Recreational Gas Planning

Recreational divers often overlook gas planning beyond simply ensuring their tank is full before a dive, but gas planning is more than just making sure you have air. It’s about ensuring you have enough gas to get back safely, handle unexpected situations, and maximize your bottom time efficiently. Without proper planning, divers risk early turnarounds, stress in emergencies, or even running out of gas at the worst possible moment.

A well-thought-out gas plan accounts for:

• Minimum Gas Reserve: Ensuring you have enough to ascend safely with a buddy if needed.
• Turn Pressure: Determining when to head back to your exit point.
• Gas Consumption Rate: Understanding your own breathing rate under different conditions.
• Emergency Scenarios: Factoring in contingencies like sharing air or dealing with an extended ascent.

By taking control of your gas plan, you improve safety, confidence, and overall dive enjoyment. This guide breaks down practical gas planning strategies so you can dive smarter—without needing technical dive training to do it.

Proper gas planning ensures safety, provides a buffer for emergencies, and allows for better dive execution.

Key Concepts in Gas Planning

What is ATA (Atmospheres Absolute)?

ATA is the absolute pressure at a given depth, including atmospheric pressure at the surface:

\[ \text{ATA} = \frac{\text{Depth (ft)}}{33} + 1 \]

For example, at 66 ft, the ATA is:

\[ \frac{66}{33} + 1 = 3 \text{ ATA} \]

Gas consumption increases proportionally with ATA. At 3 ATA, a diver breathes three times more gas per minute than at the surface.

Surface Air Consumption (SAC) and Respiratory Minute Volume (RMV)

• SAC rate describes gas consumption at the surface in pounds per square inch per minute (PSI/min) and is specific to that size cylinder. It is easy to calculate but cumbersome to use in future planning.

\[ SAC = \frac{(\frac{\text{Starting PSI} - \text{Ending PSI}}{\text{Dive Time in Minutes}})}{\text{Average Depth in ATA}} \]

• RMV rate describes gas consumption at the surface in cubic feet per minute (cuft/min) and adjusts for tank size, allowing consistent gas calculations across different cylinders.

\[ RMV = \text{SAC} \times (\frac{\text{Tank Size in cuft}}{\text{Tank Size in PSI}}) \]

• Tank Type: Aluminum 80
• Tank cuft: 80
• Starting PSI: 3000
• Ending PSI: 1000
• Dive Time: 40 minutes
• Average Depth: 30 ft (1.91) ATA

\[ SAC = \frac{(\frac{\text{3000 PSI} - \text{1000 PSI}}{\text{40 Minutes}})}{\text{1.91 Average ATA}} \]

\[ SAC = \frac{\text{50 PSI per minute}}{\text{1.91 Average ATA}} \]

\[ SAC = \text{26.18 PSI/min at the surface} \]

\[ RMV = \text{SAC} \times (\frac{\text{Tank Size in cuft}}{\text{Tank Size in PSI}}) \]

\[ RMV = \text{26.18 PSI/min} \times (\frac{\text{80 cuft}}{\text{3000 PSI}}) \]

\[ RMV = \text{26.18 PSI/min} \times \text{0.0266 Tank Factor} \]

\[ RMV = \text{0.70 cuft/min at the surface} \]

Calculating Respiratory Minute Volume (RMV) is essential for divers who want to manage their air supply efficiently and plan their dives with confidence. RMV is particularly valuable when using different tank sizes, as it provides a standardized measure of gas consumption that is independent of the specific tank in use. This is why I prefer to use RMV when discussing gas consumption instead of SAC.

Minimum Gas Reserve

The simplest Minimum Gas Reserve (MGR) calculation follows the CAT formula. This ensures enough gas for a controlled ascent with a buddy in an emergency, but it does not include a safety stop, since a safety stop is a good practice but not a necessity in an out-of-gas situation.

CAT Formula

\[ \text{MGR} = (\text{RMV} \times 2) \times \text{Average Depth in ATA} \times \text{Time to Ascend} \]

Example Calculation:

• Depth: 60 ft
• Ascent Rate: 10 ft/min
• RMV: 0.75 cuft/min
• Time to Ascend: (60ft max / 10 ft/min) + 1 min = 7 min
• Average Depth: 30 ft \(\approx \) 1.91 ATA

\[ (0.75 \times 2) \times 1.91 \times 7 = 20.1 \text{ cuft} \]

However, divers do not carry devices that display gas volume in cubic feet while underwater. Instead, we rely on an SPG (Submersible Pressure Gauge), which measures pressure in PSI. To use the SPG to monitor our remaining gas volume, we must convert cubic feet to PSI based on the tanks rated capacity and working pressure.

