Recreational Gas Planning
Gas planning is more than starting with a full cylinder. A simple plan ensures you can return safely, handle problems, and still enjoy useful bottom time. This guide covers RMV, Minimum Gas Reserve, practical turn pressure methods, and small adjustments for real conditions.
RMV: Your Gas Consumption Rate
Your Respiratory Minute Volume is the volume you breathe per minute at the surface. Planning by volume keeps the math consistent across cylinder types. If you use SAC or are unsure of the difference, read RMV vs. SAC. To compute your RMV from past dives, use the RMV calculator.
MGR: Minimum Gas Reserve
Minimum Gas Reserve is the amount needed for a calm, controlled ascent while helping a buddy who has lost gas. The CAT method gives a simple, conservative reserve. See the full steps at how we calculate MGR or run your numbers with the MGR calculator.
From Reserve to Turn Pressure
Once you convert MGR into cylinder pressure, you can set turn pressure. Turn pressure is the gauge pressure at which the team must begin the return, or begin the ascent if a direct ascent is available.
Usable Gas
Usable Gas is what remains for the dive after you subtract MGR.
\[ \text{Usable Gas} = \text{Start Pressure} - \text{MGR} \]
Example with an Aluminum 80 tank and an MGR of 800 PSI:
\[ \text{Usable Gas} = 3000\ \text{PSI} - 800\ \text{PSI} = 2200\ \text{PSI} \]
Gas Management Strategies
Pick a method that matches the environment and the team. Each method divides the same inputs differently.
1. All Usable Gas
Definition: You may use all gas that remains after MGR, then begin the ascent when the SPG reaches MGR.
Formula:
\[ \text{Usable Gas} = \text{Starting Pressure} - \text{MGR} \] \[ \text{Turn Pressure} = \text{Starting Pressure} - \text{Usable Gas} \]
Example with an Aluminum 80 tank and an MGR of 800 PSI:
\[ \text{Usable Gas} = \text{3000} - \text{800} = 2200\ \text{PSI} \] \[ \text{Turn Pressure} = 3000 - 2200 = 800\ \text{PSI} \]
- Use when: A direct ascent is always available.
- Avoid when: Workload or conditions may spike gas use on exit.
2. Rule of Halves
Definition: Split usable gas in half and reserve one half for the return.
Formula:
\[ \text{Usable Gas} = \text{Starting Pressure} - \text{MGR} \] \[ \text{Half of Usable Gas} = \left( \frac{\text{Usable Gas}}{2} \right) \] \[ \text{Turn Pressure} = \text{Starting Pressure} - \text{Half of Usable Gas} \]
Example with an Aluminum 80 tank and an MGR of 800 PSI:
\[ \text{Usable Gas} = 3000 - 800 = 2200 \text{PSI}\] \[ \text{Half of Usable Gas} = \left( \frac{2200}{2} \right) = 1100\ \text{PSI}\] \[ \text{Turn Pressure} = 3000 - 1100 = 1900\ \text{PSI} \]
- Use when: A direct ascent is available in an emergency, but you have a preferred exit, such as returning to a downline.
- Avoid when: A direct ascent is always available.
3. Rule of Thirds
Definition: Divide usable gas into thirds for out, back, and emergency.
Formula:
\[ \text{Usable Gas} = \text{Start Pressure} - \text{MGR} \] \[ \text{Third of Usable Gas} = \left( \frac{\text{Usable Gas}}{3} \right) \] \[ \text{Turn Pressure} = \text{Starting Pressure} - \text{Third of Usable Gas} \]
Example:
\[ \text{Usable Gas} = 3000 - 800 = 2200 \text{PSI}\] \[ \text{Third of Usable Gas} = \left( \frac{2200}{3} \right) = 733\ \text{PSI}\] \[ \text{Turn Pressure} = 3000 - 733 = 2267\ \text{PSI} \rightarrow 2300\ \text{PSI (rounded)} \]
- Use when: Overhead environments such as caves, wreck penetration, or ice.
- Avoid when: Open water recreational diving.
Why Thirds Are Not for Recreational Open Water
Thirds were designed for overhead environments where a direct ascent is not possible, even in an emergency. In open water this leaves extra gas unused and shorten dives without adding useful safety.
Recreational divers should use All Usable Gas or Rule of Halves and begin the ascent or return on schedule.
Adjust for Conditions
Gas use changes with workload and environment. Adjust inputs before you set turn pressure.
- Cold water: Higher consumption from thermal stress. Consider Halves even with easy exits.
- Current: Fight current early. Plan extra for the upcurrent leg.
- Task load: Photography, instruction, or navigation usually increases consumption.
Planning Checklist
- Compute MGR first, then derive turn pressure.
- Choose a strategy that matches exit options and conditions.
- Avoid fixed PSI habits. Base plans on RMV, depth, and time.
- Round to sensible SPG increments so the whole team can follow it.
- Record results. Your RMV and comfort improve with practice.