Gas planning calculates the breathing gas required for a dive, factoring in depth, time, exertion, and emergencies. It’s essential to avoid out-of-air situations, manage decompression, and ensure a safe ascent. Below are key techniques, including the Rock Bottom strategy, widely used for precise emergency reserves.
Key Concepts
1. Why Gas Planning Matters
- Safety: Guarantees enough gas for a controlled ascent, even in emergencies like buddy sharing.
- Efficiency: Maximizes dive time without risking reserves.
- Decompression: Supports planned stops or unexpected delays.
2. Basic Terms
- SAC Rate: Surface Air Consumption (psi/bar per minute) at rest on the surface.
- RMV: Respiratory Minute Volume (liters per minute), SAC adjusted for tank size.
- Turn Pressure: Pressure at which you ascend or turn back, preserving gas for return and emergencies.
- Rock Bottom: Minimum gas needed for two divers to safely reach the surface, including sharing air.
Steps for Gas Planning
1. Calculate Your Gas Consumption
- Determine SAC Rate:
- In calm conditions, note starting pressure (e.g., 3000 psi / 200 bar).
- Swim relaxed for 10 minutes at a fixed depth (e.g., 10 ft / 3 m).
- Record ending pressure (e.g., 2800 psi / 186 bar).
- Formula: SAC = (Starting Pressure – Ending Pressure) ÷ Time ÷ ATA.
- ATA = (Depth in feet ÷ 33 + 1) for saltwater.
- Example: (3000 – 2800) ÷ 10 ÷ (10 ÷ 33 + 1) = 200 ÷ 10 ÷ 1.3 = 15.4 psi/min.
- Convert to RMV: RMV = SAC × Tank Factor (e.g., 0.026 for AL80) = 15.4 × 0.026 ≈ 0.4 cu ft/min (~11 L/min).
- Adjust for Effort: Double SAC for exertion (e.g., currents, stress) = 30 psi/min or 22 L/min.
2. Plan Gas Requirements
- Depth Impact: Consumption rises with pressure. ATA = (Depth ÷ 33 + 1). Example: 66 ft = 3 ATA.
- Formula: Gas Used = SAC × Time × ATA.
- Example: 30 psi/min × 20 min × 3 ATA (66 ft) = 1800 psi.
3. Rock Bottom Strategy
- What It Is: Calculates the minimum gas two divers need to share air and ascend safely from the deepest point, including a safety stop.
- How to Calculate:
- Ascent Gas: Assume 1 minute per 33 ft (1 bar) at an elevated SAC (e.g., 60 psi/min for stress).
- 66 ft to 0 ft = 2 min avg. 33 ft (2 ATA): 60 × 2 × 2 = 240 psi per diver × 2 = 480 psi.
- Safety Stop Gas: 3 min at 15 ft (1.45 ATA): 60 × 3 × 1.45 = 261 psi per diver × 2 = 522 psi.
- Total Rock Bottom: 480 + 522 = 1002 psi (round to 1000 psi for simplicity).
- Ascent Gas: Assume 1 minute per 33 ft (1 bar) at an elevated SAC (e.g., 60 psi/min for stress).
- Key Rule: Never dip into Rock Bottom gas—turn the dive when you reach this pressure.
4. Set Turn Pressure
- Total Gas Needed: Bottom time + ascent + safety stop + Rock Bottom.
- Example (66 ft, 20 min):
- Bottom: 1800 psi.
- Ascent: 132 psi (30 × 2 × 2.2).
- Stop: 131 psi (30 × 3 × 1.45).
- Rock Bottom: 1000 psi.
- Total: 1800 + 132 + 131 + 1000 = 3063 psi.
- Example (66 ft, 20 min):
- Turn Point: Starting Pressure – (Bottom Gas + Ascent + Stop) = Turn Pressure.
- 3000 psi – (1800 + 132 + 131) = 937 psi (turn at ~1000 psi, leaving Rock Bottom intact).
- Rule of Thirds Alternative: ⅓ out, ⅓ back, ⅓ reserve (e.g., 3000 psi = turn at 2000 psi). Less precise than Rock Bottom for deep dives.
