Bauer Air
Diver's Depot
HAZMAT Classes

Earn Your Captain's License Online!
Scuba Cylinders and Title 49 of the Code of Federal Regulations

Part 173 - Shippers - General Requirements for Shipments and Packaging

Subpart A-General
173.1 Purpose and Scope
This part includes: Definitions of hazardous materials for transportation purposes; requirements to be observed in preparing hazardous materials for shipment by air, highway, rail, or water; and inspection, testing and retesting responsibilities for persons who retest, recondition, or maintain compressed gas cylinders used in the transportation of hazardous materials.
173.2 Hazardous materials classes and index to hazard class definitions.
Relative to scuba cylinders, the Class No. of the hazardous materials being considered is a"2", the Division No. is "2.2" and the name of the class or division is non-flammable compressed gas", and the 49CFR reference for definitions is 173.115
173.6 Materials of trade exceptions
When transported by a motor vehicle in conformance with this section, a material of trade (see 171.8) is not subject to any other requirements... besides those set forth or referenced in this section. (Section 6 of part 173)
173.6(a) Materials and amounts A material of trade is limited to...:
173.6(a)(2) A division... 2.2 material in a cylinder with a gross weight not over... 220 pounds
173.6(b) Packaging
173.6(b)(1) Packaging must be leak tight... and securely closed, secured against movement, and protected against damage
173.6(b)(5) A cylinder...containing Division 2.2 material must conform to packaging, qualification, maintenance and use requirements of this subchapter (subchapter C).  Manifolding of cylinders is authorized provided all valves are tightly closed.
173.6(c) Hazard Communication
173.6(c)(3) a DOT specification cylinder... must be marked as prescribed in this subchapter.
173.6(c)(4) The operator of a motor vehicle that contains a material of trade must be informed of the presence of the hazardous material... and must be informed of he requirements of this section. (173.6)
173.6(d) Aggregate gross weight
...the aggregate gross weight of all materials of trade on a motor vehicle may not exceed 440 pounds.

Subpart B - Preparation of Hazardous Materials for transportation
173.23 Previously authorized packaging
When regulations specify a packaging with specification marking prefix "DOT", a packaging marked prior to January 1, 1970, with the prefix "ICC" may be used in it's place if the packaging otherwise conforms to applicable specification requirements. The markings on older scuba cylinders contain the prefixed ICC, the markings on newer scuba cylinders contain the prefix DOT
173.23(c) After July 2, 1982, a seamless aluminum cylinder manufactured in conformance with and for use under DOT special permit (SP) or exemption (E) 6498, 7042, 8107, 8364 or 8422 may be continued in use if marked before or at the time of the next retest with either the specification identification "3AL" immediately above the special permit or exemption number, or the DOT mark... in proximity to the special permit or exemption marking.
173.34 Qualification, maintenance and use of cylinders
173.34(a) General qualification for use of cylinders
173.34(a)(1) No person may charge or fill a cylinder unless it is specified in this part (Part 173) and part 178 (All American made and/or marketed Scuba Cylinders meet this requirement) A cylinder that leaks, is bulged, has defective valves or safety devices, bears evidence of physical abuse, fire or heat damage, or detrimental rusting or corrosion, must not be used unless it is properly repaired and re-qualified as prescribed in these regulations.
173.34(b) Grandfather clause. A cylinder in domestic use previous to the date upon which the specification therefore was made effective in these regulations may be used if the cylinder has been properly tested and otherwise complies with the requirements applicable for the gas with which it is charged. Currently here are few, if any "grandfathered" cylinders being used as scuba cylinders. However, any such cylinders which continue to pass their hydrostatic retests and their five year and annual visual inspections and continue o be suitable for the gases intended to be contained may be continued to be used.
173.34(c)Cylinder Marking. Each required marking on a cylinder must be maintained so that it is legible. Retest markings and original markings which are becoming illegible may be reproduced by stamping on a metal plate which must be secured to the cylinder.
173.34(c)(1) Additional information not affecting the markings prescribed in the applicable cylinder specification may be placed on he cylinder (applies o such information as annual evidence of inspection stickers, allowing them to be placed)
173.34(c)(3) Markings required on cylinders may not be altered or removed the exception to this is that the marked service pressure may be changed but only after a lengthy and involved procedure not typically pursued by recreational scuba divers (and not allowed for cylinder which have failed their periodic hydrostatic retesting unless reheat treated and requalified in accordance with this section [frankly speaking, not something to be pursued])
173.34(d) Pressure relief device systems. Cylinders must be equipped with one of more pressure relief devices per CGA Pamphlet S-1.1
173.34(d)(1) Safety devices are not required on cylinders 12 inches in length or less (exclusive of neck) and 4.5 inches or less in outside diameter. A pressure relief device is required on a cylinder charged with nonliquified gas to a pressure of 1800psi or higher a 70 degrees F (Scuba cylinders fall into this category)
173.34(e) Periodic qualification and marking of cylinders.
173.34(e)(1)(2) Every 5 years cylinders with the following specification marks (3A, 3AA or 3AL) must be retested to 5/3 times he service pressure, or to the test pressure marked on the cylinders shoulder when such is present. (what his means is that a cylinder's markings need o be carefully scrutinized prior to the retest procedure to determine  to what pressure he cylinder is to be retested. No cylinder maybe charged (filled) unless that cylinder has been inspected and retested and the retester has marked the cylinder appropriately. The retest must be performed by an authorized retester (A person holding a current retester's identification number issued by the Associate Administrator for Hazardous Material Safety.
173.34(e)(3) Visual Inspection.. each time a scuba cylinder is retested, it must be visually inspected, internally and eternally, in accordance with CGA Pamphlets C-6, C-6.1 as applicable.
The only requirement in the Code of Federal Regulations for visual internal and external examination is at the time of retest (once every 5 years, etc) Annual visual inspections are by mutual agreement... scuba cylinder owners submit their cylinders for annual inspections to individuals known to have been trained and certified to do annual inspections.

