Scuba Cylinders 202: Alloys, Valves, Markings and other Fun Stuff

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Single cylinder photographed using iPhone Panorama setting

By Fred Stratton
Instructor & Repair Technician
fred@bubblesornot.com

It's summer!  Time to fill your cylinders and head to the water.  But first, let's build on what we learned last month in Scuba Cylinders 101.  I promised more details on valves, those cryptic cylinder markings, and safety issues with 6351-T6 aluminum alloy.

Deciphering Cylinder Markings

Think of the markings stamped into a cylinder's shoulder like a baby's birth certificate, and hydrostatic retesting stamps and annual VIP stickers like a child's vaccination record. Divers "adopt" their cylinders from the manufacturer, which is like the birth mother whose name likely will be Luxfer, Catalina, Faber or Worthington. The birth date is the original hydro date (e.g. 06 18 is June 2018). The cylinder's material, size, and service pressure are like a baby's height, weight and eye color.  The governing agency (DOT in US, TC in Canada) is like the physician overseeing the birth, ensuring that the delivery goes well. 

Cylinder Marking Rosetta Stone

  • 3 - seamless cylinder
  • A - DoT specification for carbon steel
  • AA - DoT specification for chromoly steel
  • AL - DoT specification for aluminum alloy
  • Bar - 14.5 psi (roughly one atmosphere)
  • M - metricated (Multiply this number by 14.5 to determine PSI.)
  • TC - Transport Canada
    • Formerly Canadian Transport Commission (CTC))
  • DoT - US Department of Transportation
    • Formerly Interstate Commerce Commission (ICC))
  • M4002 - Catalina Cylinders
  • M8303 - Faber Manufacturing
  • M8004 - Worthington Steel
  • REE - Rejection of Elastic Expansion
    • Limit of expansion for steel cylinders and some special permit carbon fiber cylinders to receive a + marking. (See 10% overfill.)  

10% Overfill of Steel Cylinders 

Faber 27cf steel cylinder passed recent
 hydro but wasn't eligible for 10% overfill. 
A plus sign to the right of a hydro stamp (e.g. 04 - 18+) indicates that a steel cylinder is within the manufacturer's Elastic Expansion (EE) limits during hydrostatic testing using a water jacket method. This mark lets fill station operators know that they may safely fill the cylinder to 110% of service pressure. A cylinder rated for 2400 psi with a + sign may be filled to 2640 psi (2400 x .10 = 240 +2400 = 2640) if the + is stamped next to the most recent hydro dates. Otherwise, it can be filled only to its service pressure.

The cylinder to the right is stamped with REE24. It received a + rating when manufactured and hydrostatically tested in July 2008, and it passed hydro in September 2013. However, it wasn't within REE limits, so the retesting facility (RIN 6368) didn't stamp the + on it.  I can only fill this cylinder to its service pressure of 2400. 


Sustained Load Cracking in 6351-T6 Alloy 
Aluminum scuba cylinders made from 6351-T6 alloy are subject to sustained load cracking (SLC) which usually appear as cracks in the cylinder's valve threads. Cylinders with these cracks are compromised and must be condemned immediately. Contrary to some online posts and YouTube videos, no government agency has outlawed filling 6351 cylinders.  However, many dive shops won't touch 6351 cylinders because of the alloy's confirmed metallurgical weakness and the potentially devastating effects of a cylinder rupturing during the filling process.  Watch this video to see what happens when a German compressor manufacturer intentionally overfills a scuba cylinder. 

If you have a 6351 cylinder it should have VE (Visual / Eddy) stamped to the right of the most recent hydrostatic retesting stamp to denote that the hydro facility performed both an annual visual inspection and an eddy current test (ECT) on the cylinder as required by the Department of Transportation (DoT).  If your 6351 cylinder lacks this VE stamp, slowly depressurize the cylinder and bring it to BoN for a professional assessment by a PSI-PCI trained cylinder inspector. 

The youngest 6351 Luxfer cylinder is now 30 years old.  I recommend making a lamp or a wind chime out of your old 6351 cylinders and investing in 21st century metallurgy.  

6351 Quick Reference

Catalina - All Catalina aluminum cylinders are made from 6061-T6 alloy, thus do not require ECTs.  Walk out the door if a dive shop tells you "all aluminum cylinders made before 1990 require eddy current testing." They are trying to pick your pockets...or don't know much about cylinders.  

Luxfer 3AL cylinder with hydrostatic retesting in 1987,
1992, 2001, 2002 and 2007. Note DoT-required VE stamp
in June 2007 by retesting facility A264.

Kaiser - These cylinders are uncommon. Identify by shoulder marking KAE (Kaiser Aluminum Engineering) or DoT permit codes 6575 and 6020. DoT requires these cylinders to be removed from service. 

Luxfer - Require ECT if original hydrostatic test date is 11-71 (November 1971) through 5-88 (May 1988). Luxfer switched to 6061 alloy in June 1988.

