Know your scuba cylinders

Various sizes of aluminum (top rows) and steel
cylinders (bottom rows, with black tank boots)

Scuba cylinders come in a wide range of sizes, colors, and material; each of which can be used for a variety of diving conditions, although certain types of diving may require a more specific type of cylinder.  Some issues include selecting a tank to hold primary gas for recreational diving, backup gas for extended range diving, extra bottles for special mixes for deep diving ("bottom gas") or decompression, or even specifically for non-breathing/inflation purposes only.  Beyond size alone, scuba tanks can have different buoyancy characteristics, making cylinder choice an important consideration for a given dive.

Cylinder Types:
Scuba cylinders are made from alloys of either aluminum or steel:

Aluminum cylinders are generally less expensive and lighter than steel cylinders.  Rental tanks are commonly aluminum 80's ("AL80's") which means that the cylinder holds about 80 cubic feet of gas at its working pressure of 3000 psi.  Some dive charters use smaller AL72's to limit divers' bottom time.  Decompression bottles (usually in the ballpark of AL40's) are light weight and streamlined.  Smaller cylinders like AL13-19's are sometimes filed with Argon and are used solely to inflate a drysuit (better thermal protection than air/nitrox).

Aluminum cylinders are flat on the bottom and can stand up without a plastic boot.  A boot will help protect the cylinder from scratches, however it will also trap water and debris so it is important to rinse cylinders with a boot thoroughly after diving in salt water or silty, sandy conditions.

AL80's are a great investment for open water and cavern divers who are buying their first cylinder(s).  The cost-to-volume ratio is great, and they aren't too difficult to transport to- and around a dive site, although they may be a bit long for very short divers.  An AL80 can give a diver enough bottom time to be limited only by their diving tables' No-Decompression Limits (NDL's). Aluminum 63's are shorter and generally more comfortable, at the cost of less gas and limited bottom time. The buoyancy characteristics of aluminum cylinders make them heavy (negative) at the beginning of a dive and become positively buoyant (float) as the cylinder empties, so a diver should plan to be slightly over weighted at the beginning of a dive to avoid kicking to stay down at 15-20' for a safety stop on a depleted tank.   Naturally, aluminum cylinders do not rust, but may still corrode when exposed to contaminants such as salt water, oil, etc., forming aluminum oxide.  Aluminum cylinders are more resistant to damaging amounts corrosion than rusting steel cylinders.  When aluminum oxide forms it acts as a barrier, hindering further corrosion deeper into the cylinder wall.  Steel cylinders come in two varieties: high pressure (3300-3500 psi) and low pressure (2400 to 2640 psi) cylinders (HP & LP, respectively).  Steel cylinders are a bit more expensive to purchase new, however with proper care a steel cylinder has a virtually infinite lifespan (decades old cylinders are still in proper service today).  The added weight of steel makes these cylinders negatively buoyant (heavier than water) throughout the dive, so a diver will not need as much weight than if s/he were diving a single aluminum tank.  It is common for divers using doubled steel cylinders to dive without any additional weight at all. Steel cylinders usually have a round bottom which makes then unable to stand up on their own without a tank boot.

Boyle's law (relating pressure & volume) illustrates that filling a smaller space with a higher pressure allows a cylinder to be smaller than that of its lower pressure counterpart; so a HP80 steel cylinder will be a bit shorter than a common AL80.  HP80 cylinders are are a breeze to dive since they're so small.  Further, a HP100 is closer to the size of an AL80, although the HP100 is a bit heavier and it has the benefit of around an extra 20 cubic feet of gas; good if you are a heavy breather underwater or you plan on some fun deep dives. When filling a high pressure cylinder, be sure to identify that the cylinder's working pressure is higher than 3000 psi to take advantage of the cylinder's full volumetric potential. A 3000 psi fill on an HP100 is only 85.7 ft³ - significantly less than its rated 100 cubic feet of breathing gas. A low pressure cylinder, on the other hand has a working pressure of 2400 psi. During the first five years of use the cylinder can be filled to 10% over this working pressure to 2640 psi, but this is negated after the cylinder's first hydrostatic pressure test servicing (five years from the original factory hydrostatic test stamp date). Although it is not sanctioned, some fill stations will give a "cave fill" to a low pressure cylinder: filling the cylinder beyond its rated working pressure, giving the the cylinder a capacity to hold a higher volume of gas than it is rated to hold. High pressure steel, and aluminum cylinders cannot be overfilled in this same manor.