A person who only performs visual inspections on DOT or ICC specification cylinders is not required to obtain a retester identification number. However, a compressed gas cylinder may only be rejected by a person who has obtained such a retester identification number

173.34(e)(4) Pressure retest. Each cylinder required to be retested must be retested by means suitable for measuring the expansion of the cylinder under pressure (interior hydrostatic pressure in a water jacket, or other suitable apparatus)
173.34(e)(5) Cylinder rejection
173.34(e)(5)(i) A retester shall reject a cylinder when on visual inspection, it meets rejection standards in CGA pamphlet C-6, C-6.1 as applicable
173.34(e)(5)(iii) When a cylinder is rejected, the retester shall notify the cylinder owner in writing that the cylinder has been rejected and may not be filled.

In general, scuba cylinders which have been rejected should be condemned. A cylinder can only be rejected by an approved tester.

173.34(e)(6) Cylinder condemnation.
A scuba cylinder must be condemned when: upon inspection, it meets a condition of condemnation in CGA Pamphlet C-6 or C-6.1 (as appropriate); the cylinder leaks through its wall; there is evidence of cracking the extent that the cylinder is likely to be weakened appreciable; or permanent expansion exceeds 10% of total expansion. A cylinder is condemned by having a series of X's stamped over the ICC or DOT specification number and the marked service pressure.. or the word CONDEMNED is stamped on the shoulder.. or.. a the direction of the owner he cylinder is rendered incapable of holding pressure. When a cylinder is required to be condemned, the retester shall notify the cylinder owner, in writing that the cylinder is condemned and may not be filled. A cylinder can only be condemned by an approved retester. Steel scuba cylinders condemned because of excessive permanent expansion may reheat-treated (see note above) 3AL cylinders (aluminum) condemned because of excessive permanent expansion may not be reheat-treated.
173.34(e)(7) Retester Markings
Each cylinder passing retest must be marked with the cylinder retester's identification number set in a square pattern, between the month and year of retest date. The retester's identification number must be in characters not less than 1/8 inch high. Dates of previous tests must not be obliterated.
173.34(e)(8) Recordkeeping
The Code of Federal Regulations only identifies retesters (persons holding current retester identification numbers from the associate administrator for Hazardous Material Safety) as individuals who shall maintain records (current retester identification number issuance letters, daily records of visual inspections, hydrostatic retests, etc)
173.34.(e)(16) DOT-3A or 3AA cylinders. Defines so-called "star service". "Star service does not apply to cylinders used for diving, but may apply to cylinders in dive store compressed gas banks. A cylinder made in compliance with specification DOT-3A or DOT-3AA not exceeding 125 pounds water capacity (Steel scuba cylinders meet this criterion) and removed from any cluster, bank, group, rack or vehicle each time it is filled may be retested every 10 years instead of every 5 years provided the cylinder complies with all of the following:

