DOT cylinder requalification, in plain language.

The framework

DOT-spec compressed-gas cylinders must be requalified on a recurring schedule by a person or facility holding a Requalifier Identification Number (RIN) issued by the Pipeline and Hazardous Materials Safety Administration (PHMSA). The framework lives in 49 CFR Part 180, Subpart C, which carves the work into three buckets: a visual inspection, a strength test, and the records that prove you did them.

Service gas drives the rules

The §180 framework applies to DOT-spec cylinders that are eligible for requalification in the first place. Eligibility, the marks a cylinder can carry, and the test interval all depend on what the cylinder is used for. Service gas is the load-bearing variable, and the rules are stricter than they look on paper.

Some cylinders are never requalifiable

A few categories of cylinders cannot be requalified at all:

• DOT-39 single-trip non-refillable cylinders (per spec, not refillable in the first place)
• Cylinders that have been previously condemned (per §180.205(j), the disposition is permanent)
• Cylinders that previously held Class 8 corrosive liquids without the cleaning and inspection required by §180.205(e)
• Cylinders with damage beyond the repair limits in §180.211

Acetylene is a special case. The cylinders contain porous mass and acetone solvent, so they don't undergo standard hydrostatic testing. They follow their own requalification regime focused on the porous mass condition, the acetone fill level, and external visual inspection. Service lives are long, intervals differ from standard cylinders, and the work happens at facilities equipped for that specific regime.

Service changes can void marks or eligibility

The +/★ marks are conditioned on continued service. The star mark in particular requires continuous service in non-corrosive gas. If a star-marked cylinder is moved into corrosive service, the star comes off at the next requalification regardless of test outcome. The plus mark similarly depends on the cylinder remaining in service compatible with the spec.

Service changes can also preclude requalification entirely. Moving a cylinder into oxygen or breathing- air service typically requires certified cleaning and recertification (no hydrocarbon contamination, dedicated service history); some prior corrosive services permanently restrict the cylinder from being reused for other gases. The owner's service log isn't optional context. It determines what the cylinder is legally allowed to do next.

Visual inspection (§180.205)

Every cylinder presented for requalification gets an external visual inspection before any pressure work. The inspector is looking for damage that disqualifies the cylinder from continued service: corrosion, cracks, dents, gouges, fire damage, defective threads, and unauthorized alterations. The check sounds simple but it's the step that most often produces a condemnation, and the inspector's judgment calls have to be defensible years later when an auditor reviews the record.

Each visual inspection produces a disposition: OK, condemned, or requires further evaluation. The disposition is stamped on the cylinder and recorded in the requalifier's register.

What gets recorded

• Cylinder serial number and DOT specification (e.g. 3AL, 3AA, 3HT)
• Manufacturer and date of manufacture
• Owner / customer reference
• Inspection date
• Inspector identifier (the human, not just the facility)
• Defects found, if any
• Disposition

Aluminum cylinders: a separate set of headaches

Aluminum cylinders introduce three operational realities that steel cylinders don't have. Every shop running aluminum tanks deals with at least one of them, and most deal with all three.

The 6351-T6 alloy and sustained load cracking

Per §180.209(m), DOT-3AL cylinders made from aluminum alloy 6351-T6 used in SCUBA, SCBA, or oxygen service are required to undergo eddy current examination at requalification, applied from inside the neck, to detect sustained load cracking that can extend into the neck threads. The condemnation criterion is strict: any crack in the neck or shoulder of two thread lengths or more, and the cylinder is condemned.

The 6351-T6 stock was produced roughly between 1972 and 1988 by manufacturers including Luxfer, Walter Kidde, and others; identification is by manufacture date and stamping. Industrial-gas service (CO₂, fire extinguisher, etc.) is explicitly out of scope for the eddy current rule. It applies specifically to breathing-air and oxygen service.

The annual VIP: industry standard, not DOT requirement

The dive industry treats annual visual inspection as gospel. SCUBA shops won't fill a tank without a current VIP sticker, and customers expect it. This isn't a DOT requirement; §180.205 only mandates external visual inspection at the 5-year requalification. But it's near-universal practice via CGA Pamphlet C-6.1, the industry standard that PHMSA itself references in §180.209(m). The annual VIP is an inspection-only event between requalifications, producing a dated sticker on the cylinder that shops use to gate refills.

