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What is a Technical Cylinder Visual Inspection?

Bill High, PSI, Inc.

Visual cylinder inspection (VCI) does not mean the same thing to all who claim to conduct VCIs, nor to those who write laws or guidelines about VCI.

The U.S. Occupational Safety and Health Administration (OSHA) requires all respiratory equipment, including scuba and SCBA (S/S), used by employees to be visually inspected at least every month and after each use. Also, OSHA requires that all pressurized cylinders employees may be exposed to shall be safe as determined by visual inspection.

The U.S. Department of Transportation (DOT) requires hydrostatic retesters to conduct a visual inspection as part of the retest process. The Compressed Gas Association (CGA) publishes cylinder related safety guidelines that are, in some cases, incorporated into law by reference, including CGA C6, C6.1, and C6.2. Even one cylinder manufacturer (Luxfer Gas Cylinders) and the scuba instructor associations NAUI, PADI, NASDS, and SSI have, at one time or another, prepared information about VCI. Although DOT, OSHA, and CGA stress the importance of a VCI, none tells the technician clearly just what it is or how to accomplish it.

Luxfer Gas Cylinders published an excellent guide in 1996 that clearly describes how to conduct a technical VCI on Luxfer aluminum scuba cylinders. Much of that information and those procedures parallel the program developed by PSI, Inc. during the past 15 years. A revised edition of the Luxfer SCUBA Guide was released in 1998 as well as Luxfer’s Technical Guide for inspecting SCBA cylinders. The Technical SCBA inspecting guide is printed in two volumes. Volume I deals with Luxfer aluminum and composite cylinders made before 1988 and volume II covers newer solid wall and several composite materials.

The PSI, Inc. visual cylinder inspector workshop textbook, Inspecting Cylinders, is the best single volume source for technical VCI information about steel and aluminum S/S and composite SCBA cylinders. It was first published in 1987 and is now in its greatly expanded third edition. PSI, Inc. publishes two additional cylinder inspector resource booklets: the Reprint File for scuba inspectors and the Hazmat Training Instructor Guide. PSI, Inc.’s Inspecting Cylinders and Luxfer’s aluminum cylinder inspection guide contain the details necessary for a thorough visual inspection. A technical visual cylinder inspection is a complete assessment, inside and out, using standards and damage limits. Cylinders that meet those standards are returned to service. Many cylinders that do not meet the standards can be serviced, usually cleaned or hydro retested, and returned to service. Damage or other conditions that fail to meet the allowable limits cause the cylinder to be condemned. Condemned S/S cylinders may not be reconditioned.

Tank Inspection

Technical VCI begins with a careful review of the code marks stamped into the cylinder shoulder or appearing on a composite SCBA side wall label. In the U.S., the code must include DOT or ICC markings (in Canada, CTC or TC).

The serial number MUST be legible. There must be an original hydro tester’s mark and a current mark, within 5 years for steel or aluminum cylinders and 3 years for composite SCBA. Composite SCBA must be removed from service at 15 years. All other markings should be valid and compatible. Specific information about correct and fraudulent marks is presented at PSI, Inc. workshops. The cylinder type must be correct for the intended service. For example, composite SCBA cylinders MAY NOT be used for scuba or other underwater purposes.

The cylinder exterior surface should be free of bands, boots, stickers, or other objects that might obscure damage. Cuts, dings, gouges, dents, corrosion pitting, and bulges must be compared to specific damage limits for each cylinder type. The inspector must be able to differentiate between a banana (bowed) or bulged aluminum cylinder.

The entire interior should be brightly illuminated to facilitate a critical examination of any contents or damage. Corrosion indicates moisture has been allowed into the cylinder, usually during fills in water baths or from poorly maintained compressor moisture separators. The technical inspector must estimate the depth of any corrosion pitting and compare findings to the allowable limit for the cylinder type. The PSI, Inc.-developed Master Pit Reference Plate, along with pit limit tables provided to PSI, Inc. workshop participants, simplifies this assessment.

Scuba and SCBA cylinder threads should be carefully checked for condition and minimum number. The minimum number of threads allowed varies with the type of cylinder and may range from 6 to 12 complete and continuous threads measured from the top. Thread chasers should not be routinely used to reshape cylinder threads. Certain 3AL and composite cylinders with aluminum liners warrant a special examination just below and within the threads. A small magnifying mirror and bright, directed light will aid in detecting occasional cracks that may propagate from crown folds. Training and experience are essential in locating and differentiating cracks from tool stops or other marks. The Visual Plus eddy current apparatus can be a useful supplementary tool to augment the standard mirror and light inspection by locating cracks that are sometimes difficult to see.

Valve Inspection

Although valve repair is not a part of the visual inspector's duty, several items should be inspected. In North America, all valves used by divers and fire fighters MUST have a pressure relief device, commonly called a burst disc. Both the leaded plug type and single port retainer nut type should be replaced with more effective designs. Burst disc relief pressure must not exceed the hydrostatic test pressure. Improper installation of the burst disc components may cause the device to release at a pressure much higher than the rated pressure. Valves affixed with high disc pressure ratings must not be placed in lower pressure cylinders. Several valve manufacturers recommend that this safety device be replaced at specific intervals. Many valves used in Europe have no relief device and may have metric threads. Some metric threaded scuba valves have appeared in the U.S. and are both dangerous and illegal if placed in a U.S.-made cylinder.

The cylinder-to-valve O-ring replacement policy varies among equipment manufacturers, but most technicians automatically replace the O-ring and lightly coat the threads with a lubricant having dielectric qualities and no off-gassing solvents. Two dive equipment companies, U.S. Divers Co and Scubapro, and SCBA maker, Survivair, at one time placed plastic dip tubes on their valves. None proved satisfactory and those plastic dip tubes have been recalled. Many remain on S/S valves even after more than 15 years since the first recall. Poorly trained and careless visual inspectors have failed to replace these items.

Technical, high quality visual cylinder inspections are not difficult to perform. Without proper technical training, however, an inspector has little chance of knowing or accessing the specific information necessary to properly judge cylinder condition. The rules, industry standards, and other guidelines simply are too scattered for most individuals to locate. PSI, Inc. has a single purpose and that is to search out high pressure cylinder information important to visual inspectors, fill station operators, and cylinder handlers and make it readily available in our one-day, hands-on workshops. Over 200 workshops are conducted annually in North America and at some international locations.

For more information about PSI, Inc. and cylinder safety, visit our web site at, e-mail us at, or call us at 425-486-2252. We train fire fighters, fill station operators, equipment repair technicians, hydrostatic retesters, scuba instructors, dive store staff, and others.

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