Technical diving involves carrying extra cylinders like decos or stages. More cylinders do not equal more safer, and a common issue for technical divers, even today, is breathing an incorrect gas mixture and having hypoxia or hyperoxia.

  • Deco cylinders are used on the ascent and contain higher oxygen concentrations to accelerate decompression speed.
  • Stage cylinders are used on the bottom portion of the dive and contain bottom gas. They are typically dropped (aka staged) at points at depth.
  • Travel gas is a gas used at the start of hypoxic trimix dives. The travel gas is almost always also a deco gas.

Placement Styles: Split vs. All-Left

There are two primary schools of thought when it comes to organizing your tanks:

  • Split Style (Right-Rich, Left-Lean): This method places high-oxygen “rich” gases on your right side and low-oxygen “lean” gases on your left.

  • All-Left Style: This method places every single deco and stage bottle on the left side of your body, leaving the right side empty.

The main benefit of the split style is that it separates the gases physically and reduces clutter on your left side. The idea is to prevent a diver from grabbing the wrong regulator by mistake. This setup is especially common in sidemount diving.

However, most back-mounted technical divers prefer the all-left style for one major reason: The Long Hose. In a standard technical setup, your primary regulator is on a long hose that wraps around your neck and tucks under a canister light on your right hip. If you clip a tank to your right side, you can easily trap that long hose. In an emergency, a trapped hose makes it impossible to share gas while swimming in a single-file line.

Additionally, keeping everything on the left keeps you streamlined. If you are diving inside a shipwreck or a cave, being wide makes it much harder to fit through restrictions.

MODS

No matter where you choose to put your tanks, there is one rule that overrides everything else – a team gas switch. This is where a team member verifies the cylinder from which you’re breathing and confirms it’s the correct gas for that depth.

“My deep gas is on the left, so I’ll just grab the left reg,” or “My green hose is my oxygen.” All it takes is for a hose to break on the boat, get replaced with a different color, and suddenly, a diver is relying on a color shortcut that is no longer true. This is how accidents happen.

To prevent this, technical divers use MODS before every single gas switch:

  • Mix: You and your buddy physically look at the label on the tank.

  • Open: You ensure the valve is fully turned on and check the pressure gauge.

  • Depth: You check your dive computer to make sure you are at the correct depth for that gas.

  • Switch: Your buddy confirms the depth and time, and you make the switch.

It does not matter if your hose is a certain color, or if the tank is on your left or right. The only thing that matters is the label on the cylinder. You must have one label you can see, and one label your buddy can see.

D-Rings

Before adding deco cylinders to your kit, you must make sure your harness and cylinder rigging are set up correctly. A dangling tank can hit a reef, get stuck on a wreck, or drag in the silt, ruining visibility.

  • The Shoulder D-Ring: Your top clip should attach to a D-ring on your shoulder strap. For the best fit, this D-ring should sit above your nipple line but below your collarbone (clavicle). If the D-ring is too far back toward your armpit, the tank will hang too low and fall below you into a “dangle zone.”

  • The Hip D-Ring: The bottom of the tank clips to your waist. Your hip D-ring should be placed about halfway to three-quarters of the way back between your front hip bone and your back. This keeps the bottom of the tank tucked neatly against your side.

Keep in mind that rigging can also be too tight. You still need to be able to see the neck of the cylinder during gas switches and easily reach the valve.

Leashing

If you are doing a very long or deep dive, you might need more than two stage or deco tanks. Fitting three large tanks on your left side can get incredibly crowded. This is where leashing (sometimes called butt-clipping) comes in handy.

A leash is a short piece of cord with a clip that attaches to your hip. You clip the extra tank to the leash and let it float behind you, resting near your butt. This moves the clutter away from your chest and arms, keeping your hands free.

Leashing works best with tanks that are naturally light in the water, such as those filled with a lot of helium, or tanks that are nearly empty. It clears up your profile and is especially helpful when using an underwater scooter (DPV) on multi-stage dives.

Sidemount

 In this style, your main tanks are already clipped to your left and right sides. Because your left side is already occupied by a main cylinder, you do not have the space to hang two or three more deco stages there.

Fortunately, this is perfectly fine in sidemount because of how the regulators are routed. The long hose in a sidemount setup is handled differently and does not get trapped the same way it does on a back-mounted double tank setup. While you will be wider in the water, the risk of trapping your long hose is minimized.

At the end of the day, technical diving is about managing risk through procedure. Configure your gear to keep your vital hoses free, secure your tanks to avoid the dangle, and always trust the label over your muscle memory.