Decompression Cylinders

Posted on 21. Jan, 2004 by Greg Mossfeldt in Gear

Deco Cylinders

Stage bottle rigging, as with every other equipment selection and Configuration decision, needs to account for a number of risks that are present on every dive. Some of these issues have less critical consequences, such as drag and ease of handling. Some are potentially life threatening, such as the four methods of entrapment that can detain a diver within a restriction:

1) Jamming, where the diver physically tries to move himself through a space, which is too small to accommodate him.

2) Key holing, where a diver is able to pass through a restriction in one direction but not the other, due both to the shape of the restriction and the shape and configuration of the diver's equipment.

3) Ratcheting, where a diver is able to proceed forward into a space, but unable to back out due to mechanical locking action of his body and gear, and

4) Entanglement, where the diver's movement is limited or prevented by external objects fouled on the diver's body or gear.

This text outlines a method for the selection and configuration of deco/stage cylinders arising from critical examination of these issues, and a systemic approach to minimizing the risks inherent in this activity.

Ideally, a stage cylinder should be neutral (or as close as possible) in the water. This effectively precludes the use of steel cylinders as stages. By using neutrally buoyant stages, you do not need to consider these as part of your weighting scheme. This means that you can add or subtract stages at will according to the dive plan, and have the ability to stage drop, donate or jettison a cylinder at any time without regard for the impact this would otherwise have on your buoyancy.

The specific trim characteristics of individual cylinders is quite important. Even among the available range of aluminum cylinders, we use the Luxfer S040 and Luxfer S080 exclusively. These have the best mass distribution and resultant swimming characteristics. A correctly selected stage will ride tight to the diver's body, with the bottom end higher than the neck of the cylinder. This puts it in the lee of the diver's arm and shoulder, reducing the risk of the four problems in the list above, as well as reducing drag. The cylinder will ride close to the diver, up and out of the way, instead of hanging down below with the obvious consequences.

Stages are worn only on the left side. The most important reason for this is that nothing should EVER interfere with immediate deployment of the long hose when donating gas. Since the long hose reg is our primary (always donate from the mouth), the hose needs to deploy freely from behind the diver's neck, across the chest, under the light canister and up to the right post, consistently and without obstruction. We always donate the regulator being breathed, regardless of whether it is back gas or a stage. This ensures that the gas being donated is appropriate for the depth, that the donated regulator is clear and functioning properly, and that the regulator donation procedure is consistent and does not need to be modified (enabling conditioned emergency response).

Our stages are rigged with forty-inch hoses. When in use, the hose leads from the first stage, behind the diver's head and into a standard second stage in the diver's mouth, which feeds from the right side. Donating this reg is exactly the same movement as when donating the long hose. The hose length is ideal for this, and all stage cylinders are rigged identically. Wearing stages on both sides would entail different hose lengths, different donation procedure depending on which side the reg was coming from, and different routing methods - all less than optimal.

A stage cylinder on the left side is positioned with the valve orifice facing the diver. This puts the first stage in a protected position, and leaves the valve handwheel to the left, where the diver can rest his hand in a natural position, and control gas delivery manually if necessary due to regulator failure.

Another important reason to keep all stages on the left is swimming effort. Without getting too detailed with the physics behind the explanation - imagine a flag flying in the wind. Despite the fact that the wind may be blowing in a constant direction, the flag does not stand straight out, but rather flutters as alternating vortexespass along its length. While not exactly the same principle, a diver swimming with stages slung on either side exhibits a similar phenomenon - one cylinder will swing slightly ahead of the other in alternating fashion, meaning that some of the energy in the diver's kick is wasted overcoming this tendency to yaw.

Other reasons for not wearing stages on both sides include interference with prop wash from a scooter (DPV), interference with the light canister which is worn on the right hip, and interference with your ability to operate anything in front of you or between your legs, such as harness removal, weight ditching, plugging a urine discharge fitting, or retrieval of anything on your crotch D-ring.