For example: A LP95 holds 95 cubic feet of gas @ 2640 psi. A cave fill to 3500 psi gives the cylinder 125.9 ft³. Double LP95's (190 ft³ @ 2640 psi) with a cave fill contain 252 ft³ of gas.

Dimensions:

An illustration of scuba cylinder size vs. its filled contents.
Here, a model stands next to an eighty 1 ft³ boxes, and
an aluminum 80 scuba cylinder.

Click image to enlarge
View a text version of this chart (Google Docs/Drive)
Chart recreated via Laymon, Lynn. "A Tank is a Tank."
Dive Training Magazine November 2004

 

Height	Tanks
17"	HP65
20-21"	AL63, LP66, HP80
24"	LP80, LP95, HP100, HP119
26"	AL80, AL100, LP85, LP104
28-30"	LP120, HP120, HP130

 

Diameter	Tanks
6.9"	AL72, LP85
7.25"	AL63, AL80, LP80, HP80, HP100, HP120
8"	LP95, AL100, LP104, HP119, LP120, HP130

Suggested links: Catalina Cylinders Luxfer Cylinders.com deep-six.com/page66.htm divegearexpress.com/library/tanks.shtml

Valves:

• K-valve - This is currently the standard, and most common configuration of a valve used by many cylinders which requires only one first stage regulator.  The on/off handle is located on the valve's right side (the left side of the picture, as the valve is facing the camera. Comparatively, your right hand  that you will use to operate the valve when wearing your gear will be on your friend's left if you are facing each other), and the burst disk will usually be located on the left side or in the back of the valve.  The valve stem is located below the threads on the bottom of the valve.  This extends further into the cylinder and acts as a straw: if any liquid were to be contained inside the cylinder, this stem would restrict it from being blown into a regulator when a diver is upside down in the water.  This is why it's important to inspect your cylinder annually.

K-valve with a yoke fitting

•  J-valve  - Here is an older style valve that is similar to a K-valve, except there is an additional knob on the diver's left side.  When the pressure in the cylinder is low (around 300-500 psi), this valve will start to act as if it were empty, by restricting the flow of gas to its regulator.  Turning this secondary knob opens a mechanism in the valve to deliver the remaining gas to a diver, thus warning a diver that the cylinder is almost empty if a submersible pressure gauge is not being used (not recommended - this was just old technology).

J-valve with a removable DIN-to-yoke insert

• H-valve -An H-valve is used on a single cylinder with two independent fist stage regulators, with a single octo attached to each. Using two regulators with this valve gives a diver a degree of redundancy: if one first or second stage regulator malfunctions and begins uncontrollably free-flowing or freezes over, the gas supply can be completely shut off while the second regulator set continues to receive gas from the tank and deliver breathing gas to the diver.

H-valve with a valve plug, detached valve stem,
and two DIN-to-yoke conversion plugs

This configuration is recommended for Intro Cave, Decompression, and other technical divers who are not using doubles.  This is a common configuration, however the inverse configuration is also popular: where the H-adapter will simply connect to the other side of the main valve.  The right post configuration above will not work with parts from a left post H-valve. H-valves can be converted to a K-valve by draining the cylinder (0 psi/bar), unscrewing and removing the H-adapter portion, and replacing the adapter with a valve plug (shown in the lower right of the photo above).  Another name for this is a "modular valve."  Note the thicker, longer portion of the valve that connects the H-adapter in relation to the K-valve in the previous image.  This is much stronger which allows a second regulator to be safely connected to the very high pressures contained in the cylinder. Further, a right- and left- H-valve can be connected together by replacing the H-adapter valves with an isolator valve (see below).

