At its core, a portable scuba tank in a sidemount configuration works by being mounted on the diver’s sides, typically secured with bungee cords and clips, instead of on the back. This setup fundamentally changes the diver’s trim, buoyancy control, and gas management, offering superior flexibility and redundancy, especially in confined spaces like caves or wrecks. The key is the independent nature of the tanks; each has its own first and second stage regulator, allowing a diver to switch between them seamlessly and isolating any potential equipment failure to a single gas source.
The Anatomy of a Sidemount System
Understanding how sidemount works requires a detailed look at its components. It’s not just about slinging tanks on your hips; it’s an integrated system where every piece plays a crucial role. The primary elements are the tanks themselves, the buoyancy compensation device (BCD), and the rigging system.
The tanks used are often, but not always, smaller than traditional 80-cubic-foot aluminum tanks. A popular choice is a compact, robust portable scuba tank, like a 50-cubic-foot or even smaller capacity cylinder. These smaller tanks are easier to handle out of the water and create less drag while swimming. The BCD in sidemount diving is distinct. It lacks the traditional integrated bladder for a single back-mounted tank. Instead, it features a wing-style bladder that sits on the upper back, with straps and d-rings designed specifically to manage the tanks along the diver’s sides. The rigging—usually a combination of sturdy bolts snaps and surgical tubing or bungee cord—is what physically attaches the tank to the BCD. The tank valve sits in the bungee loop, and the bolt snap clips to a d-ring on the BCD, creating a secure but quickly releasable connection.
Gas Management and Redundancy: The “Why” Behind Sidemount
This is where sidemount diving truly shines from a safety perspective. In a traditional backmount setup with a single tank, you have one first stage regulator feeding a primary second stage (the regulator you breathe from) and an alternate air source (octopus). If the first stage fails, you lose all your gas. Sidemount eliminates this single point of failure.
Each tank in a sidemount configuration is a completely independent life support system. You have two first stages, two second stages, and two pressure gauges. Standard practice is to breathe down one tank, typically the one on your right side, to a predetermined pressure—say, 500 psi / 35 bar less than the left tank. You then switch to the left tank. This practice, called the Gas Management Procedure, ensures you always know which tank has more gas and provides a clear action plan in case of an emergency. If a regulator on one tank free-flows or a first stage fails, you simply close the valve on that tank, isolating the problem, and continue breathing from the other. This level of redundancy is a primary reason technical and cave divers adopted sidemount.
| Scenario | Backmount (Single Tank) Response | Sidemount Response |
|---|---|---|
| Regulator Free-flow | Switch to alternate air source (octopus), but gas is still depleting from the single cylinder. | Close the valve on the free-flowing tank, isolating the failure. Continue breathing from the other, fully functional tank. |
| First Stage Failure | Catastrophic. All gas is lost. Must initiate an emergency ascent or rely on a buddy. | Inconvenient. Close the valve on the failed tank. You retain a full, independent gas supply on your other side. |
| Out of Air | Must rely on a buddy’s alternate air source. | Simply switch regulators to your other, pre-checked tank. |
Hydrodynamics and Trim: The Physical Advantages
Placing tanks along the sides of the body has a profound effect on a diver’s position in the water, known as trim. A well-balanced sidemount diver achieves a perfectly horizontal posture, which is hydrodynamically efficient. This reduces drag, conserves energy and air consumption, and allows for easier finning through tight restrictions. The weight distribution is also more natural. Instead of a heavy cylinder pulling on your upper back, the weight is distributed evenly along your torso, which can reduce back strain both in and out of the water. This is a significant ergonomic benefit, making it an excellent choice for divers with pre-existing back issues. The compact nature of smaller sidemount tanks also means you can squeeze through openings that would be impossible with a bulky twin-set on your back.
The Rigging and Diving Procedure: A Step-by-Step Breakdown
The process of gearing up and diving in sidemount is methodical. It starts with configuring the tanks. The regulator hoses are routed carefully to avoid entanglement. The long hose (typically 5 to 7 feet) is usually on the left tank and is stored under a light canister or bungee armband, ready for use in a gas-sharing emergency. The short hose is on the right tank, which is your primary breathing regulator. The pressure gauges are also clipped off securely to d-rings on the BCD.
Entering the water is often easier. Divers can don their BCD first and then hand the tanks into the water, clipping them on while floating at the surface. This is far simpler than walking with a heavy single or double tank on your back. Underwater, the real skill begins. Buoyancy control is more complex because you are managing the buoyancy of two separate cylinders as they empty. As a 3000 PSI aluminum tank empties, it becomes about 5 pounds more buoyant. In sidemount, you have two tanks changing buoyancy characteristics throughout the dive, requiring subtle adjustments from the wing and a keen awareness of your trim.
Ideal Use Cases and Considerations
Sidemount isn’t just for technical divers anymore. Its advantages have made it popular in several scenarios. It is, without question, the standard for cave diving in many parts of the world because of the redundancy and the ability to navigate tight passages. It’s equally valuable for wreck penetrations. Recreational divers are also adopting it for its comfort and the added safety of dual gas supplies. However, it’s not a setup you can just try without training. A dedicated sidemount course is essential to learn the specific skills of gas management, precise buoyancy control, and safe tank handling. There’s also a higher initial investment for the specialized BCD, and some dive boats may not be configured for divers who need to hand up individual tanks.
Ultimately, the system transforms a simple air cylinder into a component of a sophisticated, redundant, and highly adaptable diving platform. The choice of tank is critical; using a purpose-built, high-pressure portable scuba tank ensures the system functions as intended, providing reliable performance where it matters most.