Aluminimum 80 (80 cft / 3000 PSI):

\[ \frac{20.1}{80} \times 3000 = 754 \text{ psi} \]

\[ 754 \text{ psi} \approx 800 \text{ psi} \]

So now what?

Why Do We Round UP Gas Calculations?

SPGs are not perfectly precise and often have a margin of error of ±100 psi. Rounding gas calculations up ensures:

• Easy tracking and communication with a dive buddy.
• Compensating for small pressure gauge inaccuracies.
• Adding an extra safety margin to account for unexpected factors like increased breathing rate or delays in ascent.

Turn Pressure Calculation Methods

Once Minimum Gas Reserve is established, the next step is determining Turn Pressure—the pressure at which a dive team must turn around to ensure enough gas remains for a safe return and ascent. Because this is pressure, we must convert Minimum Reserve Gas to PSI based on whatever cylinder we are using for this particular dive.

Understanding Usable Gas

Usable Gas is the amount of gas available for the dive after subtracting the Minimum Gas Reserve. (An Aluminimum 80 for this example)

\[ \text{Usable Gas} = \text{Start Pressure} - \text{Minimum Gas Reserve} \]

\[ \text{Usable Gas} = \text{3000 PSI} - \text{800 PSI} \]

\[ \text{Usable Gas} = \text{2200 PSI} \]

Usable Gas is then allocated according to different gas management strategies.

Understanding Gas Management Strategies

Different methods exist for planning how much gas to use before turning a dive. The right choice depends on factors such as dive environment and safety margins. By selecting the right gas management strategy, divers ensure safety, efficiency, and proper team planning for every dive.

1. All Usable Gas

Definition: The diver uses all available gas except for the Minimum Gas Reserve.

Formula:

\[ \text{Turn Pressure} = \text{Minimum Gas Reserve} \]

\[ \text{800 PSI} = \text{800 PSI} \]

• Use When:
• Dives where a direct ascent is always possible.
• Avoid When:
• Dive conditions might increase gas usage unexpectedly.

2. Rule of Halves

Definition: The diver splits usable gas into two equal portions, reserving one for the return.

Formula:

\[ \text{Turn Pressure} = \text{Start Pressure} - \left( \frac{\text{Usable Gas}}{2} \right) \]

\[ \text{Turn Pressure} = \text{3000} - \left( \frac{\text{2200}}{2} \right) \]

\[ \text{Turn Pressure} = \text{3000} - \text{1100} \]

\[ \text{Turn Pressure} = \text{1900} \]

• Use When:
• Gas reserves provide a buffer for unexpected conditions.
• Strong currents or deeper dives are factors.
• Avoid When:
• Dives where a direct ascent is always possible.

3. Rule of Thirds (Not for Open Water Recreational Diving)

Definition: The diver splits usable gas into thirds—one for the way out, one for the return, and one for additional emergencies.

Formula:

\[ \text{Turn Pressure} = \text{Start Pressure} - \left( \frac{\text{Usable Gas}}{3} \right) \]

\[ \text{Turn Pressure} = \text{3000} - \left( \frac{\text{2200}}{3} \right) \]

\[ \text{Turn Pressure} = \text{3000} - \text{733}\]

\[ \text{Turn Pressure} = \text{2267}\]

\[ \text{Turn Pressure} = \text{2300}\]

• Use When:
• In overhead environments (caves, wreck penetration, ice diving).
• If direct ascent is not an option.
• Avoid When:
• In open-water recreational diving.

Though you should understand these calculations, you do not need to remember them for common dives you make. Put them in a chart somewhere.

Why the Rule of Thirds Is NOT for Recreational Diving

The Rule of Thirds is commonly misunderstood by recreational divers. While it is often presented as a gas planning method, it is designed specifically for overhead environments where direct ascent is not an option. As recreational divers, we are not supposed to be purposely going in overhead environments.

• In caves, wreck penetration, or ice diving, the Rule of Thirds ensures a diver has enough gas to exit the environment and still retain an emergency reserve.
• Using Rule of Thirds in open water often results in unnecessary gas reserves that are never used, limiting dive time.

For recreational divers, Rule of Halves or All Usable Gas is the correct approach, ensuring safety without unnecessary gas restrictions.

How to Adjust Gas Planning for Different Conditions

Gas consumption is not always constant—different environmental conditions can affect how much gas a diver uses. Properly adjusting gas planning for these conditions ensures safety and efficiency.

• Cold Water Diving: Increases air consumption because the body works harder to maintain warmth.
• Strong Currents: Increases air consumption because of higher workload as you swim against currents.

Final Thoughts on Gas Planning

• Always calculate Minimum Gas Reserve before determining Turn Pressure.
• Select the right method for your dive.
• Avoid using fixed PSI reserves—always calculate based on actual needs.
• Round up PSI values to practical increments.
• Practice gas planning in real-world scenarios.