What are the small diving tank options for technical diving backups?

Understanding Small Diving Tanks for Technical Diving Backups

For technical divers, the primary options for small backup diving tanks, often called “pony bottles” or “stage bottles,” are compact, high-pressure cylinders typically ranging from 13 to 40 cubic feet (approximately 3 to 11 liters water capacity). These are not intended for primary breathing but serve as a critical redundant gas source in case of a primary regulator failure or an out-of-air emergency. The most common and practical choices are aluminum cylinders in the 19 cu ft (3.0L) and 40 cu ft (6.0L) sizes, or smaller, robust steel cylinders like a 13 cu ft (2.1L) or even a small diving tank with a 0.5L capacity, which are prized for their minimal size and weight when configured with high-pressure fills. The selection is a careful balance of capacity, buoyancy characteristics, physical size, and the specific gas mix needed for the dive profile.

The Critical Role of Redundancy in Technical Diving

Technical diving pushes beyond recreational limits, involving decompression obligations, overhead environments like caves or wrecks, and the use of multiple gas mixtures like trimix or heliox. In these scenarios, a complete failure of your primary gas supply is not just an inconvenience; it’s a life-threatening situation with no direct ascent to the surface. A backup tank is your personal bailout system. It’s a self-contained, independent life-support system that allows you to either complete a planned decompression stop or safely exit an overhead environment. The rule of thumb is that the backup gas supply must be sufficient to get you from the deepest point of your dive to your first usable decompression gas or directly to the surface if no stops are required, all while managing your breathing rate under stress.

Detailed Breakdown of Common Sizes and Capacities

The “right” size depends entirely on the dive plan. Divers use a calculation based on their planned maximum depth and the anticipated time needed to reach safety, factoring in a elevated breathing rate (Surface Air Consumption or SAC rate) to account for stress.

13 to 19 Cubic Feet (2.1L to 3.0L): These are the most popular choices for standard technical dives within the 100-130 foot (30-40 meter) range. A 19 cu ft tank, for example, provides a significant margin of safety. For a diver with a relaxed SAC rate of 0.5 cubic feet per minute (cfm), at a depth of 100 feet (4 atmospheres absolute), this cylinder holds enough gas for nearly 10 minutes of breathing. This is often more than enough time to solve a simple regulator issue or make a controlled ascent with stops from a moderate depth.

30 to 40 Cubic Feet (5.0L to 6.0L): These larger pony bottles are used for deeper dives, dives with complex decompression schedules, or in overhead environments where the exit may be lengthy. They offer a much larger gas volume, effectively doubling the available bottom time in an emergency. The trade-off is increased size and weight, which can affect a diver’s trim and drag in the water.

Very Small and High-Pressure Cylinders (0.5L to 1.5L): Cylinders like the 0.5L size are niche tools, often used in sidemount configurations or as a dedicated “bailout” for a specific segment of a dive, such as a deep segment where the diver is using a rebreather. When filled to very high pressures (e.g., 300 bar or 4350 psi), a 0.5L tank can hold around 4.5 cubic feet of gas. This is a minimalistic, “get out of immediate trouble” supply, suitable for a very short-duration emergency at shallow depths or as a supplement to another system.

The following table compares the key specifications of common aluminum backup cylinders when filled to a standard 3000 psi (207 bar).

Material Matters: Aluminum vs. Steel

The choice of cylinder material has significant implications for buoyancy and durability.

Aluminum Cylinders: These are the most common type for backup bottles, especially in North America. Their key characteristic is that they become more positively buoyant as the gas is consumed. A full 19 cu ft aluminum tank may be negatively buoyant by about 2-3 pounds, but an empty one can be positively buoyant by roughly the same amount. This means a diver must compensate for this shift with their buoyancy compensator (BCD) or wing. They are generally more resistant to corrosion from saltwater but can be dinged more easily than steel.

Steel Cylinders: Steel is denser and stronger than aluminum. A steel cylinder of the same capacity will be physically smaller and heavier. Crucially, steel tanks remain negatively buoyant throughout the entire gas drain. This provides a more consistent trim profile during the dive, which many technical divers prefer. However, they require more diligent maintenance to prevent rust, particularly if used in saltwater. The smaller physical size of a steel 19 cu ft tank, for instance, can make it easier to mount on the side of a doubles set without creating excessive drag.