5. Adjust for Dive Profile
- Multi-Level Dive:
- Segment by depth/time:
- 10 min at 60 ft (2.8 ATA): 30 × 10 × 2.8 = 840 psi.
- 10 min at 30 ft (1.9 ATA): 30 × 10 × 1.9 = 570 psi.
- Total (with ascent/stop/Rock Bottom): 840 + 570 + 263 + 1000 = 2673 psi.
- Segment by depth/time:
- Ascent Rate: 30 ft/min standard—factor into gas and time.
Practical Example with Rock Bottom
- Dive: 66 ft, 20 min, AL80 (3000 psi), SAC 30 psi/min, saltwater.
- Gas Breakdown:
- Bottom: 30 × 20 × 3 = 1800 psi.
- Ascent (2 min, 2.2 ATA): 30 × 2 × 2.2 = 132 psi.
- Stop (3 min, 1.45 ATA): 30 × 3 × 1.45 = 131 psi.
- Rock Bottom: 1000 psi (60 psi/min stressed SAC).
- Total: 3063 psi (exceeds tank—shorten dive or use larger tank).
- Plan:
- Start: 3000 psi.
- Turn: ~1000 psi (after ~16 min at depth).
- Surface: Rock Bottom (1000 psi) intact.
Tools and Techniques
1. Dive Computer
- Tracks depth, time, and pressure (if air-integrated). Use alongside manual plans.
2. Dive Slate
- Pre-calculate: “66 ft, Turn 1000 psi, Rock Bottom 1000 psi.”
3. Buddy Coordination
- Share SAC rates and agree on the higher consumer’s Rock Bottom for safety.
Tips for Success
- Measure SAC: Test in rest and stress conditions for accuracy.
- Buffer: Add 10–20% to Rock Bottom (e.g., 1200 psi) for extra safety.
- Monitor: Check pressure every 5–10 minutes.
- Nitrox: Extends no-deco time but requires PPO2 limits (e.g., 1.4 ATA max for 32% = 112 ft).
Common Mistakes
- No Reserve: Using Rock Bottom gas—always preserve it.
- Depth Oversight: Ignoring ATA increase—recalculate for max depth.
- Buddy Neglect: Plan for two, not one, in emergencies.
Gas planning with Rock Bottom ensures you’re prepared for the worst while enjoying the dive. Combine your SAC, dive profile, and a robust emergency reserve for confidence underwater.
Here’s a safety disclaimer tailored for a scuba diving blog, written in Markdown format for easy integration. It emphasizes the importance of professional training and personal responsibility while aligning with the informational nature of your scuba-related content.
Safety Disclaimer
The information provided on this blog is intended for educational and informational purposes only. Scuba diving is an inherently risky activity that requires proper training, certification, and experience to perform safely. The content here—including guides on equipment selection, weighting, gas planning, and diving techniques—is not a substitute for professional instruction from a certified scuba diving instructor or dive professional.
- Training Required: Always complete formal scuba diving training through a recognized agency (e.g., PADI, SSI, NAUI) before attempting any dive. The techniques, calculations, and recommendations discussed may not apply to all situations and should be adapted to your specific training, equipment, and conditions.
- Personal Responsibility: Diving decisions, including gas planning, equipment use, and dive execution, are your responsibility. Verify all information with qualified professionals and adhere to safe diving practices.
- Risk Awareness: Scuba diving involves risks such as decompression sickness, equipment failure, and environmental hazards. Conditions vary by location, depth, and diver fitness. This blog does not guarantee safety or account for individual circumstances.
- Consult Experts: Before applying any advice from this blog, consult with a dive instructor, dive shop, or medical professional to ensure it suits your skill level, health, and dive plan.
- No Liability: The authors, contributors, and publishers of this blog are not liable for any injury, loss, or damage resulting from the use or misuse of the information provided. Dive at your own risk.
Stay safe, dive within your limits, and always plan your dive and dive your plan. For emergencies, follow the guidance of your training and local dive protocols.
Motus Aquaticus 
Leave a comment