Minimum Gas Reserve
The simplest Minimum Gas Reserve (MGR) calculation follows the CAT formula. This ensures enough gas for a controlled ascent with a buddy in an emergency, but it does not include a safety stop, since a safety stop is a good practice but not a necessity in an out-of-gas situation. If you are interested in seeing this calculation in action head over to our Minimum Gas Reserve Calculator.
CAT Formula
\[ \text{MGR} = (\text{RMV} \times 2) \times \text{Average Depth in ATA} \times \text{Time to Ascend} \]
Example Calculation:
- Depth: 60 ft
- Ascent Rate: 10 ft/min
- RMV: 0.75 cuft/min
- Time to Ascend: (60ft max / 10 ft/min) + 1 min = 7 min
- Average Depth: 30 ft \(\approx \) 1.91 ATA
\[ (0.75 \times 2) \times 1.91 \times 7 = 20.1 \text{ cuft} \]
However, divers do not carry devices that display gas volume in cubic feet while underwater. Instead, we rely on an SPG (Submersible Pressure Gauge), which measures pressure in PSI. To use the SPG to monitor our remaining gas volume, we must convert cubic feet to PSI based on the tanks rated capacity and working pressure.
Aluminimum 80 (80 cft / 3000 PSI):
\[ \frac{20.1}{80} \times 3000 = 754 \text{ psi} \]
Round Up:\[ 754 \text{ psi} \approx 800 \text{ psi} \]
So now what?
The Minimum Reserve Gas for specific depth with a specific RMV will always be the same when expressed as cuft per min. You can calculate Minimum Reserve Gas once and store it somewhere. If you then typically dive certain sized tanks, you can also pre-calculate PSI for a specific tank as well.
Why Do We Round UP Gas Calculations?
SPGs are not perfectly precise and often have a margin of error of ±100 psi. Rounding gas calculations up ensures:
- Easy tracking and communication with a dive buddy.
- Compensating for small pressure gauge inaccuracies.
- Adding an extra safety margin to account for unexpected factors like increased breathing rate or delays in ascent.