Rock Bottom Gas Management for Technical Diving

Hello! Welcome back to your technical diving course.

In our last lesson, we determined what gases to carry for an efficient decompression. We learned to calculate optimal high-oxygen mixes like EAN50 and pure oxygen, based on their Maximum Operating Depth (MOD). Now, we need to answer the next crucial question: how much of this gas, and our bottom gas, do we actually need to carry?

This lesson addresses one of the most critical safety concepts in technical diving. We will move beyond simplistic rules like "save 50 bar" and learn how to apply rock bottom gas management principles to technical dive planning. This is a fundamental shift in mindset from recreational diving, where you plan for the worst-case scenario first, and then build your dive plan with the gas that remains.

1. The Rock Bottom Philosophy: Plan for the Worst, Dive the Rest

In recreational diving, you might have learned the "rule of thirds" (a third of your gas out, a third back, and a third in reserve) or to simply surface with 50 bar left in your tank. These rules are arbitrary and can be dangerously inadequate for technical diving. A 50 bar reserve might be plenty for a shallow dive, but it's not nearly enough to get you and a buddy to the surface from 50 meters.

Technical diving adopts a more rigorous approach called Rock Bottom gas management.

• Definition: Rock Bottom is the absolute minimum amount of gas required for two divers to safely ascend from the deepest point of the dive while sharing gas from a single source, completing all required decompression stops.

The core idea is to calculate the gas needed for this worst-case emergency first. This calculated pressure is your "rock bottom" reserve. You are never, under any circumstances, allowed to use this gas for the normal part of the dive. The gas you have above this reserve is your "usable" gas for the dive itself.

As the video explains, this method puts you in control. You know with certainty that you have enough gas for an emergency, and you can then make an informed decision about whether the remaining "usable" gas is sufficient for your planned dive.

2. How to Calculate Your Rock Bottom Reserve

Calculating your Rock Bottom pressure involves modeling the entire ascent from the deepest point of the dive under emergency conditions. We can break this down into a clear, step-by-step process.

The fundamental formula we'll use is:

$\text{Gas Volume (L)}=\text{Time (min)}\times \text{Gas Consumption Rate (L/min)}$

Of course, the gas consumption rate changes with depth (pressure), so we need to account for that. The full formula for the volume of gas needed for the ascent is:

$\text{Volume (L)}=\text{Total Ascent Time}\times (\text{Stressed SAC}\times 2)\times \text{Average Pressure}$

Let's break down each component of this formula.

Step 1: Determine the Total Ascent Time

This is the total time it takes for two divers to get from the maximum depth to the surface. We must account for every phase of the ascent:

• Problem Response (1 minute): A standard assumption is that it takes one minute at the maximum depth to recognize the out-of-gas emergency, for the donor to provide their alternate regulator, and for the team to stabilize before starting the ascent.
• Ascent to First Stop: A safe ascent rate is crucial. A common rate used in technical diving is 9-10 meters per minute.
• Decompression Stops: The time spent at all required decompression stops must be included.

Step 2: Determine the Emergency Gas Consumption Rate

In an out-of-gas emergency, both divers will be under stress. Their breathing rates will be elevated.

• Stressed SAC Rate: A conservative Surface Air Consumption (SAC) rate of 30 litres/minute is a widely used standard for planning purposes.
• Two Divers: Since both divers are breathing from a single source, we must double this rate. The combined surface consumption rate is therefore 60 L/min.

Step 3: Determine the Average Pressure (ata)

Gas consumption is proportional to ambient pressure. To simplify the calculation for the entire ascent, we use the average pressure.

• Average Depth: A simple and effective way to find the average depth is (Max Depth + Surface Depth) / 2.
• Average Pressure: Convert this average depth to absolute pressure (ata). Average Pressure = (Average Depth / 10) + 1.

Step 4: Calculate the Total Volume and Convert to Pressure

Once you have the total volume in litres, you convert it to the pressure (bar) it represents in your specific cylinder.

• Pressure (bar): Rock Bottom Pressure = Total Volume (L) / Cylinder Size (L)

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Worked Example

Let's watch a detailed walkthrough of this calculation. The following video calculates the Rock Bottom reserve for a 30-meter dive.