The "sell instead of inspect" trade-off

This part isn't regulation; it's industry culture. Many dive shops will condemn a marginal aluminum cylinder rather than do the proper visual + eddy current work. Three reasons:

• Liability fears around aging aluminum stock, especially older 6351-T6
• Time and tooling overhead for a thorough inspection
• Margin. Selling a new $200 cylinder is faster and more profitable than billing the customer $30 for an inspection

The result: older aluminum cylinders pulled from service prematurely, and customers paying for new tanks they didn't strictly need. A facility with the documentation infrastructure to inspect properly, eddy-current test when required, and stand behind the result has a real customer-loyalty advantage over the "just buy a new one" shops.

Eddy current as a value-add (VIP+)

Here's the upside. Many shops have already invested in the eddy current equipment and the inspector training to handle the 6351-T6 work. That equipment doesn't have to sit idle between mandated jobs. Plenty of shops offer eddy current testing as a value-add, often branded VIP+, on cylinders where it isn't strictly required, including non-6351-T6 aluminum and even steel, charging a small premium for the additional confidence.

For the customer, VIP+ on a non-mandated tank is reassurance: there's no hidden neck crack waiting to grow. For the shop, it's a higher-margin service line that uses equipment already on the floor. The bottleneck isn't the test. It's the documentation: an eddy current report has to be tied to the cylinder's service history, retained per §180.215, and surface again at audit time alongside the matching VIP and hydro records.

Requalify supports this end-to-end. Eddy current records attach to the cylinder timeline alongside visual and hydro records, annual VIP entries, and SP coverage. Records are versioned, backed up, access-controlled, and exportable as part of the audit packet, so the shop captures the value of the upsold service without inheriting a parallel filing system to maintain.

Hydrostatic testing (§180.209)

Hydrostatic ("hydro") testing pressurizes the cylinder with water to a defined test pressure, measures the expansion under load, and uses the elastic and permanent expansion values to judge whether the cylinder is still fit for service.

Test pressure

Test pressure is determined by the cylinder's DOT specification and service pressure. For a typical 3AA cylinder, test pressure is 5/3 of service pressure. SP-permitted cylinders carry their own rules; the SP itself is the source of truth and the test pressure must match the version of the SP in effect at the time of test.

Expansion measurements

A hydrostatic test produces three expansion values: total expansion (the cylinder's response under full test pressure), permanent expansion (what doesn't come back when pressure is released), and elastic expansion (the difference). The reject threshold is expressed as a percentage:

Permanent Expansion ÷ Total Expansion × 100 ≤ 10%

A cylinder exceeding 10% is condemned. Some SPs allow different thresholds; again, the SP version at test time is what governs.

Disposition codes

These are the dispositions a hydrostatic test produces. PASS and CONDEMN are the regulatory floor; the variants with marks reflect the additional qualifications a cylinder may earn at requalification:

• PASS: within rejection limit, returned to service on the standard interval and standard service pressure
• PASS+: qualifies for the "plus" mark, allowing the cylinder to be filled 10% above marked service pressure
• PASS+★: qualifies for both plus (10% overcharge) and star (10-year retest interval) marks
• CONDEMN: exceeds rejection limit, removed from service permanently

The plus (+) and star (★) marks

Plus and star are two separate qualificationsthat get stamped on cylinders meeting additional criteria during requalification. They govern different things and shouldn't be confused: a cylinder can have one, both, or neither.

Plus (+): 10% pressure overcharge

Per 49 CFR §173.302a, a cylinder marked with "+" can be filled to 110% of its marked service pressure. A 3000 PSI cylinder carrying the plus mark can be charged to 3300 PSI. This is a capacity benefit for the cylinder owner, not a schedule benefit. The mark requires the cylinder to be a DOT 3A or 3AA spec and to pass the hydrostatic test under the tighter elastic-expansion criteria the spec defines.

Star (★): 10-year retest interval

Per 49 CFR §180.209(g), a cylinder marked with "★" can be requalified at 10-year intervals instead of the standard 5-year cycle. This is a schedule benefit for the requalifier and the owner, not a pressure benefit. To qualify, the cylinder must be a DOT 3A or 3AA, be in continuous service for non-corrosive gas, and meet the permanent-expansion threshold at every requalification.

