Normoxic Trimix Dive Planning

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

In our previous lessons, you learned how to select an appropriate trimix for a given depth and how to analyze the resulting decompression obligation. We've covered the "what" (the gas) and the "why" (the decompression). Today, we connect all the pieces to answer the most critical question in technical diving: "How do I plan and execute this dive safely?"

This lesson is the capstone of our module. We will synthesize everything you've learned to build a complete dive plan from the ground up.

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

• Develop a complete theoretical dive plan for a normoxic trimix profile, including gas selection, volume calculations, and contingency planning.

We will work through a practical case study, making the decisions and calculations a technical diver performs before ever getting near the water.

1. The Dive Plan Framework: A Case Study

Every technical dive starts with a clear objective. For this lesson, we will plan a dive to a hypothetical wreck.

• Dive Objective: Explore a wreck at 55 meters (180 feet).
• Planned Bottom Time: 25 minutes.

This is a classic normoxic trimix profile. To build our plan, we need to define the following components:

1. Gas Selection: The best bottom gas and decompression gases.
2. Decompression Schedule: The ascent profile required.
3. Gas Volume Calculations: How much of each gas we need.
4. Contingency & Team Plan: How we manage emergencies and dive as a team.

Let's begin.

2. Step 1: Gas Selection

First, we must choose our breathing gases. Our goal is to balance oxygen safety (avoiding toxicity), nitrogen narcosis (maintaining a clear head), and decompression efficiency.

2.1. Bottom Gas

We need a "best mix" for 55 meters. Let's apply the principles you've learned.

• Oxygen (PO2): A partial pressure of oxygen of 1.4 bar is a standard maximum for the bottom phase of an open-circuit technical dive.
• Nitrogen (END): We'll target an Equivalent Narcotic Depth (END) of 30 meters to ensure mental clarity.

Now, let's do the math, which is clearly laid out in the "A Comprehensive Guide to Technical Dive Planning" article you've seen before.

Let's apply that logic to our 55-meter dive:

1. Ambient Pressure (P): At 55 meters, P = (55 / 10) + 1 = 6.5 bar.

2. Calculate Oxygen Fraction (FO2):
   $F{O}_2=\frac{\text{Target }P{O}_2}{P}=\frac{1.4}{6.5}\approx 0.215$
   We'll round this down to 21% Oxygen. This gives us a PO2 of $0.21\times 6.5=1.365$ bar, which is safely below our 1.4 limit.

3. Calculate Nitrogen Fraction (FN2):

   • An END of 30 meters means we want the partial pressure of nitrogen (PN2) to be what it would be breathing air at 30 meters.
   • PN2 at 30m (4 bar) = $0.79\times 4\text{ bar}=3.16\text{ bar}$.
   • Now, find the fraction of nitrogen that gives us this PN2 at our actual depth of 55 meters.
     $F{N}_2=\frac{\text{Target }P{N}_2}{P}=\frac{3.16}{6.5}\approx 0.486$
     We'll round this to 48% Nitrogen.

4. Calculate Helium Fraction (FHe): The remainder is helium.
   $FHe=1-F{O}_2-F{N}_2=1-0.21-0.48=0.31$
   So, we need 31% Helium.

Our chosen bottom gas is Trimix 21/31.

2.2. Decompression Gases

To accelerate our decompression, we'll use gases with high oxygen content. The standard choices are:

• EAN50 (50% Nitrox): Switched at its Maximum Operating Depth (MOD) of 21 meters.
• 100% Oxygen: Switched at its MOD of 6 meters.

3. Step 2: Decompression Schedule

As you learned in the last lesson, we would now input our dive profile (55m for 25 min) and gas choices (Tx 21/31, EAN50, O2) into a dive planning software using conservative Gradient Factors (e.g., GF 40/80).

For our planning purposes, let's assume the software generates the following decompression schedule:

Depth (m)	Stop Time (min)	Gas
24	1	Trimix 21/31
21	3	EAN50
18	3	EAN50
15	4	EAN50
12	6	EAN50
9	8	EAN50
6	20	Oxygen
Total Decompression:	45 minutes

Now we have a complete ascent plan. The next crucial step is to calculate if we can carry enough gas to complete it.

4. Step 3: Gas Volume Calculations

This is the heart of technical dive planning. It's not enough to have a plan; you must have the gas to execute it, especially if something goes wrong.

First, let's establish a baseline for gas consumption. A diver's gas consumption rate is their Respiratory Minute Volume (RMV). For planning purposes, we'll use these standard values:

• Working RMV: 20 litres/minute (for a calm diver during the main part of the dive).
• Stress RMV: 30 litres/minute (for calculating emergency gas needs).

4.1. Bottom Gas: The "Rock Bottom" Reserve

The most important calculation is the Rock Bottom reserve. This is the absolute minimum gas required for two divers to ascend safely from the most critical point of the dive (55 meters, just before ascent) while sharing air from a single source.

Now, let's perform the detailed calculation for our dive, following the methodology for technical diving.

The emergency scenario involves:

1. Problem Solving: 2 minutes at the bottom (55m) to sort out the emergency and start sharing air.
2. Ascent: Ascending together from 55m to the first gas switch at 21m.