• Isolator valve - A mechanism that connects two cylinder valves together is called an isolator valve or a crossbar (no valve).  This valve gives a diver the option of shutting off the connection between the two tanks, thus isolating their gas supplies independently in an emergency, offering a high degree of control and redundancy.  Connecting two valves together with an isolator valve is called a "manifold."
  

Isolator valve

If one of a diver's first or second stage regulators fail, that regulator's valve can be shut off while still giving a diver access to breathing gas both cylinders via the backup regulator.  Gas will pass through the isolator valve to the working regulator.  If a leak develops that remains uncontrolled by operating the right or left valve, the isolator valve can be closed to "isolate" or preserve gas in at least one of the non-malfunctioning cylinders.

• Manifold - The common "double" cylinder configuration connects two cylinders together with a manifold, which is the combination of a "right" valve", a manifold, and a "left valve."

A manifold is a combination of a right, isolator, and left valve.
Photo courtesy Cave Adventurers

• Y-valve -A simplified version of the H-valve is the Y-valve, which allows two independent first stage regulators to be connected to a single cylinder.  This configuration cannot be combined with any other type of valve or cylinder.

Y-Valve

Suggested links: http://www.divegearexpress.com/gas/mvs.shtml

DIN vs. yoke fittings:

DIN K-valve with a removable yoke insert.
Photo courtesy AquaViews.net

Scuba regulators attach to cylinder valves in one of two ways

1. A yoke regulator fits over a valve and is held in place when the regulator is pressurized.

2. A DIN regulator screws into a valve and is held in place with threads that are tightened further when the regulator is pressurized.

A yoke-based regulator is usually less expensive and is great for open water diving.  When you begin to dive overhead environments or use high concentrations of oxygen (above 40%), it becomes important to use a regulator with a DIN fitting since DIN valves are less likely to become dislodged from the cylinder; above or below the surface. Converting between the two usually only goes one way.  A DIN-to-yoke adapter can be attached to a DIN regulator to fit a yoke cylinder. Some yoke regulators have a conversion kit (which are usually pretty expensive for some reason) where the A-clamp can be completely replaced with a new DIN fitting.

DIN to yoke adapter for a DIN regulator.

Suggested links: http://www.nigelhewitt.co.uk/diving/din.html

Painting / finishing:
There are a number of reasons why painting a scuba cylinder is a bad idea.

• Scratches or chips in paint or other finishing coatings allows water to become trapped between the metal of the cylinder and the coating itself.  This can quickly lead to rust or corrosion of the cylinder
• Stickers can trap water against the cylinder as well.  Whenever possible, use small stickers, and avoid large stickers as tank wraps.  Mesh warps are acceptable, as water does not become trapped against the cylinder, and the plastic sleeve provides a degree of scratch/clanking protection
• Using heat to cure paint onto metal cylinders will weaken the metal and become an extremely dangerous hazard.  Never use heat in any part of a painting process
• Paint covers corrosion and damage.  Some scuba shops/filling stations will refuse to fill an after-market painted cylinder
• In order for a cylinder to pass annual visual inspection, the surface must be free of after-market paint and decals

There's a dilemma I have with this issue. On the one hand, diving shouldn't be about looking super cool.  On the other hand, if you're spending hundreds of dollars on expensive equipment, you might as well invest in something that makes you happy every time you see it.  Certainly, a shot-blasted or brushed aluminum tank isn't as flashy as a red one, but it'll get the job done.  For full disclosure, I purchased two candy apple red, glossy coated Aluminum 80's for my first set of tanks, and my girlfriend bough pink tanks!  Our technical gear are shot-blasted double steel 95's, and she has a single white steel 95s with some very small stickers for decoration and a pink tank mesh.

Suggested links: xsscuba.com/tank_lux_tech_paint.html