Minimum Gas Reserve

When divers want to calculate a turn pressure, they need to know how much gas must be held in reserve in case a buddy runs out at the deepest point of the dive. The "CAT" method (Consumption × Average depth × Time) gives a simple, repeatable way to figure this out. Instead of tracking every second of depth change, you assume two divers with a standard breathing rate, add one extra minute at the bottom for problem solving, then work out how long it takes to ascend to the surface. By using the pressure at half of the maximum depth as a stand-in for the whole ascent, the math becomes quick enough to do on a slate or in your head while still giving a conservative answer. This balance of realism and simplicity is what makes the shorthand so practical under real dive conditions.

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: 100 ft

• Depth: 100 ft
• Ascent rate #1: 30 ft/min from 100 ft to 50 ft
• Ascent rate #2: 10 ft/min from 50 ft to surface
• Half-depth pressure: \( \text{ATA} = 1 + \frac{50}{33} = 2.52 \)

Step 1: Joint RMV = \( 0.75 \times 2 = 1.50 \) cu ft/min.

Step 2: Multiply by pressure at half depth: \( 1.50 \times 2.52 = 3.78 \) cu ft/min.

Step 3: Total time = 1 min (emergency at depth) + (100→50 at 30 ft/min = 1.67 min) + (50→0 at 10 ft/min = 5 min) = 7.67 min.

Step 4: Multiply: \( 3.78 \times 7.67 \approx 28.9 \) cu ft.

Convert Cubic Feet to PSI for an AL80

An AL80 contains 77.4 cu ft at 3000 psi. Use proportional scaling:

\[ \text{MGR}_{\text{PSI}} = \frac{28.9}{77.4} \times 3000 \approx 1120 \ \text{psi} \]

Round up to a practical SPG value: \( \approx 1150 \ \text{psi} \).

Always round up to a minimum of 500 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.