Let's summarize the parameters from that video's example:

• Max Depth: 30 meters
• Ascent Profile & Time:
  • Problem Response: 1 min @ 30m
  • Ascent from 30m to 15m (@ 10m/min): 1.5 min (rounded to 2 min)
  • Ascent from 15m to 9m (@ 3m/min): 2 min
  • Stop at 9m: 1 min
  • Stop at 6m: 2 min
  • Stop at 3m: 3 min
  • Total Ascent Time: 1 + 2 + 2 + 1 + 2 + 3 = 11 minutes
• Emergency Gas Consumption:
  • Stressed SAC: 30 L/min
  • For two divers: 60 L/min
• Average Pressure:
  • Average Depth: 30m / 2 = 15m
  • Average Pressure: (15 / 10) + 1 = 2.5 ata

Now, we plug these into the formula:

$\text{Volume (L)}=11\text{ min}\times 60\text{ L/min}\times 2.5\text{ ata}=1650\text{ Litres}$

Finally, convert this volume to pressure for a 15-litre tank:

$\text{Rock Bottom Pressure}=\frac{1650\text{ L}}{15\text{ L}}=110\text{ bar}$

This is the Rock Bottom reserve. For this dive plan, you must begin your ascent before your pressure gauge reads 110 bar.

3. From Reserve to Dive Plan: Usable Gas and Turn Pressure

Calculating the Rock Bottom reserve is the first half of the process. The second half is using that information to plan your dive.

1. Calculate Usable Gas: This is the amount of gas you can actually use for the dive itself.
   $\text{Usable Gas (bar)}=\text{Starting Pressure}-\text{Rock Bottom Pressure}$
   For our example, with a 200 bar fill: 200 bar - 110 bar = 90 bar.

2. Calculate Turn Pressure: This is the gauge reading that signals the end of the bottom phase of the dive. The calculation depends on the dive profile.

   • Direct Ascent (e.g., drift dive): You ascend from your furthest point. The usable gas is for the bottom time. Your turn pressure is your Rock Bottom pressure. You start your ascent when your gauge hits 110 bar.
   • Out-and-Back (e.g., shore dive, wreck penetration): You must return to your starting point before ascending. You must split your usable gas. The simplest method is the "rule of halves": half for the journey out, half for the journey back.
     $\text{Turn Pressure}=\text{Rock Bottom}+\frac{\text{Usable Gas}}{2}$
     In our example: 110 bar + (90 bar / 2) = 110 + 45 = 155 bar. You must turn the dive when any team member's gauge reads 155 bar.

4. Team Planning and Conservatism

Two final, critical points on gas management:

• Team First: All gas planning must be based on the diver with the smallest gas supply. If you are diving with a 15L tank but your buddy has a 12L tank, all Rock Bottom and turn pressure calculations must be based on the 12L tank. The team is only as strong as its weakest link.
• Conservatism: The parameters we used (1 min response, 30 L/min SAC) are common standards, but you can add conservatism. For example, you could assume a higher stressed SAC rate (e.g., 35 or 40 L/min) or add a small pressure buffer (e.g., 10-20 bar) to your final Rock Bottom calculation.

The article "Scuba Gas Planning" discusses this idea of adding conservatism for stressed breathing.

Conclusion

Mastering Rock Bottom gas management is non-negotiable for safe technical diving. It replaces arbitrary rules with a robust, scenario-based calculation that ensures you and your team have the gas necessary to handle a worst-case failure.

Key Takeaways:

• Rock Bottom is a gas planning philosophy that prioritizes calculating the reserve for a worst-case, out-of-gas emergency first.
• The calculation determines the gas volume needed for two divers to ascend from the maximum depth on a single gas source, including all decompression.
• The key variables are Total Ascent Time, a Stressed SAC Rate (x2), and the Average Pressure of the ascent.
• The calculated Rock Bottom pressure is your absolute minimum reserve. The gas above this is "usable" for the dive.
• Turn pressure is calculated based on the Rock Bottom reserve and the dive profile (direct ascent vs. out-and-back).
• All team gas planning must be based on the diver with the smallest gas volume.

Preview of the Next Lesson:

We now have a solid understanding of our gas reserve. In the next lesson, we will use the "usable gas" portion of our cylinder to calculate the bottom gas volume requirements for a planned technical dive profile. This will allow us to determine how much bottom time we can achieve with a given set of cylinders.