Our stages (Luxfer S040 or S080) are rigged as follows:

Choke a bolt snap on a bight of braided synthetic line. Six inches from this clip (the needed slack), tighten a stainless hose clamp on the cylinder (over the line). This clamp should be the same distance from the break in the cylinder (where the spherical tank neck meets the cylindrical profile of the tank wall) as the distance between the diver's chest and hip D-rings. Thread a length of hose (garden hose, fuel hose, whatever - just to provide a positive tactile underwater handle) over the line, cut to the correct length to fit between the hose clamp and the upper clip. The upper clip is tied tight to the cylinder right at the break. Finally, the line is tied to itself, taught around the base of the valve.

In practice, the upper clip is clipped on to the diver's left chest D-ring. This keeps the top of the cylinder tight to the diver's body. The lower clip is then clipped to the hip D-ring. The slack in this lower clip allows the tail end of the cylinder to float up into its protected position, and also allows the cylinder to ride in its natural position of least resistance when the diver is in motion. This is especially important when carrying multiple stages in the same position, and the closer these cylinders are to being neutrally buoyant, the better they will ride. Typically, up to three cylinders are carried in this position, with additional stages being towed from the upper clip only at the hip ring. When diving with additional stages of bottom gas, one may opt to carry these in the functional position, with deco gases being towed from the hip, and only brought forward as necessary.

Stages are typically stacked with the highest oxygen inboard, and the lowest (deepest) decompression gas on the outside. Note that this has absolutely nothing to do with identification of a gas, and should NEVER be relied upon as such. Gas identification should never be made on the basis of position. The purpose of ordering the bottles in this fashion is to make it more convenient to hand off used bottles to support divers to clean yourself of unnecessary gear, and also to make it easier to jettison a bottle if you have to - stages of bottom gas obviously are the most likely to be jettisoned for some reason.

Stage cylinders are marked with only the diver's name, and the Maximum Operating Depth (MOD). The MOD is based on maximum PPO2 for the gas being breathed - typically 1.6 ATA for decompression gases at 120 fsw and shallower, and 1.4 ATA for deep decompression gas or stages of bottom gas. We use no other colour schemes, stickers, or markings of any kind. Keep it simple - the MOD is immediately identifiable (three inch high characters on either side of the stage bottle in a contrasting color - reflective numeral stickers are great for this) by both the diver wearing it, and any buddy who may (should) be looking at you. Stages are carried charged, but with the valve turned off. This prevents unintentional gas loss due to a burping stage reg. A diver can read the SPG on a stage reg at a glance at any time during the dive, so if a reg burps and the pressure drops, it can be recharged with a quick on/off with the valve. By keeping the valve off, though, you ensure that you will not accidentally misidentify a reg and switch to the wrong gas without noticing - obviously a much greater risk.

All stages, as with every other bottle, use a 300 bar DIN valve for the best possible connection to the reg (resistant to o-ring extrusion, impact, etc.) Valve handwheels are preferably the softer rubber type (such as Sherwoods) for maximum tactility and impact resistance. Cylinders are preferably white for maximum visibility, but in any case must conform to the marking standards above. Also, tank boots are not used. Tank boots create a high risk of ratcheting, as well as contributing to corrosion, and being entirely unnecessary anyway. As with everything, keep your stages clean and unlikely to be an entanglement risk.

To employ a stage, the diver need only identify the bottle to be switched to by the MOD number, deploy the reg, trace with his hand from the first stage, along the hose to the second which is then placed in the mouth, turn on the bottle and breathe. If the stage continues to deliver gas - you are on the right gas. Simple, hard toscrew up, and clean since you don't need second stage jackets, color coding, or any other convoluted identification scheme that includes the regulator as an identifying factor. Taking the reg out of the equation goes a long way to preventing misidentification accidents.


* January 21/03