Gas Planning Guide
Scuba diving is an exhilarating adventure immersing divers within the captivating underwater world. Rigorous planning, especially in gas management, is key to having a safe dive. Through this guide, we unravel the crux of scuba diving safety — segments and minimum gas planning.
Pressure: A Fundamental Diving Factor
Pressure is a core concept in the science of scuba diving, fundamentally influencing both physiological processes and equipment functionality. It becomes more relevant with increased depth underwater due to the weight of the column of water exerting pressure on the diver.
At sea level, we live under the pressure of the Earth's atmosphere, which is measured as 1 bar or 1 atmosphere (ATM). However, when we dive, for every 10 meters of depth in seawater, this pressure increases by another 1 bar or 1 ATM. So, at a depth of 10 meters, the pressure is 2 bars (or 2 ATM), at 20 meters it's 3 bars, and so on - a direct correlation between depth and pressure underwater.
This increase in pressure impacts the body and the gas, which we breathe from our scuba tanks. The gas becomes denser as pressure increases, meaning that the gas molecules are compressed and brought closer together. This results in increased gas consumption the deeper we go.
Having a good grasp of how pressure works is vital for scuba divers. It influences various aspects of the dive including dive planning, equalization, decompression, and the calculation of gas needs.
Segments
Segments in scuba diving approximate a diver's gas consumption at different depths. This rule takes into account the increased pressure and density of gas breathed underwater.
The consumption rate depends on the diver's gas tank capacity, their average surface gas consumption rate, fitness level, stress, and temperature. These rates are calculated typically over 5 minutes to aid in-water calculations.
With pressure increasing approximately by 1 atm every 10 meters in seawater, the gas consumption rate also rises, being the surface segment times the pressure at the given depth.
Understanding and using segment rules effectively aid in dive planning and underwater gas consumption tracking. They also verify readings from your diving equipment, ensuring dive safety.
Different scuba tanks have different volumes, thus they provide different amounts of gasflow at the same pressure. Segments vary with the tank size. The larger the tank, the more the gas it holds, and thus the lower the bar consumed per 5 minutes at a given pressure.
Here is a table displaying the hypothetical consumption of different tank sizes at different depths:
| Surface | 10m (2 atm) | 20m (3 atm) | 30m (4 atm) | 40m (5 atm) | |
|---|---|---|---|---|---|
| 10l single | 10 bar | 20 bar | 30 bar | 40 bar | 50 bar |
| 12l single | 8.5 bar | 17 bar | 25.5 bar | 34 bar | 42.5 bar |
| 15l single | 7 bar | 14 bar | 21 bar | 28 bar | 35 bar |
| 18l single | 5.5 bar | 11 bar | 16.5 bar | 22 bar | 27.5 bar |
| 10l twin set | 5 bar | 10 bar | 15 bar | 20 bar | 25 bar |
| 12l twin set | 4.5 bar | 9 bar | 13.5 bar | 18 bar | 22.5 bar |
| 15l twin set | 3.5 bar | 7 bar | 10.5 bar | 14 bar | 17.5 bar |
| 18l twin set | 3 bar | 6 bar | 9 bar | 12 bar | 15 bar |
Note: The values are hypothetical, meant for illustrative purposes only, and may differ based on several factors including the diver's fitness level, stress, temperature, and more. This table helps to understand how different tank volumes will impact gas consumption at varying underwater pressures.
Minimum Gas
The Minimum Gas concept gives guideline for the gas needed to abort a dive safely from any depth and safely ascend to the surface with a buffer. The calculation assumes two divers, accounting for the possibility of gas supply sharing.
Minimum gas is computed as:
Minimum Gas in liters = Average Pressure (Ap) * Ascent Time (t) * Consumption (c) * 2 (divers)
These components represent:
- Average Pressure (Ap): The mean of pressure at current depth and surface.
- Ascent Time (t): The safe ascend time to the surface plus one minute for contingencies.
- Consumption (c): The per minute liter consumption of gas, typically around 20 liters for a comfortable dive.
- 2 (divers): Accounting for a potential gas-sharing scenario.
Minimum gas is eventually converted to tank pressure:
Pressure in bars = Volume of gas needed (in liters) / Total volume of the tanks (in liters)
Final values are then rounded to the nearest multiple of five for simplicity and safety.
Here is a table displaying the minimum gas of different tank sizes at different depths:
| 10m (2 atm) | 20m (3 atm) | 30m (4 atm) | 40m (5 atm) | 50m (6 atm) | 60m (7 atm) | 70m (8 atm) | |
|---|---|---|---|---|---|---|---|
| 10l single | 30 bar | 65 bar | 110 bar | 180 bar | 250 bar | 335 bar | 450 bar |
| 12l single | 25 bar | 55 bar | 90 bar | 150 bar | 210 bar | 280 bar | 375 bar |
| 15l single | 20 bar | 45 bar | 75 bar | 120 bar | 170 bar | 225 bar | 300 bar |
| 18l single | 15 bar | 35 bar | 60 bar | 100 bar | 140 bar | 185 bar | 250 bar |
| 10l twin set | 15 bar | 30 bar | 55 bar | 90 bar | 125 bar | 170 bar | 225 bar |
| 12l twin set | 15 bar | 25 bar | 45 bar | 75 bar | 105 bar | 140 bar | 190 bar |
| 15l twin set | 10 bar | 20 bar | 35 bar | 60 bar | 85 bar | 110 bar | 150 bar |
| 18l twin set | 10 bar | 20 bar | 30 bar | 50 bar | 70 bar | 95 bar | 125 bar |
Safety Precautions
The minimum gas calculation is a precaution for worst-case scenarios, detailing how much gas should be kept in reserve for safety. Always view it as a minimum requirement, not a maximum limit.
Remember to:
- Do a pre-dive safety check.
- Monitor your gas regularly during the dive.
- Ascend slowly and carefully.
- Account for dive conditions, your exertion, and other factors in your gas planning.
Effective gas management can make your dives safer and more enjoyable. Take the time to understand and use these guidelines and calculations. This will help ensure your safety while exploring the wonders beneath the waves. Dive safe and have fun!