  • Was manufactured after December 31, 1945
  • Used exclusively for air or certain other gases (argon, helium, nitrogen, oxygen or permitted mixtures)
  • Passes the CGA Pamphlet C-6 hammer test prior to each refill
  • Is stamped with a five-pointed star following the test date
  • Is dried inside immediately following hydrostatic testing
  • The cylinder is not used for underwater breathing

Some hydro-station and air fill station personnel misinterpret the "Star Service" description to mean that certain steel cylinders ("old") ones must be taken out of service. They are mistaken.
173.34(f) Cylinders subjected to the action of fire. Such cylinders must not again be placed in service until they have been properly reconditioned. Aluminum scuba cylinders subjected to the action of fire shall not be reheat treated and must be removed from service. Certain steel cylinders subjected to the action of fire shall be hydrostatically retested, other steel cylinders shall be reheat treated and reconditioned as specified in paragraph G.
173.34(g) Reheat treatment. The description is lengthy, and the process is involved. It will be safer and simpler to condemn such a cylinder (one which was subject to the action of fire) and remove it from service.
173.34(h) repair by welding or brazing of specification DOT-3A, DOT-3AA, ...cylinders.. describes a lengthy procedure not applicable to steel scuba cylinders and not at all applicable to aluminum ones. Frankly speaking, any scuba cylinder deemed to require repairs by welding or brazing should instead be condemned and removed from service.

Subpart C - Definitions, Classification and Packaging
Class 1
173.50 Class 1 - Definitions
An explosion is defined as an extremely rapid release of gas and heat. When a scuba cylinder under pressure ruptures, there is no release of heat. Although such incident can not technically defined as "explosions" they will always be described as such

Subpart D - Definitions, Classification, Packing Group Assignments and Exceptions for (certain) Hazardous Materials
173.115 Class 2 Division 2.2 - Definitions
Division 2.2 (Non-flammable, non-poisonous compressed gas-including compressed gas and oxidizing gas) For the purpose of this subchapter, a non-flammable, non-poisonous compressed gas (Division 2.2) means any mixture which-
173.115(b)(1) exerts in the packaging an absolute pressure of 40.6psia or greater at 68 degrees f. The gases in scuba cylinders referenced in this book meet this definition
173.115(d) Non Liquefied compressed gas. A non-liquefied compressed gas mean a gas, other than in a solution, which in a packaging under the charged pressure is entirely gaseous at a temeperature of 68 degrees f. The gases in scuba cylinders referenced in this book meet this definition
173.115(i) Service Pressure. The expression service pressure means the authorized pressure marking on the packaging. For example, for a cylinder marked "DOT3A1800", the service pressure is 1800psi

