Standards & Safety • Cylinder Testing

Understanding ISO 18119 / EN 18119 — Cylinder Inspection & Testing

By Dive Technician — October 8, 2025 • Reading time: ~6 minutes

ISO 18119 / EN 18119 provides the framework and procedures for the periodic inspection and testing of seamless steel and aluminium-alloy transportable gas cylinders and tubes — including scuba cylinders. These tests keep cylinders serviceable and safe by detecting damage, corrosion, or material fatigue before a failure occurs.

What the Standard Covers

The standard applies primarily to seamless steel and aluminium-alloy cylinders. While it sets out the core test and inspection steps, some cylinder types — notably composite cylinders and acetylene cylinders — may follow additional or separate standards. Always check the cylinder manufacturer’s data and local legislation for specific requirements.

Why Periodic Testing Matters

Cylinders store gas at very high pressures and are exposed to mechanical stress, corrosion and environmental conditions. Periodic inspection and testing are essential to:

• Detect defects before they cause catastrophic failure.
• Verify that the cylinder still meets original design limits.
• Document compliance for audits and insurance.
• Safely extend the service life where appropriate.

Key Stages of the Testing Process

Testing intervals vary by country, cylinder material and usage, but inspections commonly occur every 2.5 to 5 years (ISO/EN Standard Every 1 Year and Hydrostaic Testing Every 5 Years). The main stages include:

1. Visual Inspection

External inspection: Technicians inspect the exterior for dents, bulges, deep scratches, corrosion or paint damage that could indicate underlying problems. Paint may be removed to inspect the metal beneath.

Internal inspection: Using bright illumination and inspection mirrors (or borescopes where required), the cylinder interior is checked for corrosion, pitting, contamination, or cracking — especially in high-stress areas such as the shoulder and base.

Valve & neck inspection: The valve is removed, cleaned and inspected. Neck threads are checked for wear or deformation and, if re-tapped, verified with appropriate gauges.

2. Hydrostatic Test

The hydrostatic test is the principal structural test:

• The cylinder is filled with water and placed in a pressurised test jacket.
• Internal pressure is raised to a specified test pressure (for many scuba cylinders this can be near 345 bar/5,000 psi depending on the cylinder’s rating and national rules).
• Permanent expansion is measured. Excessive permanent expansion — commonly above a specified percentage or absolute limit (5% for EN/ISO Standards) and a (10% for DOT Standards) of the total expansion.— indicates failure and a reason to condemn the cylinder.

What the Limit Means 

• Total Expansion: This refers to the total elongation, or strain at break, a material can undergo before it fractures.
• 5%: This is the absolute value that the total expansion must not exceed ISO/ EN Standards.
• Absolute Limit: It sets a hard upper boundary for a material’s deformability.
• ISO 6892-1 Standard: This standard outlines the methodology for performing uni-axial tensile tests to determine characteristic values like strain at break.
• Tensile Testing: A specimen of the material is subjected to a tensile (pulling) force until it breaks.
• Strain Measurement: During the test, the amount of elongation (expansion) is measured.

3. Eddy Current Testing (for Certain Aluminium Alloys)

Cylinders made from alloys such as 6351 (used historically for some scuba cylinders) are vulnerable to sustained-load cracking. For these alloys an eddy current test is often required by legislation and manufacturer guidance to detect fine cracks in the neck and shoulder region that visual inspection might miss.

4. Valve Servicing & Replacement

Valves are overhauled with oxygen-compatible materials where required, and any damaged valves or burst discs are replaced with certified components that match cylinder ratings and usage (e.g., oxygen service vs. breathing air).

How Other Standards Differ

• ISO 11623: This is a standard for the periodic inspection and testing of composite gas cylinders. 
• ISO 10462: This standard specifies requirements for the periodic inspection of acetylene cylinders. 
• ISO 10464: This standard provides requirements for the periodic inspection and testing of welded aluminium alloy cylinders. 

Key Takeaway

The core difference is that ISO 18119 is for periodic maintenance of specific types of gas cylinders (seamless steel and aluminium alloy), while other standards cover different types of cylinders or different inspection procedures. When referring to ISO 18119, it’s important to note the base standard (e.g., 2018) and any amendments, as these clarify or update the requirements. 

After the Test: Certification, Stamping & Records

Stamping: Cylinders that pass testing are permanently stamped with the test date, the testing station’s ID and other required marks (sometimes including a “+” or other notation depending on national practice).

Records: Maintain full records of inspection, test results and certificates. These records support traceability, regulatory compliance and insurance claims.

Rejection: Cylinders that fail are condemned — typically by stamping, marking or physically altering the neck or body to prevent further use. Condemned cylinders must be removed from service and disposed of safely.

Practical Notes for Dive Technicians & Operators

• Know your alloys: Check the cylinder material stamp (e.g., 6351, 6061, steel grades) and follow the specific testing path for that alloy.
• Follow manufacturer guidance: If the cylinder maker provides testing intervals or special inspection steps, follow those along with ISO/EN requirements.
• Use accredited facilities: Choose test houses authorised by national regulators and with the right equipment (hydrostatic rigs, eddy current probes, borescopes and certified thread gauges).
• Keep owner records: Encourage cylinder owners/operators to keep their inspection records in a safe, accessible place — digital backups are recommended.

Quick reminder: Standards and national requirements can evolve. Always confirm the latest regulatory intervals and testing requirements with your national authority and the cylinder manufacturer.

Final Thoughts

Adhering to ISO 18119 / EN 18119 is about more than compliance — it’s a critical safeguard for life and limb. Regular inspection, appropriate testing (including eddy current where needed), careful valve servicing and accurate records are the backbone of a safe cylinder programme.

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