Which Tanks? Pony Cylinders
Cylinder Boots Recommendations
A "Pony" cylinder is a 3-litre backup to your main gas supply but with a very limited volume. If you should be in the unfortunate situation of suffering a gas failure in your main tank at a moderate depth (35 metres), then it should be adequate to get you back to the surface, in what is a very stressful situation. It is however unlikely to allow you to complete more than a couple of minutes stops on the way. This is why we do not recommend this set-up for decompression diving.
Before you commit yourself to buying a pony cylinder, ask yourself why you want one. If you think you need a pony cylinder, then you almost certainly need a twin-set due to the limited gas supply it contains. Pony cylinders should never be counted on as part your redundant gas supply (reserve).
With regards to the choices available in respect of the pillar valves and operating pressure, the considerations are much the same as when buying a "Single Tank" but the choice of alloy is different.
There are two primary options when choosing to mount a pony cylinder:
"Back" Mounting is where the pony cylinder is mounted next to your main tank behind your head. There are two ways of doing this:
"Side" Mounting is where the pony cylinder is mounted on your left-hand side running beneath your arm. It uses two bolt-snaps, which are secured, in place. These bolt snaps are simply clipped off to attachment points on your buoyancy compensator.
To Back Mount or To Side Mount:
Back Mount
Advantages
Side Mount
Advantages
Removes Clutter away from your upper torso. Places the cylinder in the lee of your arm, reducing the total drag. You can split the wing and dry suit inflation systems so that the wing comes off the main cylinder and the suit off the pony bottle, ensuring redundancy for buoyancy. The diver can easily turn the valve on or off, if necessary "feathering" the valve if it should free flow. (Turn valve on, take breath, turn valve off etc) The regulator hose placement can be standardised in the way that twinset rigs are set up. (Pony left, main tank right.) You can remove the cylinder if it should become entangled in something and cut it free yourself. Usually less expensive. If your dive partner should run out of air, then it can be removed and passed to them, enabling each of you to make an independent ascent. Disadvantages Disadvantages Moves the weight away from your centre of gravity leaving you potentially unbalanced. The ability to mount the cylinder properly depends on which buoyancy compensator you own. You cannot pass it to your dive partner if they should run out of gas. This adds the complication of a dual ascent in an otherwise stressful and difficult situation. You cannot split the inflation systems since this would compromise your ability to pass the tank across in an emergency. This should not be a major issue PROVIDED that you are not over-weighted Cannot be removed if it becomes entangled. Usually more expensive. There is a link to rigging Side mounts on:
http://www.bentleytech.com/cave/stage/index.htmlAs before Cylinders can be made from either Steel or Aluminium.
The buoyancy characteristics of steel "pony" cylinders are such that they will leave you unbalanced in the water, unless you adjust your weighting to compensate. It is also necessary to remove some weight (about 2kg dependent on the cylinder), to ensure that you aren't over-weighted. If you do remove this weight, it is not subsequently possible to remove the cylinder in the water without risking making an uncontrolled ascent to the surface. You must also readjust your weighting if you should subsequently choose to dive without one.
In contrast, aluminium "pony" cylinders are almost neutrally buoyant in the water. Consequently they can be added or removed from your equipment configuration with little or no necessary adjustment. They are particularly suited to being removed whilst in the water. Aluminium cylinders are the better option.
Ahem, just one question, why? These tanks never rest on their bottoms, save your money. If you really want to think about it check out Which Tanks? Single Cylinders. Whovere came up with this idea for pony cylinders has clearly never dived!
If you or your dive partner need to use a pony cylinder, then it will be in an emergency situation. It is very important that the regulator you chose is of a sufficiently high calibre that it will act efficiently on demand, without adding additional stress to what is already a very stressful situation. In effect you need to give it the same consideration as if you were buying your main regulator. If you make the right choice it will also be possible to use it as part of a set-up for more advanced diving, if that option should ever catch your fancy
There are three alternatives available:
Normal Gauge on standard length hose - If the cylinder is back mounted then this will enable you to see how much pressure remains in the tank during the dive but this extra gauge may be confused with the one from your main tank. If side mounted, then it simply adds an extra length of hose, which may get in the way or become entangled. This option is consequently not recommended.
Button Gauges - These are about the diameter of a 1p coin and screw directly into the regulator's first stage. They allow the user to check the pressure in the tank before the dive and have the benefit of reducing the clutter but are difficult to read accurately, impossible during a dive if the cylinder is back-mounted. These are the better option if you have to back mount the cylinder and don't want the extra hassle of checking the contents with another regulator in advance of each dive.
Normal Gauge on 6" hose - This allows you to see how much gas you have throughout the dive and check that it hasn't mysteriously "fallen out", but only if the cylinder is side mounted. Since it is on a short hose, it is incredibly difficult to confuse it with your normal gauge. This is the recommended option for side mounted cylinders. A plastic gauge is used as opposed to a brass/glass gauge since the reduced weight helps the bottle remain neutrally buoyant.
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Overall the best option is a 3 litre 232bar aluminium cylinder with right handed DIN "MDE" or Scubapro Pillar valves. (Right handed means that the handle will be on the right hand side with the valve orifice facing directly away from you.) It is best to side mount the cylinder with one of the regulators and plastic gauges (on a 6" hose) as recommended elsewhere in this guide. Reading the section Which Decompression Cylinder? will likely assist your decision making process. If it is not possible to side mount the cylinder, due to your type of buoyancy compensator, then the stainless steel clamps made by "Portland Oceaneering" or "SDS Watersports Ltd" are far more user-friendly than the less expensive cylinder bag. |
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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
