 200-foot) range, we must choose our gas fractions (FO2, FHe, FN2) to satisfy:

1. Oxygen Limit (PO2): The partial pressure of oxygen at the target depth must not exceed a safe limit. For the bottom portion of a dive, this is typically 1.4 ata.
2. Narcosis Limit (END): The narcotic effect of the nitrogen in the mix should be manageable. A common target is to keep the Equivalent Narcotic Depth (END) at 30 meters (100 feet) or less.
3. Normoxic Requirement: The oxygen fraction (FO2) must be at least 18% (0.18).

Worked Example: A Dive to 60 meters (200 feet)

Let's design a mix for a 60-meter dive.

• Target Depth: 60 m
• Ambient Pressure: $(60/10)+1=7$ ata
• Max PO2: 1.4 ata
• Target END: 30 m

Step 1: Determine the Oxygen Fraction (FO2)

• The mix must be normoxic, so: ${\text{FO}}_2\ge 0.18$
• The PO2 at depth must be $\le 1.4$ ata. This means:
  ${\text{FO}}_2\le \frac{{\text{Max PO}}_2}{\text{Pressure}}=\frac{1.4}{7}=0.20$
• So, our oxygen percentage must be between 18% and 20%.

Step 2: Determine the Nitrogen Fraction (FN2)

• We want our END to be no more than 30 m. The formula for END is $\text{END}=({\text{FN}}_2\times \text{Depth in ata}/0.79)-10$. We can rearrange this to find the maximum FN2 for our target END at our target depth:
  ${\text{FN}}_2\le \frac{(\text{Target END}+10)\times 0.79}{\text{Depth}+10}=\frac{(30+10)\times 0.79}{60+10}=\frac{40\times 0.79}{70}\approx 0.45$
• So, our nitrogen percentage must be 45% or less.

Step 3: Select a Mix and Verify

We have our constraints: FO2 is 18-20% and FN2 is $\le$ 45%. Let's try to build a mix.

A very common "standard" mix for this depth range is Trimix 18/45. Let's see if it fits our criteria.

• FO2: 18% (0.18)
• FHe: 45% (0.45)
• FN2: 100% - 18% - 45% = 37% (0.37)

Now, let's check this mix against our safety limits for a 60m dive:

• Is it normoxic? Yes, 18% is $\ge$ 18%.
• What is the PO2 at 60m?
  ${\text{PO}}_2={\text{FO}}_2\times \text{Pressure}=0.18\times 7=1.26\text{ ata}$
  This is well below our 1.4 ata limit. Excellent.
• What is the END at 60m?
  $\text{END}=\left(\frac{{\text{FN}}_2\times (\text{Depth}+10)}{0.79}\right)-10=\left(\frac{0.37\times 70}{0.79}\right)-10\approx 32.8-10=22.8\text{ meters}$
  An END of ~23 meters is very clear-headed and well below our 30-meter target.

Conclusion: Trimix 18/45 is an ideal and widely used normoxic mix for dives to 60 meters.

What about a 45-meter dive?

Using the same logic, you'll find that a common choice is Trimix 21/35 (21% O2, 35% He, 44% N2). At 45 meters (5.5 ata), this mix gives you a PO2 of 1.16 ata and an END of ~21 meters, both very conservative and safe values.

4. Standard Mixes in the Real World

You might wonder why divers often use "standard" mixes like 18/45 and 21/35 instead of calculating a custom blend for every dive. The reasons are primarily logistical: it simplifies gas blending, planning, and communication within a dive team.

The following forum thread from ScubaBoard shows a real-world example of a diver asking this exact question and receiving advice that confirms our calculations.

This practical context shows that the calculations we just performed lead directly to the gas choices made by technical divers in the field.

Conclusion

Let's summarize the key takeaways from today's lesson:

• Trimix is a breathing gas of oxygen, helium, and nitrogen used to manage oxygen toxicity and nitrogen narcosis on deep dives.
• Normoxic Trimix is a category of Trimix with enough oxygen (18-21%) to be safely breathable at the surface. It is the standard choice for dives in the 45-60 meter range.
• Choosing an appropriate mix involves a balancing act: keeping the PO2 below 1.4 ata at depth, maintaining a clear head with an END below 30-40m, and ensuring the mix is normoxic.
• For the 45-60 meter range, Trimix 21/35 and Trimix 18/45 are common, safe, and effective standard mixes that meet these criteria.

In our next lesson, we will build on this knowledge. Now that you know what mixes to use, we will explore the trade-offs involved, such as cost and thermal properties, and begin to analyze the decompression obligations that result from using these gases.