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Dive Theory – Scuba Tanks and Cylinders

Due to the importance of the Scuba cylinder we thought its important to give them their own section, the information on these pages are extra to the dive equipment theory requirements but this information compliments and enhances the knowledge already given about scuba tanks or cylinders.

Cylinder Safety

  • Cylinders are heavy so utilize proper handling procedures; i.e. do not use your back to do the lifting
  • Avoid dropping cylinders – a broken valve can turn a cylinder into a missile with 200bar or 3000 psi of thrust.
  • Treat cylinders with respect. A scuba tank or cylinder filled to 200 bar or 3000 psi has over a million pounds of kinetic energy.
  • Although rare somewhere in the world one scuba cylinder explodes each year, usually with disastrous results.
  • Store cylinders vertically either full or with 20 bar / 100 -300 psi in them. In case of a fire, a partially-filled cylinder can explode. A full cylinder will burst the safety and drain the cylinder.
  • Transport cylinders in your car lying crosswise. This way a rear end collision will not shear the valve and create a missile having 3000 pounds of thrust.
  • Do not put your skin against the orifice and turn on the air. This can create a traumatic air embolism (air forced into the body under pressure).
  • When a rubber scuba / cylinder boot is used, make sure it has an open bottom to allow for maintenance and avoid corrosion potential.
  • After each use check the condition of the cylinder valve o-ring and change if suspect.
  • Turn the valve a little and sniff the air. If it smells bad, it is bad. Don’t use it., suspended particles of compressor oil can make you sick.

Filling Cylinders

  • All aluminum cylinders must be stamped with a 3AL designation to be serviceable. The only legal marking for an aluminum cylinder is 3AL
  • Steel cylinders are made under specification 3AA. If a hydro stamp is followed by a + sign, it may be filled 10% over rated pressure.
  • No cylinder can be filled if any of its “markings” have been altered or obliterated in any way.
  • Verify current evidence of inspection (visual) within the last year.
  • Verify a current hydro date and that all stampings on the cylinder are readable and have not been
    tampered with.
  • No cylinder can be filled without a proper pressure relief device installed (burst disk).
  • All aluminum cylinders must be stamped with a 3AL designation to be serviceable.
  • The only legal marking for an aluminum cylinder is 3AL.

Safety First

  • Industry wide we have at least one catastrophic failure per year. Most years there are three or four
  • More than 80% of these failures occur during the fill process
  • To avoid potential problems get the tank inspected Hydostatically every 5 years or according to local laws
  • Scuba cylinders may be carried by their valves.
  • Visual Inspections must be carried out every year, also if you every suspect somethings wrong, inspect!
  • You must be trained in the hazards of dealing with higher % of oxygen in air

Valve Inspection

  • Annual inspection
  • Remove the old O-ring and cut it
  • Clean any lubricants from the threads
  • Verify that the dip tube is metal. Plastic dip tubes are a health hazard as they can disintegrate and clog the air in valve the or
    regulator. Replace the plastic with metal. If not replaceable, change the valve.
  • Verify that the burst disk assembly is a legal one (i.e. holes in the side)
  • If valve looks in good condition, replace O ring and apply a very small amount of 111 to the bottom three threads
  • Nitrox cylinder need you to get the requisite training to service valves for nitrox use
  • Using higher percentages of oxygen and pressure will accelerate the degradation of any O-rings and require special lubricants, components and procedures are needed to service nitrox valves

Visual Inspections

  • Remove the cylinder boot and any covering.
  • Remove any previous EOI stickers as well as any other customer-applied stickers if you have any doubt whatsoever what may be under them.
  • Lay the tank on a flat surface and, using a straight edge, look for any bulges or dents on the side of the cylinder. Be sure to rotate the cylinder a full 360 degrees so that the entire surface has been inspected.
  • Inspect the exterior of the tank for any abnormal indications and note them on your report.
  • If inspecting a steel cylinder, lightly strike the cylinder with a small hammer while holding the tank by the valve and off the ground. It should produce a bell like ring.
  • Using a tank vise, remove the valve from the cylinder.
  • Sniff the air in the cylinder for any strange odors.
  • Strange and offensive odors may require the cylinder to be cleaned.
  • Using a light wand, inspect the interior of the cylinder for any indications. Pay particular attention to the crown area of the cylinder.
  • The inside of aluminum tanks will have different finishes depending on the manufacturer and year of production. They can be an almost black (Alrock) finish, dull gray, or shiny. There may also be some streaking of the finish. These are all normal.
  • The inside of a steel cylinder is usually a dull mat gray. There may be some flash rust showing as a very light reddish coating. Of note is obvious rust deposits. These must be removed.
  • Using a 2x right angle mirror and light, slowly inspect the threads. On aluminum cylinders made of 6351 T6 be very critical in the test. It is advisable to utilize an eddy current tester for crack detection if one is available.
  • Verify the correct number of continuous good threads starting from the top.
  • Verify that the O-ring surface is smooth.
  • Apply a very small (pea size) amount of approved lubricant to the bottom three threads. Do not apply lubricant to the O-ring.
  • Install the valve on the cylinder and torque to manufacturer’s specifications.
  • Finish any documentation used by your organization.
  • Apply a current EOI sticker to the cylinder. NEVER DATE AN EOI STICKER TO EXPIRE AFTER A DUE HYDRO DATE.
  • There are other stickers and labels which may be on a cylinder. For filling purposes the EOI sticker is the primary one. It should make no claims or warranties. (i.e. never state the vip is good for any period of time.)
  • Other stickers can alert you to possible gas mixes for which your fill station is not equipped to supply air fills. Again DO NOT ADD STANDARD GRADE E AIR TO NITROX CYLINDERS.


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