The basic principle
A hydrostatic test pressurizes a cylinder with water (water-jacket method or proof-pressure method) to a defined test pressure higher than its working pressure, then measures how the cylinder body expands under load and how much expansion remains after pressure is released.
Two values come out of the test:
- Total expansion (TE): how much the cylinder body grew under full test pressure
- Permanent expansion (PE): how much of that growth didn't spring back when pressure was released
Elastic expansion is implied: TE − PE. The cylinder passes if its permanent expansion is small relative to the total: the metal's grain structure is still recovering elastically and isn't taking on permanent deformation.
Test pressure by spec
Test pressure is set by each cylinder spec section in 49 CFR Part 178. The most common ratios:
| Spec | Test pressure ratio | Spec section |
|---|---|---|
| 3A | 5/3 of marked service pressure | §178.36 |
| 3AA | 5/3 of marked service pressure | §178.37 |
| 3AL (aluminum) | 5/3 of marked service pressure | §178.46 |
| 3HT | 5/3 of marked service pressure | §178.44 |
| 3T | 5/3 of marked service pressure | §178.45 |
| 4B / 4BA / 4BW | 2× marked service pressure | §178.50 / §178.51 |
The math: a 3AA cylinder marked at 2400 psi service pressure tests at 4000 psi (2400 × 5/3). A 3AL marked at 3000 psi tests at 5000 psi. A 4BA at 240 psi tests at 480 psi.
Test pressure is held for the duration specified in the test method (typically 30 seconds at full pressure, with the exact procedure in §180.205(g)) while expansion is measured. The cylinder is then depressurized and the permanent expansion is read off.
Expansion measurements
Modern hydrostatic testing uses the water jacket method. The cylinder is filled with water and sealed in a jacket, also filled with water and connected to a graduated burette. As the cylinder expands under test pressure, it displaces water out of the jacket into the burette. The volume displaced is the cylinder's expansion.
Two readings are taken:
- Total expansion: burette reading at full test pressure (cylinder fully expanded)
- Permanent expansion: burette reading after pressure is released and the cylinder has finished springing back
Both are recorded in cubic centimeters (cc). For a typical SCUBA-size aluminum cylinder, total expansion might be 14–18 cc and permanent expansion might be 0.4–1.0 cc, depending on the cylinder's age and history.
The 10% rejection threshold
The standard rejection threshold for DOT 3A and 3AA cylinders is:
Permanent Expansion ÷ Total Expansion × 100 ≤ 10%
A cylinder above 10% is condemned. Below 10%, it passes. This ratio is called the permanent expansion percentage in some literature, or just the reject percentage.
The logic: in a healthy cylinder, the metal grain structure is fully elastic. Under pressure, the cylinder expands; when pressure releases, it springs back almost entirely. A small amount of permanent set is normal and acceptable, but a large amount means the metal has work-hardened, fatigued, or been weakened by corrosion or hydrogen embrittlement. That cylinder is on a path toward eventual failure under pressure, and the 10% threshold is where the regulators draw the line.
For 3HT and some other specs, the threshold may differ. The spec section is the source of truth. Aluminum cylinders use the same 10% rule under §178.46.
Plus and star math
The plus and star marks have additional qualifying criteria beyond just "passing the test."
Plus eligibility
Per §173.302a, the plus mark requires the cylinder to meet the spec-defined elastic expansion criteria for plus eligibility, in addition to passing the basic hydrostatic test. The qualifying expansion values are defined in the cylinder's spec section (e.g. §178.37 for 3AA). For most 3A/3AA cylinders, this works out to a permanent expansion percentage tighter than the 10% rejection threshold.
Star eligibility
Per §180.209(g), the star mark requires plus eligibility, plus continuous service in non-corrosive gas, no body welds, manufacture date after 1945, and the same permanent-expansion threshold (PE/TE ≤ 10%) at the most recent requalification. Star is essentially "plus, plus continued good behavior over time."
Full coverage in the plus and star marks article.
SP overrides
Special Permit (SP) cylinders may have their own test pressure formulas, rejection thresholds, and qualification rules. The SP document is the source of truth, and the version of the SP in effect at the time of test governs the math.
Common SP overrides:
- Test pressure ratios different from 5/3 (sometimes higher, sometimes lower)
- Different permanent expansion thresholds (some SPs are tighter than 10%)
- Mandatory eddy current or ultrasonic examination at requalification
- Different retest intervals than the standard 5 or 10-year cycles
Don't default to standard math on an SP cylinder. Pull the SP document, check the version date, apply the rules in that revision.
Gauge accuracy
Per 49 CFR §180.205(g), the test pressure indicator must be accurate within 1% of the test pressure. For a 4000 psi test that's ±40 psi. The gauge must be calibrated against a master gauge traceable to a national standard, and the calibration has to be current.
Master gauges are typically calibrated annually against a deadweight tester or equivalent reference, with the calibration documented and kept on file. Working gauges (the ones at the test bay) are checked against the master more frequently, also documented.
Without a current calibration on the gauge that read the test pressure, the test record is not defensible at audit. The math is only as accurate as the measurement, and the measurement is only as accurate as the gauge. See the calibration chains article for the records side.
