Scuba Cylinder Code Markings

Bill High, PSI, Inc

Letter and number groups which appear on the shoulder of a cylinder tell the knowledgeable reader much about the cylinder's features and limitations. In North America, some of the code is required by federal regulation, while occasionally, inspectors or hydrostatic retester's add their own reference marks. Federal regulations have changed several times in our nearly 50 year diving history so older cylinders may carry any of the once valid marks.

The current designations (as of August, 1997) result from regulations modified in October, 1996. The Department of Transportation (DOT) specifies in those regulations how newly manufactured cylinders will be marked and what information must be included. For example, conventional steel cylinders shall have DOT-3AA followed by the service pressure, a serial number, the manufacturer's name or identifying symbol and the initial hydrostatic tester's
date of test and identification mark. In 1987, the Pressed Steel Tank Co. (PST) began producing a high density alloy (not 3AA) cylinder under an exemption (E9791). Those cylinders were sold with brand names Sherwood Genesis or U.S. Divers HP3.5. Beginning in 1997, only U.S. Divers may distribute the PST E9791 cylinder.

DOT had no appropriate cylinder specification in place when aluminum cylinders were introduced into North America in 1971. Luxfer USA (now Luxfer Gas Cylinders), Walter Kidde, Norris Industries and Kaiser were granted special permits and those permit numbers appeared on their cylinders in place of an equivalent to 3AA. Only Luxfer and Walter Kidde remained in business and, when the 5-year special permit expired, were granted exemptions using the same numbers, that is E6498 and E7042. Both Norris and Kaiser permits SP6688 and SP6576 expired so those cylinders are now illegal. In July of 1982, the DOT and CTC granted 3AL status to certain special permit and exemption cylinders, including scuba cylinders made by both Luxfer (SP6498, E6498) and Kidde SP7042, E7042). North American hydro retester's are required to apply a stamp 3AL mark above the allowed cylinder exemption and SP numbers. Those cylinders are technically illegal and should not be filled until that mark has been applied. Many retester's' have failed in their responsibility to both apply the mark and apply it only to eligible aluminum cylinders.

For the most part, Canada (Canadian Transport Commission CTC or its new name Transport Canada TC) and the U.S. DOT have equivalent marks and requirements. Cylinders marked with both DOT and CTC or TC may be filled and serviced in both countries. Beginning in about 1993, Canada required its cylinders to be marked 3ALM and pressure shown in atmosphere (bar) while DOT only requires 3AL and pressure in psi.

Other marks may be included at the manufacturer's option. Luxfer Gas Cylinders includes an S followed by a number representing the volume. Both Luxfer and Catalina Cylinders also identify the volume by specific letters groups incorporated into the serial number.

Of the 8 or more present or past U.S. cylinder manufacturers, divers have most often purchased cylinders made by PST, Norris, Walter Kidde Co. Luxfer Gas Cylinders Coyne and Catalina Cylinders. Two international companies, Faber of Italy (Scuba Pro and OMS brands) and Heiser of Austria (Beauchat brand) make steel cylinders for sale in the U.S.
Occasionally, cylinder distributors apply their own brand name to a manufacturer's cylinder. The Luxfer 3AL appeared as U.S. Divers, Dacor, Sherwood, and Dolphin.

Surplus Air with A Plus

Each 3AA steel cylinder receives an initial hydrostatic test to 5/3 service pressure. That test is documented on the cylinder crown by imprinting the month, year and tester's mark. New cylinder testers use a brand mark, while North American retester's must have a registered mark consisting of a letter and 3 numbers. When a new original test brand mark on a 3AA cylinder is followed by a plus mark (+), the cylinder is eligible for an additional pressure 10% in excess of the marked service pressure. A 2250 psig cylinder with plus (+) may be filled for the valid period of that test to 2475 psig.

Hydrostatic retester's are also authorized to retest a plus marked cylinder to determine whether the wall elastic expansion is low enough to retain the 110% fill. Most retester's choose not to get involved. Some facilities are unwilling to expend the extra effort or don't understand the process, so the steel cylinder owners seldom see a plus (+) after the original test. Since
the plus mark is only valid for the duration of the hydro test, unless renewed along with the next retest, the cylinder must not be surplus filled. A hydro retest may, at any time, re-certify an eligible cylinder if he follows the prescribed protocol.

Aluminum cylinders and the PST high-density steel 3500 psig (E9791) cylinders are not eligible for plus marks and none should appear. The plus (+) mark on these cylinders is illegal and the improperly marked cylinder must be removed from service.

Ten Year Hydro Interval

Certain 3AA steel cylinders, including those containing air, are eligible for a 10 year interval between hydrostatic retests. This provision is for storage cylinders held in a cluster. Many air fill stations assumed that their cascade system fell within this DOT category and the typical 5 year test interval could be extended. The law is written and interpreted very precisely. It applies to cylinders stored in clusters AND REMOVED FROM THE CLUSTER WHEN REFILLED. At reach refill, the cylinder must receive a hammer test. Although the hammer test is a poor substitute for a quality visual inspection, it might identify cylinders with gross damage or contamination. Since most air fill station operators pump the air directly from the
compressor to the cascade group and the cylinder cluster is not broken down and subjected to the hammer test, these air storage system do not qualify for star (*) service. Dive store cylinders must be retested each 5-years. If a storage bank is broken down and returned to the air supplier for refilling, then the cylinder owner must request in writing for the retester to affix the star (*) following his retest mark.

Readable Marks

All required marks must be readable at the time of filling or at retest. Some steel cylinders receive a durable coating of zinc (galvanizing) which fills portions of the marks and reduces legibility. In the early 1970s, some steel cylinders were covered with vinyl plastic that also tended to obscure the code marks. If required marks especially the initial hydro date and serial number, cannot be read, the cylinder must be removed from service. Any cylinder bearing code marks suspected of being altered or obliterated must be condemned.

Illegal Marks

A cylinder must be removed from service if there is any cause to believe that false marks appear or marks are applied by un-authorized persons. A plus mark applied by a cylinder owner to deceive the fill station operator is illegal. A fraudulent reproduction of a retester's mark can land the perpetrator in prison and subject to a considerable fine. Both of these actions have occurred in the U.S. One illegally marked but not tested cylinder exploded.
The fill station operator lost his leg and the man who applied the false hydro mark faces a prison term.

Well-maintained cylinders have a long service life. We have seen numerous code changes over the years and we can expect further changes to cylinder codes. Codes are the permanent record of that cylinder's features so it is vital that cylinder markings are kept intact, never altered and always exposed for reference by the visual inspector, fill station operator and hydrostatic retest facility.

For more information about cylinder safety, read the book INSPECTING CYLINDERS
by William L. High available from:

1997-1998 PSI Inc. all rights reserved.

Created by Rudy Wittwer