Calculating Total Gas Requirements for Diving

Hello! Welcome to your next lesson in technical dive planning.

In our previous lessons, we've built the essential skills for gas planning piece by piece. We learned to calculate the gas needed for the bottom phase of a dive and how to determine the volume required for a given decompression schedule.

Today, we will put all those pieces together. This lesson addresses the learning outcome: Integrate bottom gas, travel gas, and decompression gas volumes with contingency reserves to determine total gas requirements. We will assemble a complete, holistic gas plan for a technical dive, moving from individual calculations to a final, integrated strategy that tells you exactly how much of each gas you need and which cylinders will carry it.

1. The Components of a Total Gas Plan

A complete gas plan for a technical dive accounts for every phase of the dive and every gas you will breathe. Before we can sum everything up, we need to be clear on what those components are.

As you just read, our total gas requirement is a combination of:

• Bottom Gas: Used for the deepest part of the dive. This is typically carried in your largest cylinders (e.g., back-mounted twins).
• Travel Gas: Used during descent and ascent when the bottom gas is hypoxic (has too little oxygen to be safely breathed at shallow depths).
• Decompression Gas(es): One or more oxygen-rich mixes used to accelerate decompression during the ascent. These are usually carried in separate "stage" cylinders.

To determine the total volume for each, we must also calculate and integrate our Contingency Reserve.

2. The Foundation: Planning for the Worst-Case First

In recreational diving, you might have learned to end your dive with a fixed reserve pressure (e.g., 50 bar). Technical diving requires a more robust approach. We don't calculate our dive and then see what's left for an emergency; we calculate the gas for the worst-case emergency first and then plan our dive with the gas that remains. This emergency reserve is often called "Rock Bottom" or "Minimum Gas".

The worst-case scenario is typically defined as a complete gas failure for one diver at the deepest point of the dive, requiring the other diver to share gas for the entire ascent, including all decompression.

This "Rock Bottom" volume is not an extra bottle you carry. It is the minimum pressure you must have remaining in your primary (bottom gas) cylinders when you decide to end the dive. Your turn pressure will be this Rock Bottom pressure plus the gas needed to get back to your ascent point.

3. Integrating the Plan: A Worked Example

Let's build a complete gas plan for a hypothetical dive. This will integrate all the concepts we've discussed.

The Dive Plan:

• Depth: 45 meters
• Bottom Time: 20 minutes
• Gases:
  • Bottom Gas (Back Gas): Trimix 18/45 (18% O₂, 45% He). This is hypoxic.
  • Travel Gas: Nitrox 32 (32% O₂). Used from surface to 30m.
  • Deco Gas 1: Nitrox 50 (50% O₂).
  • Deco Gas 2: 100% Oxygen.
• Decompression Schedule:
  • Ascent to 21m. Switch to Nitrox 50.
  • 2 min @ 21m
  • 4 min @ 12m
  • 5 min @ 9m
  • Switch to 100% O₂.
  • 10 min @ 6m
• SAC Rates:
  • Working SAC (Bottom): 20 L/min
  • Deco/Travel SAC: 15 L/min
  • Emergency SAC (Stressed): 30 L/min

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Step 1: Calculate Rock Bottom Reserve (Minimum Gas)

This is the gas for two divers sharing gas from 45m, using a stressed SAC of 30 L/min each (total 60 L/min), and completing the full decompression schedule on the donor's back gas (Tx 18/45).

1. Problem Solving (1 min @ 45m):

   • Pressure: $45/10+1=5.5\text{ ata}$
   • Volume: $60\text{ L/min}\times 5.5\text{ ata}\times 1\text{ min}=330\text{ L}$

2. Ascent to First Stop (45m to 21m @ 9m/min):

   • Time: $(45-21)/9=2.67\approx 3\text{ min}$
   • Avg. Depth: $(45+21)/2=33\text{ m}$ -> $4.3\text{ ata}$
   • Volume: $60\text{ L/min}\times 4.3\text{ ata}\times 3\text{ min}=774\text{ L}$

3. Decompression Stops (on Back Gas):

   • 2 min @ 21m (3.1 ata): $60\times 3.1\times 2=372\text{ L}$
   • 4 min @ 12m (2.2 ata): $60\times 2.2\times 4=528\text{ L}$
   • 5 min @ 9m (1.9 ata): $60\times 1.9\times 5=570\text{ L}$
   • 10 min @ 6m (1.6 ata): $60\times 1.6\times 10=960\text{ L}$

4. Total Rock Bottom Volume:
   $330+774+372+528+570+960=3534\text{ Litres}$
   This is the absolute minimum gas required in the back-gas cylinders to handle the worst-case scenario.

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Step 2: Calculate Planned Gas Consumption (for one diver)

This is the gas needed for one diver to execute the dive as planned, using the appropriate gas for each phase and a relaxed SAC rate.

