Recreational Gas Planning

Tyler, Fran and John planning a cenote dive in Tulum, MX

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:

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)

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

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

A full calculation after a dive would look something like this:

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

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

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


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

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

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

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

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:

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

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):

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

Round Up:

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

So now what?

The Minimum Reserve Gas for specific depth with a specific RMV will always be the same when expressed as cuft per min. You can calculate Minimum Reserve Gas once and store it somewhere. If you then typically dive certain sized tanks, you can also pre-calculate PSI for a specific tank as well.

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:


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:

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

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


2. Rule of Halves

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

Formula:

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

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

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

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


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:

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

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

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

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

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


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.

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.

Final Thoughts on Gas Planning