Subpart G - Gases; Preparation and Packaging
173.300a Approval of independent inspection agency
Any person who does not manufacture cylinders.. and... is not directly or indirectly controlled by any person or firm which manufactures cylinders... may apply to the Department of Transportation for approval as an independent inspection agency for the purpose of performing cylinder inspections and verifications... require by part 178 (A very involved process)
173.301 General Requirements for shipment of compressed gases in cylinders...
173.301(b) Ownership of a container. A container charged with a compressed gas must not be shipped unless it was charged by or with the consent of the owner of the cylinder
173.301(c) Retest of Container. A container for which prescribed periodic retesting has become due must not be charged and shipped until such retest has been properly made.
173.301(d)(1) Manifolding containers in transportation. Manifolding of compressed gas cylinders is allowed for containers of argon, air, helium, nitrogen (or) oxygen, provided that each container is equipped with a pressure relief device [per 173.34(d)]
173.301(e) Container Pressure. The pressure in the container at 70 degrees f must not exceed the service pressure for which the container is marked or designated except as provided in 173.302(c) [173.302(c) describes the "10% overfill" allowed for some steel cylinders
173.301(f) Container pressure a 130 degrees f. The pressure in he container at 130 degrees f shall not exceed 5/4 times the service pressure.
173.301(h) Compressed gas containers. Compressed gases must be in metal containers built in accordance with DOT specifications in effect at time of manufacture.
173.301(i) Foreign cylinders in domestic use. Foreign cylinders are allowed in the USA provided they were manufactured, inspected and tested in accordance with the applicable DOT specification set forth in part 178
173.301(j) Charging of foreign cylinders. If the foreign cylinder is not manufactured, inspected and tested in accordance with DOT/ICC specifications it can only be charged for export.
173.302 Charging of cylinders with non-liquefied compressed gases.
Detailed requirements. Cylinders of compressed gases must be shipped, per 173.301 and 173.305 in the following containers:
173.302(a)(1) Specification 3A and 3AA (to mention only two). Applies to cylinders of compressed air, cylinders of compressed oxygen, cylinders of compressed helium, cylinders of compressed nitrogen and cylinders of compressed argon.
173.302(a)(5) Authorizes the use of aluminum cylinders (DOT 3AL) for compressed air, argon, helium, nitrogen and oxygen (to mention only a few)
173.302(a)(5)(i) In oxygen service, 3AL aluminum cylinders must be equipped only with brass or stainless steel valves
173.302(a)(5)(ii) In oxygen service, aluminum cylinders (DOT3AL) must be equipped only with straight threads in the neck opening.
173.302(a)(5)(iii)(iv) In oxygen service, aluminum cylinders (3AL) must be cleaned in compliance with the requirements of Fed Spec R-C-901b (Aug 7, 1967), paragraphs 3.7.2and 3.8.2 Cleaning agents equivalent to those specified in RR-C-901b may be used provided they are not capable of reacting with oxygen. One cylinder selected at random from a group of 200 or less, cleaned at the same time, must be tested for oil contamination in accordance with RR-C-901b paragraph 4..4.2.3 and meet the standard of cleanliness specified, and when used in oxygen service, the pressure in the aluminum cylinder may not exceed 3000psig at 70 degrees f
173.302(b) Filling limits. The pressure in the container at 70 degrees f must not exceed the service pressure for which the container is marked or designated. The only exception to this filing limit is provided in 173.302(c) which follows below.. and which does not apply to 3AL aluminum cylinders. 3AL aluminum scuba cylinders must not be overfilled.
173.302(c) Special filling limits for specifications 3AA cylinders. This is the regulation which permits 10% over-fill of certain steel cylinders. Steel cylinders may be over-filled by 10%, provided:

  1. Such cylinders are equipped with frangible disc safety relief devices (without fusible metal backing) having a burst pressure not exceeding the minimum prescribed test pressure
  2. Such cylinders were last tested or retested by the water jacket method
  3. for DOT 3AA cylinders the average wall stress shall not exceed 67,000PSI or the maximum wall stress shall not exceed 73,000 psi maximum. These figures in no way represent the pressure of gas in the cylinder, This wall stress is the internally (within the steel itself) distributed  force-per-unit-area mechanical reaction of the steel resulting from he deformation (stretching of the steel)
  4. An external and visual examination made at the time of test or retest shows the cylinder to be free from excessive corrosion, pitting or dangerous defects.
  5. That a plus sign (+) be added following the test date marking on the cylinder to indicate compliance with (c)(2), (c)(3) and (c)(4) above.

173.305 Charging of cylinders with a mixture of compressed gas and other material. Applies to nitrox, trimix etc
Detailed requirements. If the mixture is a compressed gas, that mixture must be treated as a compressed gas in accordance with these regulations
173.306 Limited Quantities of compressed gases
173.306(a) Limited quantities of compressed gases (air, argon, helium, nitrogen and oxygen, to name a few) are excepted from labeling (except when offered for transportation by air). In addition, shipments are not subject to subpart F of 172 (placarding), nor to part 174 (Carriage by rail) except 174.24, nor to part 177 (Carriage by public highway) except 177.817. Each package may not weigh more than 66 pounds gross weight

EKS Enterprises, Inc., dba and, is an authorized Bauer distributor.
Disclaimer: and are not affiliated with Bauer Compressors, Inc.
Bauer, Oceanus, BP II, Dive Mate, Maxi Verticus, Mini Verticus, Securus, Capitano II, Junior II, Mariner II, Unicus III, Mini Unicus, TCom Lite, TCom, Vertecon, Triplex, and Classic II are all trademarks owned by Bauer Compressors, Inc. of Northfolk, Virginia.