Recreational Gas Planning
Gas planning is more than starting a dive with a full cylinder. It ensures you can return safely, handle problems, and still enjoy useful bottom time. This guide explains RMV, Minimum Gas Reserve, turn pressure methods, and how to adjust plans for conditions. All examples use clear equations so you can apply them on your next dive.
Gas Consumption Rate
You cannot plan gas reliably without your personal Respiratory Minute Volume (RMV), the volume you breathe per minute at the surface. If you use SAC or do not know the difference, read RMV vs. SAC to see why RMV is preferred for planning. If you need help computing it, try the RMV calculator.
Minimum Gas Reserve
The simplest Minimum Gas Reserve (MGR) follows the CAT formula. It covers a controlled ascent with a buddy in an emergency and does not include a safety stop. For the detailed math, see how we calculate MGR. To run the numbers for your dive, use the Minimum Gas Reserve Calculator.
Turn Pressure Calculation Methods
Once you set MGR in pressure units for your cylinder, compute turn pressure. Turn pressure is the pressure at which the team must start the return so enough gas remains for the exit and ascent.
Understanding Usable Gas
Usable Gas is the amount of gas available for the dive after you subtract MGR. Example with an Aluminum 80:
\[ \text{Usable Gas} = \text{Start Pressure} - \text{Minimum Gas Reserve} \]
\[ \text{Usable Gas} = 3000 \ \text{PSI} - 800 \ \text{PSI} = 2200 \ \text{PSI} \]
Allocate usable gas according to one of the strategies below.
Understanding Gas Management Strategies
Choose a strategy based on environment and risk tolerance. Each method below uses the same inputs but divides usable gas differently.
1. All Usable Gas
Definition: Use all gas that remains after MGR.
Formula:
\[ \text{Turn Pressure} = \text{Minimum Gas Reserve} \]
\[ 800 \ \text{PSI} = 800 \ \text{PSI} \]
- Use When: A direct ascent is always available.
- Avoid When: Workload or conditions may spike gas use.
2. Rule of Halves
Definition: Split usable gas in half and reserve one half for the return.
Formula:
\[ \text{Turn Pressure} = \text{Start Pressure} - \left( \frac{\text{Usable Gas}}{2} \right) \]
\[ \text{Turn Pressure} = 3000 - \left( \frac{2200}{2} \right) = 3000 - 1100 = 1900 \ \text{PSI} \]
- Use When: You want a buffer for current, depth, or navigation uncertainty.
- Avoid When: A direct ascent is always available and you value maximum bottom time.
3. Rule of Thirds (not for open water recreational diving)
Definition: Divide usable gas into thirds: out, back, and emergency.
Formula:
\[ \text{Turn Pressure} = \text{Start Pressure} - \left( \frac{\text{Usable Gas}}{3} \right) \]
\[ \text{Turn Pressure} = 3000 - \left( \frac{2200}{3} \right) \approx 3000 - 733 = 2267 \ \text{PSI} \rightarrow 2300 \ \text{PSI (rounded)} \]
- Use When: Overhead environments such as caves, wreck penetration, or ice.
- Avoid When: Open water recreational diving.
For quick reference on the examples above, keep a small chart with common start pressures and MGR values.
Why the Rule of Thirds is not for Recreational Diving
The Rule of Thirds is designed for overhead environments where a direct ascent is not possible. It ensures exit gas plus an emergency reserve when you must return the way you came. In open water, it often leaves unused reserve and shortens dives without adding useful safety. Recreational teams should use All Usable Gas or Halves as appropriate to conditions.
How to Adjust Gas Planning for Conditions
Gas use changes with workload and environment. Adjust inputs before you set turn pressure.
- Cold water: Higher consumption due to thermal load. Consider a more conservative strategy.
- Current: Plan extra for upstream work. Align the route so you fight current early.
Planning Checklist
- Compute MGR first, then derive turn pressure.
- Pick a strategy that matches the dive and team.
- Avoid fixed PSI reserves. Base plans on RMV, depth, and time.
- Round pressures to practical increments for your SPG.
- Practice planning on recent dives and record results.