Calculation:

• Gas for Problem Solving (2 divers, stress RMV):

  • Volume = Time × (Diver1 RMV + Diver2 RMV) × Pressure
  • Volume = 2 min × (30 + 30 L/min) × 6.5 bar = 780 Litres

• Gas for Ascent to 21m (2 divers, stress RMV):

  • Ascent time = (55m - 21m) / 9 m/min ≈ 3.8 min. We round up to 4 minutes.
  • Average depth during ascent = (55 + 21) / 2 = 38m. Average pressure = 4.8 bar.
  • Volume = 4 min × (30 + 30 L/min) × 4.8 bar = 1152 Litres

• Total Rock Bottom Reserve:
  $\text{Reserve}=780L+1152L=\text{1932 Litres}$

This is the non-negotiable reserve volume that must be left in your tanks when you begin your ascent.

4.2. Total Bottom Gas and Turn Pressure

Now we can determine our total gas need. We'll assume we are using a standard twinset of two 12-litre cylinders (2x12L) filled to 200 bar.

• Total Starting Volume: 24 Litres × 200 bar = 4800 Litres.

The gas available for the dive itself is divided into two parts: gas for the descent and bottom phase, and gas for the planned ascent and initial decompression stops on bottom gas. A common and safe approach is the Rule of Thirds:

• 1/3 for the descent and first half of the bottom time.
• 1/3 for the second half of the bottom time and ascent to the first gas switch.
• 1/3 as a safety reserve.

However, our calculated Rock Bottom reserve (1932 L) is more precise than a simple "third" (4800 / 3 = 1600 L). We must always use the larger, more conservative number. So, our reserve is 1932 Litres.

• Usable Gas for the dive:
  $\text{Usable Gas}=\text{Total Volume}-\text{Reserve}=4800L-1932L=\text{2868 Litres}$

This 2868 litres covers your descent, 25 minutes at the bottom, and the ascent to your first stop at 24m. Let's check if it's enough (using a working RMV of 20 L/min):

• Descent: ~2 min at avg depth of 27.5m (3.75 bar) = 2 * 20 * 3.75 = 150 L
• Bottom Time: 25 min at 55m (6.5 bar) = 25 * 20 * 6.5 = 3250 L
• Total Needed: 150 + 3250 = 3400 L

Our calculation shows we need 3400 L, but we only have 2868 L of usable gas. This means a 25-minute dive is not feasible with 2x12L cylinders. We would need larger cylinders (e.g., 2x15L) or to shorten our planned bottom time. This is a critical discovery during the planning phase, not underwater!

For the rest of this exercise, let's adjust our planned bottom time to 15 minutes.

• Bottom Time Gas: 15 * 20 * 6.5 = 1950 L
• Total Needed: 150 L (descent) + 1950 L (bottom) = 2100 L.
  This is well within our 2868 L of usable gas.

4.3. Decompression Gas Volumes

This calculation is for a single diver. We need enough of each deco gas to complete the planned stops, plus a safety margin. We'll calculate the required gas and then double it for safety.

• EAN50 Volume:

  • Our schedule has a total of 24 minutes of stops on EAN50, from 21m to 9m.
  • Average depth for these stops is ~15m (2.5 bar).
  • Required Volume = 24 min × 20 L/min × 2.5 bar = 1200 Litres
  • Total to Carry: 1200 L × 2 = 2400 Litres. An 11L cylinder filled to 220 bar would be perfect.

• Oxygen Volume:

  • Our schedule has 20 minutes at 6m (1.6 bar).
  • Required Volume = 20 min × 20 L/min × 1.6 bar = 640 Litres
  • Total to Carry: 640 L × 2 = 1280 Litres. A 7L cylinder filled to 200 bar would suffice.

5. Step 4: Contingency & Team Planning (Gas Matching)

A plan is useless if it doesn't work for the entire team. The entire dive must be planned around the diver with the smallest gas volume. This is called gas matching.

The key takeaway is that even if you have larger cylinders than your buddy, you must turn the dive when they reach their turn pressure. Your plan is only as strong as its weakest link. Each diver calculates their own turn pressure that corresponds to the team's agreed-upon gas usage for the first half of the dive.

Finally, a complete plan includes an emergency and evacuation plan, as noted in the Divesoft article. This includes having emergency oxygen on site, knowing contact information for emergency services and the nearest recompression chamber, and having a clear plan for managing an incident.

Conclusion

Today you have constructed a complete theoretical dive plan for a normoxic trimix dive. You have seen how a simple change in planned bottom time can make a dive feasible or unfeasible, and how critical it is to make that discovery on paper.

Key Takeaways:

• A dive plan integrates gas selection, decompression planning, and volume calculations.
• Gas selection is a balance between managing PO2, END, and cost/thermal properties.
• Rock Bottom is a calculated, non-negotiable gas reserve based on a worst-case emergency scenario for two divers.
• Your usable gas is what remains after subtracting your Rock Bottom reserve from your total volume. Your dive time is limited by this usable gas.
• Decompression gas volumes must be calculated to cover the entire planned deco with a significant safety margin.
• Gas Matching is essential for team safety. The dive plan must be based on the diver with the smallest gas capacity, and all divers must adhere to the turn time/pressure.

Preview of the next lesson:
You have now mastered the theory of planning dives in the 45-60 meter range. With this solid foundation, you are ready to look deeper. In the next module, we will begin our study of Hypoxic Trimix Operations, exploring the new procedures and more complex planning required for dives beyond 60 meters, where your bottom gas is no longer breathable at the surface.