Which Decompression/Stage Cylinder?
Many of the considerations relating to "Pony" cylinders also apply to "Decompression" cylinders, but with a number of elements, which can seem more prescriptive. This is due to the risks involved in getting it wrong.
Decompression cylinders are used when a diver carries multiple gas mixes. Typically they contain either 50% or 100% Oxygen to speed up the decompression process. These gas mixes must not be breathed deeper than either 21 or 6 metres respectively, otherwise the diver risks suffering convulsions and drowning due to Oxygen toxicity. It is consequently imperative that he not confuse his gas supplies - accidentally breathing the wrong one could be fatal!
As a result of this, the decompression cylinder must be side-mounted, which allows the diver to see exactly which regulator is attached to which bottle. He can also then leave it turned off until the time that it is needed - this provides an extra safeguard and means that the valve can be "feathered" in some failure modes.
The tanks must be clearly marked with letters at least 3" high on the side to indicate the maximum operating depth (MOD), it must be possible for both the diver and his buddy to read this.
The cylinder chosen, should be one which holds a volume of gas sufficient to meet both your own and your dive partner's decompression requirements, plus a small reserve. This is so that if one diver should lose his decompression gas due to a failure then he can remain on "back-gas". The other diver simply completes his stops and then passes the bottle to his partner after he returns to his "back-gas"
A 7-litre bottle is usually sufficient for more advanced diving up to the mid 40 metre range, containing 50% Nitrox, a second 7-litre cylinder can be added for dives up to the mid 60 metre range containing Oxygen. This assumes run times of about 60 minutes, however this will depend on your gas consumption and that of your buddy.
The bottle may have to be passed from one diver to another, consequently there can only be one real choice - aluminium. The buoyancy characteristics are such that the cylinder will go from slightly negative to slightly positive so that there is a minimal overall change to a diver's buoyancy.
Steel cylinders will leave a diver badly unbalanced unless he moves additional weight to the other side to offset it. He must also carry excessive buoyancy to counteract the negative buoyancy of these additional tanks. They create problems which do not exist with aluminium cylinders. Decompression stages must never be an integral part of a weighting system.
Your regulator must be O2 cleaned for use with rich nitrox mixes. It is also very important that the regulator be reliable and subject to the same considerations as when buying your back gas regulators.
If you intend moving into the more extreme end of the diving spectrum then it is also important that your regulators are all interchangeable under any circumstance, this would flood any regulator if you were forced to swap it under water. Diaphragm style first stages like the DS4 are more inclined to be damaged in this event than piston stages like the Mk 20, so the latter may be a better option for stage regulators.
Some people find that a First stage with a swivel provides a more convenient hose routing. Others consider the extra O'ring a potential failure point and consequently avoid them. The choice is yours.
You will need a 40" hose on your regulator if you own a swivel first stage like the Mk 25 or 42" if you own a non swivel first stage.
You must not use the colour of a regulator to identify what the gas mix is, mouthguards or any other such complication, but should visually identify it both before and after starting to breath off it. If you accidentally attach the wrong regulator to the wrong bottle it might otherwise kill you!
The yellow hose in the picture was added to ease the visual tracing of bottle to regulator by your buddy in poor visibility since he should also check it as an extra safeguard. Needless to say black against black can be less than obvious.
It has absolutely nothing to do with colour coding!
See the Section "Decompression Gas Switch Drill" for more information on how to swap bottles during a dive.
This can be the same "plastic" gauge as included within the gauges' section but it should be attached to a short 6" hose. This allows you to conveniently monitor your gas supply throughout your stops. A plastic gauge is used as opposed to a brass/glass gauge since the reduced weight helps the bottle remain neutrally buoyant. The gauge is tied back on itself to facilitate easy reading and to reduce wear on the hose through continuous flexing.
There are some links on rigging side mounts on:
http://www.bentleytech.com/cave/stage/index.html
http://www.gue.com/equipment/tanks-4.shtmlAdditionally there are two further pages included within this site providing a template on how to mark up your decompression cylinders properly with both your name and the depth at which they can be breathed from:
BITZ' Decompression Cylinder Marking Template for 50% Enriched Air Nitrox
BITZ' Decompression Cylinder Marking Template for 100% Oxygen
|
 |
Diving can be a hazardous sport and the information contained within this site is not intended to replace formal training.
Furthermore no responsibility will be accepted for any accidents, injuries or financial disadvantage, howsoever caused.
Copyright © H Beasley 01 August 2001 All Rights Reserved
Note
a Dive store may not be prepared to fit items from different manufacturers
for "insurance" reasons (eg. Scubapro Mk25 with an Apeks
TX50) however the Internal pressure is perfectly compatible and
there is no reason not to do so as any "good" dive technician
will be able to tell you.
