Gas Management and Volume Considerations
When you connect a portable scuba tank to a drysuit, the first and most critical consideration is gas management. A drysuit requires a dedicated gas source for inflation to counteract the increasing pressure of the water as you descend and to maintain thermal protection by preventing suit squeeze. Using your primary tank’s gas for both breathing and suit inflation is standard, but a secondary, smaller tank changes the equation entirely. The core question becomes: does the portable tank have enough gas volume to handle both your buoyancy needs and serve as a potential emergency breathing gas source?
Let’s break down the numbers. A typical drysuit inflation can require a significant amount of air, especially during the initial descent or in cold water where you might wear more undergarments. A common rule of thumb is that a drysuit might consume between 0.5 to 1.5 cubic feet of gas per minute of inflation, depending on the diver’s depth, undergarment thickness, and how well they are trimmed. A small, 0.5-liter portable tank, like the portable scuba tank, holds approximately 0.6 cubic feet of gas when filled to 3000 PSI. This volume is sufficient for several short inflation bursts but would be depleted rapidly if used continuously or for a major buoyancy adjustment at depth. The following table illustrates the approximate number of inflation “bursts” you might get from a 0.5L tank at different depths, assuming a 2-second inflation from the low-pressure inflator:
| Depth (feet/meters) | Approximate Gas Used per 2-second Burst (cu ft) | Estimated Number of Bursts (from 0.6 cu ft) |
|---|---|---|
| 33 ft / 10 m | ~0.1 | 5-6 |
| 66 ft / 20 m | ~0.2 | 2-3 |
| 100 ft / 30 m | ~0.3 | 1-2 |
This data clearly shows that a small portable tank should be viewed primarily as a backup buoyancy device, not a primary one. Its gas supply is too limited for routine buoyancy control. The prudent approach is to use your main buoyancy compensator (BC) for primary buoyancy and the drysuit for trim, reserving the portable tank’s gas for drysuit inflation only in a contingency, such as a failure of the main air supply. This strategy maximizes the utility of the small gas volume.
Regulator and Valve Configuration
The hardware connecting the tank to your drysuit is just as important as the gas inside. You cannot simply screw a standard high-pressure scuba regulator onto a small tank and expect to inflate your suit. Drysuits are inflated via a low-pressure connection, identical to the one on your BC inflator. Therefore, the portable tank must be outfitted with a dedicated regulator system that provides a low-pressure output.
There are two main configurations for this:
1. Miniature First Stage with LP Hose: This is the most robust and reliable method. A compact first-stage regulator is attached to the tank’s valve. This first stage steps the high-pressure tank air down to intermediate pressure (typically 135-150 PSI above ambient pressure). A standard low-pressure hose then runs from this first stage to the drysuit’s inflation valve. This setup mimics the functionality of your primary regulator but on a smaller scale. It’s efficient and allows for smooth, controlled inflation.
2. Integrated Inflator/Regulator Assembly: Some systems, particularly those designed for surface marker buoys (SMBs) or buoyancy aids, combine the regulator and inflator mechanism into a single unit that screws directly onto the tank valve. While compact, these can be less durable and may not provide the same fine control as a separate first-stage regulator. You must ensure any such unit is rated for scuba use and is compatible with your drysuit’s inflation valve.
The valve on the portable tank itself is also a key factor. Many small tanks come with a simple on/off valve, sometimes with a “J”-type handle for easy operation. For safety, a pressure gauge or a reserve indicator is highly recommended. This allows you to monitor your remaining gas supply, a critical piece of information if this tank is also intended as a backup breathing gas source.
Dive Planning and Emergency Procedures
Integrating a portable tank into your dive plan requires a significant shift in your emergency drills and mental rehearsals. You are introducing a second, independent life-support system, and you must be prepared to use it correctly under stress. Your dive plan must explicitly state the purpose of the portable tank. Is it solely for backup drysuit inflation? Or is it also an emergency air source (EAS) for breathing?
If it’s for breathing, you must have a second-stage regulator attached to it, either via a “Y” or “H” valve from the first stage or via a separate pony bottle regulator. This adds complexity and weight. You and your buddy must practice gas-sharing scenarios that include switching to this independent air source. A common drill involves simulating a primary air failure at depth, switching to the portable tank’s regulator, and then using its limited gas to perform a controlled ascent, potentially using the same gas for minimal suit inflation if needed.
The most critical procedure to practice is ditch and don. If you need to use the portable tank as an emergency air source, you must be able to access it quickly. This typically means it should be mounted in a way that it can be easily removed from its holder and brought to your mouth. Many divers use a sling setup or a quick-release bracket. You should practice this drill until it becomes second nature. Remember, in a real emergency, your fine motor skills will deteriorate; your setup must be simple and intuitive.
Physical Integration and Diver Trim
Where and how you mount the portable tank on your body has a direct impact on your safety and comfort underwater. An improperly mounted tank can throw off your trim, create drag, and become a serious entanglement hazard. The goal is to keep the tank as streamlined and close to your body’s center of mass as possible.
Common mounting locations include:
Backmount, below the Primary Cylinder: Using special brackets, the small tank can be secured to the backplate or band between your primary tank and the plate. This keeps it centralized and minimizes impact on trim, but it can be harder to reach in an emergency.
Sidemount on the Upper Chest or Hip: Slinging the tank under your arm, similar to a sidemount configuration, makes it very accessible. However, it can affect lateral balance and requires careful hose routing to prevent snagging. You’ll need to adjust your weighting to compensate for the weight distribution shift.
Butt Mount: Attaching the tank to the back of your harness near your buttocks is another option for centralizing weight. Accessibility can be an issue, and it may interfere with leg movement.
Regardless of the location, you must use sturdy, reliable boltsnaps or quick-release mechanisms. The hose from the tank to your drysuit inflator must be the correct length—long enough to allow movement but short enough to avoid loops. A good practice is to use a hose retainer or bungee loop to secure the excess hose. After donning your gear on the surface, have your buddy check for any obvious snag points or loose hoses before you enter the water.
Environmental and Operational Factors
The conditions you plan to dive in will heavily influence the viability of using a small portable tank with your drysuit. Cold water diving presents the greatest challenge and the most significant need for a backup system. In water below 10°C (50°F), the risk of regulator freezing increases. If the first stage on your primary tank freezes and free-flows, you lose your air supply for both breathing and suit inflation. In this scenario, a completely independent portable tank with its own regulator becomes a vital safety tool. However, you must ensure the regulator on the portable tank is also environmentally sealed or suitable for cold water use to prevent it from freezing as well.
Depth is another major factor. As shown in the gas management table, the usefulness of the small tank’s gas volume diminishes rapidly with depth. A tank that might give you three or four inflation bursts at 20 meters might be emptied in one long burst at 40 meters. Therefore, this setup is generally better suited for recreational diving within the 18-30 meter (60-100 foot) range. For deeper technical dives, a larger pony bottle (e.g., 3-liter or 13 cubic feet) is a more appropriate choice to ensure an adequate gas supply for both breathing and buoyancy during a decompression ascent.
Finally, consider the type of diving. In overhead environments like wrecks or caves, where a free ascent to the surface is impossible, a backup gas source is mandatory. While a 0.5L tank is insufficient for cave diving, it could be a component of a more complex redundant system on a shallow wreck dive, provided the primary goal is backup inflation rather than sustained breathing. Always adhere to the rule of thirds for gas management in overhead environments, which a tank this size cannot satisfy for primary breathing gas.