Decompression Gas Volume Calculation

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

In our last session, we mastered the calculation of bottom gas requirements, learning how to determine the gas needed for the deepest phase of our dive and how to calculate our maximum allowable bottom time based on our usable gas supply.

Today, we will address the final major component of planned gas consumption: the gas required for decompression. This lesson directly addresses the learning outcome: Calculate decompression gas volume requirements based on a planned decompression schedule. By the end of our session, you'll be able to take a given decompression schedule and precisely calculate the volume of gas needed to complete it safely.

1. The Role of Decompression Gas

Before we calculate the volume, let's clarify what decompression gas is and why it's a distinct part of our plan. During the bottom phase of a dive, your body tissues absorb inert gases (like nitrogen and helium) from your breathing mix. To avoid decompression sickness (DCS), you must ascend slowly and make planned stops at specific depths to allow these gases to safely "off-gas" from your tissues. The gas you breathe during these stops is your decompression gas.

While you can decompress using your bottom gas (the gas you breathe at depth), technical divers often carry separate cylinders with gas mixtures optimized for decompression. These mixes typically have a higher percentage of oxygen, which accelerates the off-gassing process.

2. The Calculation: Same Formula, New Application

The good news is that you already know the fundamental formula for this calculation. It's the exact same one we used to calculate bottom gas volume:

$\text{Gas Volume (L)}=\text{SAC Rate (L/min)}\times \text{Ambient Pressure (ata)}\times \text{Time (min)}$

When calculating decompression gas, we simply apply this formula to each segment—or "stop"—of our planned decompression schedule. A decompression schedule is a list of depths and the time you must spend at each.

A key consideration is that your Surface Air Consumption (SAC) rate during decompression is often lower than your "working" SAC rate on the bottom. Decompression is a period of rest, not exertion. For planning, it's common to use a more relaxed SAC rate. For our examples, we'll use a working SAC of 20 L/min for the bottom phase and a deco SAC of 15 L/min for the ascent and decompression stops.

3. Worked Example: Calculating Decompression Gas

Let's calculate the gas required for a specific decompression schedule. Imagine your dive planning software has generated the following schedule for a dive:

• Stop 1: 3 minutes at 15 meters
• Stop 2: 7 minutes at 9 meters
• Stop 3: 15 minutes at 6 meters

We will calculate the gas needed for each stop and then sum them to find the total. Our diver's deco SAC rate is 15 L/min.

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Step 1: Calculate gas for the 15-meter stop.

• Pressure at 15m = $(15/10)+1=2.5\text{ ata}$
• Gas Volume = $15\text{ L/min}\times 2.5\text{ ata}\times 3\text{ min}=112.5\text{ Litres}$

Step 2: Calculate gas for the 9-meter stop.

• Pressure at 9m = $(9/10)+1=1.9\text{ ata}$
• Gas Volume = $15\text{ L/min}\times 1.9\text{ ata}\times 7\text{ min}=199.5\text{ Litres}$

Step 3: Calculate gas for the 6-meter stop.

• Pressure at 6m = $(6/10)+1=1.6\text{ ata}$
• Gas Volume = $15\text{ L/min}\times 1.6\text{ ata}\times 15\text{ min}=360\text{ Litres}$

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Step 4: Calculate total decompression gas.

Now, we simply add the volumes from each stop:

$\text{Total Deco Gas}=112.5\text{ L}+199.5\text{ L}+360\text{ L}=672\text{ Litres}$

So, for this planned schedule, you need a minimum of 672 litres of decompression gas.

4. Planning for Contingencies

In technical diving, planning for the minimum is not enough. We must always carry a reserve for unforeseen circumstances, such as:

• Needing more decompression time than planned.
• Dealing with an equipment issue.
• Sharing gas with a teammate during decompression.

A common and robust method for calculating this reserve is to apply a multiplier to your planned gas volume.

As the handout explains, a standard approach is to carry 1.5 times the planned gas volume. This is equivalent to the "rule of thirds": one third for the planned use, one third for contingencies, and one third for your buddy's contingencies.

Let's apply this to our example:

$\text{Total Deco Gas with Reserve}=672\text{ Litres}\times 1.5=1008\text{ Litres}$

To safely execute this decompression plan, you should carry at least 1008 litres of decompression gas.

5. From Gas Volume to Cylinder Pressure

The final step is to translate this required volume into a practical measure: the pressure you need in your decompression cylinder. This depends on the size of the cylinder you choose to carry.

The formula is:
$\text{Required Pressure (bar)}=\frac{\text{Total Gas Volume (L)}}{\text{Cylinder Volume (L)}}$

Let's say you plan to use a 7-litre cylinder for your decompression gas.

$\text{Required Pressure}=\frac{1008\text{ L}}{7\text{ L}}\approx 144\text{ bar}$

This means you must start the dive with at least 144 bar in your 7-litre decompression cylinder to meet the requirements of your plan, including the contingency reserve. If you chose an 11-litre cylinder, you would need:

$\text{Required Pressure}=\frac{1008\text{ L}}{11\text{ L}}\approx 92\text{ bar}$

This final step makes your abstract volume calculation a concrete, verifiable number you can check on your pressure gauge before you dive.

Conclusion

You are now equipped to calculate the gas requirements for the decompression phase of a technical dive. This is a critical skill that bridges the gap between a theoretical decompression schedule and the practical reality of gas management.

Key Takeaways:

• Decompression gas is calculated using the same Volume = SAC × Pressure × Time formula as bottom gas.
• The calculation is performed for each decompression stop individually, and the results are summed.
• It is crucial to use a SAC rate appropriate for the relaxed state of decompression.
• A significant contingency reserve (e.g., multiplying planned gas by 1.5) must be added to account for potential problems.
• The final required gas volume is converted into a minimum starting pressure for the specific decompression cylinder being used.

Preview of the Next Lesson:

We have now learned how to calculate gas for our Rock Bottom reserve, our bottom phase, and our decompression phase. In the next lesson, we will learn how to integrate bottom gas, travel gas, and decompression gas volumes with contingency reserves to determine total gas requirements. We will assemble all these individual pieces into one complete, holistic gas plan for a multi-stage technical dive.