Minimum Gas Reserve
Minimum Gas Reserve ensures you have enough gas to ascend with a buddy during an emergency. The CAT formula gives a simple, repeatable way to compute that reserve. This guide shows the exact math with a worked example, then converts cubic feet to PSI for an AL80, and explains why rounding up is standard practice.
CAT Formula
\[ \text{MGR} = (\text{RMV} \times 2) \times \text{Average Depth in ATA} \times \text{Time to Ascend} \]
This covers a controlled ascent with a buddy and intentionally excludes a safety stop. If you want to run custom numbers quickly, try the Minimum Gas Reserve Calculator.
Worked Example: 60 ft at 10 ft/min
- Depth: 60 ft
- Ascent rate: 10 ft/min
- RMV: 0.75 cu ft/min (per diver)
- Time to ascend: \( 60/10 + 1 = 7 \) min (include a 1 min overhead buffer)
- Average depth in ATA: \( \text{ATA} = 1 + \frac{30}{33} = 1.909\bar{09} \approx 1.91 \)
Step 1: Team RMV under CAT is \( 0.75 \times 2 = 1.50 \) cu ft/min.
Step 2: Apply average depth: \( 1.50 \times 1.9091 = 2.8636 \) cu ft/min at depth.
Step 3: Multiply by time: \( 2.8636 \times 7 = 20.045 \) cu ft.
MGR in volume: \( \approx 20.0 \) cu ft.
Convert Cubic Feet to PSI for an AL80
An AL80 holds 80 cu ft at 3000 psi. Use proportional scaling to convert volume to pressure.
\[ \text{MGR}_{\text{PSI}} = \frac{20.045}{80} \times 3000 = 751.7 \ \text{psi} \]
Round up to a practical SPG value: \( 751.7 \rightarrow 800 \ \text{psi} \).
Why We Round Up
- SPG readability and communication are cleaner at round increments.
- Small gauge error and rounding tolerances are covered by rounding up.
- Extra margin accounts for brief delays or a higher breathing rate under stress.