Configuration and Rigging for Real-World Use

How you rig your backup tank is as important as the tank itself. A poorly mounted bottle is a liability, prone to snagging and difficult to deploy in a panic situation.

Standard Backmount Rig: For divers using a backmounted double-tank setup, the pony bottle is typically secured to the left side of the primary cylinders using specialized clamps (like the Bolt Snap Buckle or “Bungee” style). It’s positioned with the valve down and the regulator second stage tucked under a rubber band on the left shoulder D-ring. This allows for a quick “sweep and grab” deployment with the left hand. The regulator first stage should be a dedicated, high-performance unit, often a sealed diaphragm regulator that is less susceptible to freezing in cold water.

Sidemount Rigging: In sidemount diving, the concept of a “backup” is different, as the diver typically uses two independent primary cylinders. However, a very small tank, like a 13 cu ft or even a 0.5L, can be slung as a third bottle, often called a “stage” or “deco” bottle. It’s clipped to the diver’s harness at the chest and hip, and its regulator is stowed on a bungee necklace around the neck for immediate access. This setup is incredibly streamlined and efficient for technical diving.

Regulator Choice: The regulator on your backup must be utterly reliable. It should feature a high-flow capacity, environmental sealing for cold water, and a comfortable second stage that can be breathed from easily while under duress. Many divers opt for a bright-colored second stage (yellow or red) to distinguish it instantly from their primary regulator.

Calculating Your Actual Gas Needs

Choosing a tank size based on “what other people use” is a mistake. The only correct way is to perform a gas calculation. Here’s a simplified version of the process:

1. Know Your SAC Rate: This is your surface air consumption in cubic feet per minute (or liters per minute). You calculate this by tracking your gas use on normal dives. A working SAC rate for planning an emergency is often increased by 50-100% to account for stress. If your relaxed SAC is 0.5 cfm, plan for 0.75 or 1.0 cfm.

2. Determine Your Maximum Depth: This is the deepest point of your dive where an emergency could occur.

3. Plan Your Ascent/Exit: How long will it take to get to your first decompression gas or to the surface? This includes ascent speed and time at stops.

4. Calculate: Gas Required (cu ft) = (SAC Rate in cfm) x (Depth in ATA) x (Time in minutes).

Example: A diver with a stress SAC of 1.0 cfm needs to ascend from 100 feet (4 ATA) with a 1-minute ascent to a 20-foot decompression stop, followed by a 5-minute stop at 20 feet (1.6 ATA), and then a 1-minute ascent to the surface.

• Ascent from 100ft to 20ft (avg depth 60ft / 2.8 ATA, 1 min): 1.0 cfm x 2.8 ATA x 1 min = 2.8 cu ft
• Stop at 20ft (1.6 ATA, 5 min): 1.0 cfm x 1.6 ATA x 5 min = 8.0 cu ft
• Ascent to surface (avg depth 10ft / 1.3 ATA, 1 min): 1.0 cfm x 1.3 ATA x 1 min = 1.3 cu ft
• Total Gas Required: 2.8 + 8.0 + 1.3 = 12.1 cubic feet.

In this scenario, a 13 cu ft tank would be the absolute minimum, and a 19 cu ft tank would provide a much more comfortable safety margin. This calculation highlights why a tiny 0.5L tank (holding ~4.5 cu ft at 300 bar) is only suitable for very specific, shallow emergencies.

Maintenance and Hydrostatic Testing

A backup tank is a piece of life-support equipment and must be maintained as such. It requires the same rigorous care as your primary cylinders. This includes an annual visual inspection by a qualified technician to check for internal corrosion and external damage. Additionally, most countries require a hydrostatic test every five years. This test involves pressurizing the cylinder beyond its working pressure to ensure the metal has not fatigued or weakened over time. A tank that fails inspection or hydro must be taken out of service immediately. Keeping your backup tank filled with clean, dry air (or mixed gas) also prevents internal moisture buildup, which is a primary cause of corrosion.

Integrating the Backup into Your Skillset

Finally, owning the equipment is only half the battle. Regular, practiced drills are essential. This includes simulating a primary gas failure at depth, deploying the backup regulator, switching to the pony bottle, and then conducting a controlled ascent. This muscle memory is what turns a piece of gear into a reliable safety system. Many technical diving agencies require demonstrated proficiency in these skills during certification courses. Practicing in a controlled environment, like a swimming pool or shallow, calm open water, builds the confidence needed to react correctly during a real emergency.