1. Travel Gas (Nitrox 32): Descent from surface to 30m @ 18m/min.

   • Time: $30/18\approx 1.7\text{ min}$
   • Avg. Depth: $30/2=15\text{ m}$ -> $2.5\text{ ata}$
   • Volume: $15\text{ L/min}\times 2.5\text{ ata}\times 1.7\text{ min}=64\text{ L}$

2. Bottom Gas (Tx 18/45): Descent from 30m to 45m, plus 20 min bottom time.

   • Descent (30m to 45m @ 18m/min): Time $\approx 1\text{ min}$, Avg. Depth $37.5\text{ m}$ -> $4.75\text{ ata}$
     • Volume: $20\text{ L/min}\times 4.75\text{ ata}\times 1\text{ min}=95\text{ L}$
   • Bottom Time (20 min @ 45m):
     • Volume: $20\text{ L/min}\times 5.5\text{ ata}\times 20\text{ min}=2200\text{ L}$
   • Total Planned Bottom Gas: $95+2200=2295\text{ L}$

3. Deco Gas 1 (Nitrox 50): Ascent from 45m to 21m, plus stops at 21m, 12m, 9m.

   • Ascent (45m to 21m @ 9m/min): Time $\approx 3\text{ min}$, Avg. Depth $33\text{ m}$ -> $4.3\text{ ata}$
     • Volume: $15\text{ L/min}\times 4.3\text{ ata}\times 3\text{ min}=194\text{ L}$
   • Stops:
     • 2 min @ 21m (3.1 ata): $15\times 3.1\times 2=93\text{ L}$
     • 4 min @ 12m (2.2 ata): $15\times 2.2\times 4=132\text{ L}$
     • 5 min @ 9m (1.9 ata): $15\times 1.9\times 5=143\text{ L}$
   • Total Planned Nitrox 50: $194+93+132+143=562\text{ L}$

4. Deco Gas 2 (100% O₂): Stop at 6m.

   • 10 min @ 6m (1.6 ata): $15\text{ L/min}\times 1.6\text{ ata}\times 10\text{ min}=240\text{ L}$

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Step 3: Integrate and Allocate to Cylinders

Now we combine the planned gas with reserves to find the total gas needed in each cylinder. For stage-carried gases (travel, deco), a common contingency is the "Rule of Thirds" (or multiplying by 1.5), which we discussed in the last lesson.

1. Back Gas (Tx 18/45):

   • Total Required = Planned Bottom Gas + Rock Bottom Reserve
   • Total = $2295\text{ L}+3534\text{ L}=5829\text{ Litres}$
   • Cylinder Choice: Twin 12L cylinders (24L total).
   • Required Pressure: $5829\text{ L}/24\text{ L}=243\text{ bar}$. You would need your twinset filled to at least 243 bar.

2. Travel Gas (Nitrox 32):

   • Total Required = Planned Gas $\times$ 1.5
   • Total = $64\text{ L}\times 1.5=96\text{ Litres}$
   • Cylinder Choice: A small 3L or 7L stage cylinder would be sufficient. For a 7L cylinder, you'd need $96/7\approx 14\text{ bar}$. This is a very small amount, so this gas might be combined with a deco gas in some plans, but we'll keep it separate for clarity.

3. Deco Gas 1 (Nitrox 50):

   • Total Required = Planned Gas $\times$ 1.5
   • Total = $562\text{ L}\times 1.5=843\text{ Litres}$
   • Cylinder Choice: A 7L stage cylinder.
   • Required Pressure: $843\text{ L}/7\text{ L}=121\text{ bar}$.

4. Deco Gas 2 (100% O₂):

   • Total Required = Planned Gas $\times$ 1.5
   • Total = $240\text{ L}\times 1.5=360\text{ Litres}$
   • Cylinder Choice: A 7L stage cylinder.
   • Required Pressure: $360\text{ L}/7\text{ L}=52\text{ bar}$.

4. The Final Integrated Gas Plan

The result of all these calculations is a clear, actionable plan. Here it is in a summary table:

Gas Type	Planned Use (L)	Contingency Reserve (L)	Total Volume (L)	Cylinder(s)	Min. Fill (bar)
Back Gas (Tx 18/45)	2,295	3,534 (Rock Bottom)	5,829	Twin 12L (24L total)	243
Travel Gas (N₂ 32)	64	32 (1/3 for reserve)	96	7L Stage	14
Deco Gas 1 (N₂ 50)	562	281 (1/3 for reserve)	843	7L Stage	121
Deco Gas 2 (100% O₂)	240	120 (1/3 for reserve)	360	7L Stage	52

This table represents the complete integration of all gas requirements for the dive. It tells you not just what you plan to breathe, but ensures you are carrying enough gas to solve the most likely worst-case emergency.

Conclusion

You have now learned how to construct a complete gas plan for a multi-stage technical dive. This process is the heart of technical dive planning, transforming theoretical profiles and schedules into a practical and safe strategy.

Key Takeaways:

• A total gas plan integrates bottom gas, travel gas, and decompression gases.
• The plan is built on the foundation of a "Rock Bottom" reserve, calculated for a worst-case gas sharing emergency from the deepest point.
• The gas required for the planned dive profile is calculated for a single diver with a normal SAC rate.
• Total gas for back-mounted cylinders is the sum of planned use and the Rock Bottom reserve.
• Total gas for stage cylinders includes the planned volume plus a contingency reserve (e.g., rule of thirds).
• Finally, these total volumes are converted into minimum fill pressures for the chosen cylinders.

Preview of the Next Lesson:

We have now mastered the "how much" of gas planning. In our next module, we will dive deeper into the "what." The next lesson will focus on the first outcome of Normoxic Trimix Operations: Define normoxic trimix and calculate appropriate mixes for dives in the 45-60 meter range. We will learn how to select the optimal percentages of oxygen, helium, and nitrogen for a specific dive plan.