Cylinders

 Standard 232 Bar 7, 10, 12 and 15 litre steel cylinders. The size of cylinder you require depends on your SAC (surface air consumption) and the type of diving you will be doing. There are tall thin and dumpy cylinders and the vertically challenged divers tend to use dumpy cylinders so the cylinder does not hang below there bottom.
 
Standard 232 Bar 3 litre steel cylinder.This cylinder is usually called a pony cylinder and is connected to the main cylinder by a pony clamp or pony bands but sometimes it is side slung
 
Dumpy 232 Bar 3 litre steel cylinder. This cylinder is usually called a pony cylinder and is connected to a dumpy main cylinder by a pony clamp or pony bands but sometimes it is side slung
 
Steel twinsets using standard 232 Bar 7, 10, and 12 litre cylinders. This setup is used when more gas is required to do longer or deeper dives. The cylinders are connected by Stainless steel twinning bands and twinset tank boots. and an isolating manifold. Less weight is required on your weight belt as the twinset is heavier.
 
Isolated manifolds allows you the option of isolating the cylinders in the event of a neck ring blowing on a cylinder. All other failures IE first stages or hoses blowing being protected by turning off the valve in question. A cable connector called a slob knob can be connected the the middle valve to make isolation of the two cylinders easier for the less subtle diver. Some divers have the cylinders upside down to make the valves easier to get too.
 
To follow
232 bar DIN cylinder valve with 5 threads has a 25mm depth. If you screw a 232 bar or a 300bar DIN regulator in here it winds down the 5 threads and then compresses the O-ring and is ready to dive. There is a dimple on the back so an A-clamp regulator just will fit with an insert.
 
To follow
300 bar DIN cylinder valve with 7 threads has a 35mm depth and no dimple on the back so an A-clamp frame just will not fit. If you screw a 232bar regulator in here it is too deep for the O-ring to tighten against the bottom and it will not seal so there is no danger of blowing your 232bar regs up on 300bar pressure. Also the hole at the bottom of the threaded section is narrower than the 232 bar.
 
The 232bar DIN insert is used to convert DIN to A-clamp. It has a captive O-ring at the inside end and the outside end has one to seat the A-clamp. If you screw it into a 300bar DIN fitting it just keeps screwing in untill it seals down inside the valve where the A-clamp cannot seal on it. This is to prevent 232bar A-clamp regulators being fitted to 300bar cylinders.
 
Stainless steel twinning bands used to hold two cylinders together for a twinset. Weights can also be placed between the cylinders for trim or to replace the weight belt.
 
Suit Inflation bottle usually a 1.5 litre Aluminium cylinder and is connected to the main cylinder by a pony clamp or pony bands The cylinder is usually called an argon bottle but rarely have they argon in them, its usually air. This saves expensive gasses being used to fill the suit. Use a pressure relief valve in the regulator, which will protect you in the event of a first stage failure.  If you don't use a PRV and the first stage creeps you will be constantly filling your suit or you will blow the dry suit hose.

Summary

When you are buying a cylinder you will probably have rented or borrowed a few cylinders and know which size you require. Its a good idea to have a pony cylinder as a completely separate gas supply is advised. Remember that oxygen and nitrox cylinders will have to have a M26 thread after August 2008 and shops will have to have M26 connections to fill them. Make sure the boot on the bottom is not the plastic spiky one as they scrape floors and tear car boots. Its also a good idea to have protective mesh over the cylinder to protect the paintwork. The valve should have a cover to keep out dirt and grease when the cylinder is not being used. You can get cylinders that have a 300 bar working pressure but the jury is out on whether the increased cost and weight is worth the small extra amount of free air they can give. The cylinder valve should be DIN as it is generally known to be the better fitting.

Cylinder testing

On the cylinder is a date stamp, so look out for it when you buy a cylinder and make sure its not a year old. From the new date you have 2.5 years until it requires a Visual test. 2.5 years after the visual date and you'll require a hydro test. 2.5 years from this and you'll need another visual, and so on and so forth. If you want to use nitrox you'll need your cylinder O2 cleaned every 12 months, in addition to the other tests above. There is a slight exception to this supposedly, and thats if the cylinder is being filled by a membrane system then it doesn't need O2 cleaning.

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