Together: PASS+★

A cylinder with both marks gets the maximum operational benefit: 10% overcharge and the 10-year interval. Each requalification has to demonstrate continued compliance with both sets of criteria, or the affected mark comes off and the cylinder reverts (to standard service pressure, or back to the 5-year cycle, depending on which qualification was lost).

Tracking which cylinders carry which marks across years of requalification, and pulling the marks correctly when a test fails the tighter criteria, is one of the bigger documentation headaches in the workflow, and one of the things software is meaningfully better at than a spreadsheet.

Records and retention (§180.215)

Section §180.215 sets two different retention rules for two different record types. The distinction is important and easy to miss:

Routine requalification records (§180.215(b))

For daily records of visual inspection, pressure (hydro) test, eddy current examination, and ultrasonic examination, the requalifier must retain the record until either the expiration of the requalification period or until the cylinder is again requalified, whichever occurs first. In practical terms: at least 5 years for a standard cylinder, or 10 years for a cylinder carrying the star (★) mark.

Repair, rebuild, and reheat-treatment records (§180.215(c))

Records covering welding or brazing repairs, rebuilding, or reheat treatment are held to a much longer standard: at least 15 years by the approved facility. These records survive even after the cylinder itself has been re-tested or condemned.

What the regulation actually says

The minimum retention is the floor, not the ceiling. Most audits ask for more, and most defensible compliance postures keep records substantially longer than required. Requalify defaults to 10-year retention on every record. Well past the routine-record minimum, approaching the 15-year repair-record requirement, and comfortably past most state-level retention overlays.

Records that must be kept

• Cylinder identification (serial number + DOT spec + manufacturer)
• Date of inspection or test
• Test pressure used (where applicable)
• Specified service pressure
• Disposition (with marks, if applied)
• Identification of the requalifier (the RIN, plus the human inspector)
• Any deviations from standard procedure, with explanation
• The applicable Special Permit version, if the cylinder is SP-permitted

The Requalifier Identification Number (RIN)

Every facility doing this work holds a RIN issued by PHMSA. The RIN identifies the facility on every record and on the stamp impression that goes onto cylinders. RINs are renewableand have an expiration; testing performed after the expiration date isn't valid. A facility that misses its renewal can end up with weeks or months of tests that don't count, and the cylinders involved have to be re-tested by an active RIN holder before they can return to service.

Special Permits (SPs)

An SP modifies the standard requirements for a defined class of cylinders. SPs cover cylinders that don't fit a stock DOT specification, or that need different procedures because of size, material, or service. SPs are versioned; PHMSA periodically revises them.

The record is a snapshot of that point in time. Each test documents which SP version was in effect when the test ran, and that record stands. A test performed under SP-X revision 3 doesn't become invalid when revision 4 is published. It was valid under the rules that applied at the time of test, full stop.

What changes is everything downstream: future tests on cylinders covered by that SP must comply with the new revision, and the affected cylinders typically need to be surfaced for the inspector at their next requalification so they know the rules shifted between this requalification and the last one.

Tracking which SP version applied at the time of each test, and surfacing affected cylinders when a revision lands, is one of the bigger record-keeping headaches in the workflow.

What this all looks like in software

Requalify takes the §180 framework and bakes it into the data model:

• Visual inspection records carry every required §180.205 field at entry
• Hydrostatic records auto-calculate elastic expansion and rejection percentage
• Plus and star marks are applied only when the test data passes the underlying criteria
• 6351-T6 aluminum cylinders in SCUBA / SCBA / O₂ service are flagged for eddy current at every requalification, per §180.209(m)
• Annual VIP records (CGA C-6.1) live on the same cylinder timeline as the 5-year requalification records
• RIN expiration triggers a 30-day banner before approval lapses
• SP revisions tag the affected cylinders for review when PHMSA publishes a change
• 10-year retention is the default, well past the §180.215(b) minimum for routine records
• The audit packet exports as a single PDF with cylinder lineage, inspector roster, calibration chain, and equipment provenance

The result: the spreadsheet becomes a system of record, the audit becomes a download, and the requalifier becomes the operator they actually trained to be, not a part-time data-entry clerk.