What each mark does
The plus and star marks live on the cylinder shoulder and authorize different things. They're commonly confused, especially because both are written next to the requalification date.
| Mark | What it authorizes | Authority |
|---|---|---|
| Plus (+) | Filling to 110% of marked service pressure (capacity benefit) | 49 CFR §173.302a |
| Star (★) | Requalification at 10-year intervals instead of 5 (schedule benefit) | 49 CFR §180.209(g) |
A cylinder can have one mark, both, or neither. Both marks are conditional on the cylinder continuing to meet the qualifying criteria at every requalification. When a cylinder fails to qualify at a subsequent test, the affected mark is removed even if the cylinder passes the basic hydrostatic test.
Plus (+)
Per 49 CFR §173.302a, a cylinder marked with "+" is authorized to be charged to 110% of its marked service pressure. A 3000 psi cylinder with the plus mark can be filled to 3300 psi. A 3AA 2400 with the plus can go to 2640.
The plus mark is restricted to:
- DOT 3A and DOT 3AA cylinders only
- Cylinders that pass the hydrostatic test under the tighter elastic-expansion criteria the spec defines for plus eligibility
- Cylinders in compatible service per §173.302a (the regulation specifies which gases the overcharge is authorized for)
The mark is applied at requalification by the RIN holder. It's a capacity benefit: the same cylinder, filled higher. It does not change the retest interval.
Star (★)
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 cylinder owner: half the test work, same regulatory standing.
The star mark is restricted to:
- DOT 3A and DOT 3AA cylinders only
- Cylinders that also qualify for the plus mark (this is the part most commonly missed)
- Cylinders in continuous service for non-corrosive gas
- Cylinders manufactured after 1945
- Cylinders without body welds since manufacture
- Cylinders that meet the permanent-expansion threshold at the most recent requalification
Star requires plus. You can have plus without star (common). You cannot have star without plus.
Together: PASS+★
A cylinder that qualifies for both marks gets the maximum operational benefit: 10% overcharge plus the 10-year interval. The combined mark is typically written as "+★" or "+ star" in records, sometimes as "PASS+★" or "PASS+SAR" in disposition columns.
Each requalification has to demonstrate continued compliance with both sets of criteria. If the cylinder fails plus criteria at the next test, both plus and star come off (because star requires plus). If the cylinder fails star criteria but still passes plus (for example, by switching to corrosive service while still meeting expansion limits), only the star comes off and the cylinder reverts to a 5-year cycle with the plus retained.
How a cylinder qualifies
Both marks are earned at hydrostatic requalification. The qualification logic at the test bay:
- Cylinder undergoes external visual inspection per §180.205. Pass before any pressure work.
- Cylinder is hydrostatically tested per §180.209.
- Permanent expansion (PE) and total expansion (TE) values are recorded.
- Rejection threshold check: PE / TE × 100 ≤ 10%. Above 10%, the cylinder is condemned.
- Plus eligibility check: cylinder is DOT 3A/3AA, in compatible service, meets the spec's plus criteria.
- Star eligibility check: cylinder qualifies for plus, plus the additional star requirements (non-corrosive service, no body welds, manufactured after 1945, etc.).
If the cylinder qualifies for both, the requalifier applies both marks alongside the date and the RIN stamp on the cylinder shoulder. If only plus, only the plus stamp. If neither, just the date and RIN.
When marks come off
Marks are conditional on continued qualification. They come off when:
- The cylinder fails the qualifying criteria at the next requalification. Star requires that the cylinder still meets non-corrosive-service and expansion-threshold criteria. Plus requires the cylinder to still be eligible per §173.302a.
- The cylinder is moved into incompatible service. A star-marked cylinder used in corrosive service loses the star regardless of test outcome.
- The cylinder undergoes a body weld or rebuild. Star requires no body welds since manufacture, so a rebuilt cylinder loses star.
- The cylinder is condemned. All marks become irrelevant; the cylinder is out of service permanently.
Removing a mark physically is done by stamping over the existing mark per the marking standards in §180.213. The removal is recorded in the requalifier's records as part of that test's disposition.
SP variations
Special Permit (SP) cylinders may have their own rules that override the standard plus and star framework. An SP can:
- Authorize plus marking on cylinders that aren't standard 3A or 3AA
- Authorize a different retest interval (longer or shorter than 10 years) without the standard star qualification
- Prohibit plus or star marking entirely for cylinders covered by the permit
- Define different qualifying criteria (alternative expansion thresholds, additional inspection requirements)
When the cylinder is SP-permitted, the SP version in effect at the time of test is the source of truth for what marks (if any) apply. Don't apply standard marks to an SP cylinder by reflex; check the permit text for the version active at the test date.
Common confusion
Plus does not extend the retest interval
This is the single most common mistake at audit. The plus mark authorizes overcharge, not a longer test cycle. A cylinder with only plus is on the standard 5-year requalification schedule. Adding the star is what extends to 10.
Star alone is not a thing
A cylinder cannot carry the star without also carrying the plus. If you see a star without a plus on a cylinder, either the plus stamp is hidden under paint, the cylinder was incorrectly marked, or the cylinder has been altered in a way that requires re-evaluation.
The marks are physical, not paper-only
Both marks must be stamped on the cylinder. A record that says "PASS+★" without the corresponding physical stamp on the cylinder isn't complete. The cylinder's marks govern what fillers can do with it; the records explain how those marks got there.
Customers don't always understand the difference
Customers ask for "the plus stamp" to mean "a 10-year tank" because they've heard the term plus and don't distinguish. Educate them at the counter: plus is more pressure, star is longer between tests. They're different services and the cylinder has to qualify for each.