Norris Industries - Manufactured cylinders using  6351 alloy under DoT permit 6688. DoT requires these cylinders to be removed from service. Identify Norris cylinders by an "N" inside a diamond stamped into the cylinder shoulder. 

Walter Kidde - Manufactured cylinders from 6351 alloy 1-72 through 12-89.  Most Walter Kidde scuba cylinders have been taken out of service as their DoT permits have expired. Identify these cylinders by WK stamped into the shoulder. 

This article can’t possibly cover the range of cylinders. If in doubt, bring your cylinders into BoN for a professional determination on their viability for continued use. 

Aluminum Oxygen Cylinders

DAN O2 kit with M9 cylinder

Most divers will never need to use their DAN O2 kits in live scenarios. The kits stays nice and clean in its watertight Pelican case. So, this means that an expired hydro is no big deal, right?  Wrong. Your O2 kit cylinder is likely a Luxfer M9 or Jumbo D made of 6061 aluminum, meaning DoT 3AL specification rules apply: a hydro retest is required every five years.

If your O2 cylinder is due for a hydro, organize practice rescue scenarios during your next dive outing. It will refresh your Rescue Diver and Emergency First Aid skills and help you empty your cylinder before you take it in for hydro. 




Cylinder Valves

J and K Valves

Left: J valve in dive start position.
Right: Reserve mechanism activated by pulling rod down.

These valves were Item J and Item K in the 1962 Aqualung Scuba catalog, and the names stuck. The J valve contains a spring-loaded valve that closes airflow from the cylinder to the diver at around 500 psi, at which point the diver would pull a rod, activating the reserve mechanism which opened the airflow. The ubiquitous submersible pressure gauge (SPG) has made J valves an obsolete curiousity.

The K valve has no reserve mechanism, and doesn't need one because the Recreational Scuba Training Council long ago included the SPG as a mandatory piece of dive kit.


Yoke versus DIN

Dive Rite DIN valves on Faber
27cf 3AA rebreather cylinders

Dive Rite DIN valve with yoke insert
Most recreational scuba divers mate their regulators to their cylinders using yoke fittings although DIN (Deutsches Institut fur Normung or German Institute for Standardization) valves have become more widespread for good reason; they contain deep female threads which accept the first stage regulator via a threaded male fitting on the first stage body. Regulators using DIN fittings have a more substantial, tightly fitting O-ring that seats deeply within the DIN valve. DIN fittings can be pressurized to 300 bar (~ 4500 psi) while yoke fittings generally are limited to 200 bar (~ 3000 PSI).

Many cylinders now come with DIN valves and yoke inserts requiring only an 8mm Allen key to install.  Divers can purchase a yoke adapter for their DIN regulators to use while traveling. (Yoke-style valves are the norm for many dive boat operations.)  

Valve knobs can be changed to suit a diver's tactile preference or to help identify quickly what cylinder may contain oxygen, such as the green knob in the photo to the left. (This is in addition to the large green OXYGEN sticker on the side of the cylinder.) 

All Right, Class.  Quiz time!

Armed with the knowledge above, here are some cylinder and corresponding questions.  I'll provide answers in the next post. 

This cylinder was manufactured under a DoT exemption E6498. A subsequent hydro retest facility properly stamped 3AL above E6498.



Questions:
  1. When was this cylinder manufactured?
  2. Is it steel or aluminum?
  3. Is it made of 6351-T6 alloy or 6061-T6 alloy?
  4. What is its service pressure?
  5. What company manufactured it?
  6. What is R69125?


Questions:
  1. When was this cylinder manufactured?
  2. Is it steel or aluminum?
  3. Is it made of 6351-T6 alloy or 6061-T6 alloy?
  4. What is its service pressure?
  5. What company manufactured it?
  6. What testing or inspection is required of this cylinder in addition to annual visual inspections and 5-year hydros?




Explain all the markings on this cylinder.
TC-3AL M207, DOT-3AL 3000, LS9470, LUXFER, 03-04, 2-10, S040

Explain all the markings on this cylinder.
TC-3AL M207 BAR, DOT-3AL 3000, CATALINA, BJ06396, M4002, 09-93, S063

The Last Word: Buying Used Scuba Cylinders
Caveat Emptor!  "Let the buyer beware" is great advice, particularly when buying life support equipment. Consider their external condition (pits, gouges), valve appearance (peeling chrome),  functionality (bent or reluctant hand wheel), and history (current VIP and hydro). 

Have BoN perform a professional cylinder inspection before you pay for used cylinders. New cylinders cost little more than they did 35 years ago and provide peace of mind from diving modern equipment. 

* My gratitude to Odessa Stratton for editing and improving this article. *

Coming Attractions

Answers to the Scuba Cylinder Quiz

Fill Station Operations: Does filling cylinders immersed in water make a difference?

All images: Fred Stratton 

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