Why Is Permanent Volumetric Expansion (PE) Limited to 5% in Europe and Britain?
ASSET Technical Guidance Note
Published by ASSET – Association of Scuba Service Engineers and Technicians
A frequent technical question raised during gas cylinder testing is:
“Permanent volumetric expansion must not exceed 10% of total expansion — so why do Europe and Britain often apply a 5% limit?”
This is a very good question, and the confusion is common — even among experienced test stations.
Short Answer
The reason you hear both figures is simple:
- 10% is the generic ISO maximum allowable limit, and
- 5% is a stricter national or sector-specific limit applied in UK practice and much of Europe, particularly for CO₂ and fire-protection cylinders.
Both values are technically correct — they originate from different regulatory layers.
1. Where the 10% Rule Comes From
The 10% permanent volumetric expansion limit originates from ISO-based gas cylinder standards, including:
- EN 1968 (historical)
- EN ISO 18119 (current)
The ISO acceptance principle is:
Permanent volumetric expansion ≤ 10% of total expansion
This represents the absolute technical maximum permitted under the harmonised ISO framework.
If a cylinder exceeds 10% permanent expansion:
- ❌ Automatic failure
- ❌ Mandatory condemnation
2. Why 5% Is Applied in the UK and Much of Europe
This is the critical point.
EN ISO 18119 explicitly allows national authorities and sector-specific schemes to apply more conservative acceptance criteria.
This flexibility exists to account for:
- Local regulatory expectations
- Risk profiles of specific cylinder applications
- Public safety considerations
3. UK and European Practice Explained
🇬🇧 United Kingdom
In the UK, accepted industry practice — reflected in expectations from:
- HSE guidance
- Inspection and certification bodies
- Insurers
- Fire-safety servicing schemes
commonly applies:
Permanent volumetric expansion ≤ 5% of total expansion
The reasons include:
- Older cylinder populations in service
- A conservative national safety culture
- Alignment with CO₂ and fire extinguisher servicing standards
- Insurer and enforcement expectations
As a result, many UK test stations treat:
>5% = FAIL
even though ISO would technically allow values up to 10%.
🇪🇺 Europe (Especially CO₂ Cylinders)
Across much of Europe:
- CO₂ cylinders used in fire protection, beverage, and industrial service
- Are tested under national pressure equipment regulations
These frequently adopt 5% as a practical condemnation threshold, particularly for:
- High-cycle cylinders
- Cylinders susceptible to work hardening
- Older seamless steel CO₂ cylinders
4. Fire Extinguisher Cylinders — A Key Driver for the 5% Rule
Many technicians first encounter the 5% rule when working with CO₂ fire extinguishers.
Fire-safety servicing standards are:
- More conservative than diving or industrial gas standards
- Designed for worst-case public-use scenarios
- Closely aligned with insurer risk assessments
Consequently, a cylinder that technically passes ISO at 7–8% permanent expansion may still be:
- Rejected by fire-safety schemes
- Unacceptable to insurers
- Expected to be condemned during audits
5. How EN ISO 18119 Addresses This
EN ISO 18119 does not state:
“You must always accept cylinders up to 10%.”
Instead, it establishes that:
- 10% is the upper technical limit (maximum).
- National regulations and sector rules may impose stricter limits
In practical terms:
- 10% = ISO ceiling
- 5% = UK / EU applied operational limit
6. ASSET Best Practice for Test Stations
ASSET recommends that test stations clearly document both:
- The inspection standard applied (EN ISO 18119)
- The national or sector-specific acceptance criteria used
For UK and EU CO₂ cylinders, ASSET-aligned best practice is:
- Apply ≤ 5% permanent expansion
- Never pass a cylinder exceeding 10%
- Document acceptance criteria explicitly
Example wording:
“Hydrostatic test carried out in accordance with EN ISO 18119. Acceptance criteria applied in line with national practice: permanent volumetric expansion ≤5% of total expansion.”
This approach is:
- ✔ Technically correct
- ✔ Legally defensible
- ✔ Aligned with ASSET training principles
- ✔ Insurer and auditor safe
7. Bottom Line
- 10% is the ISO maximum allowable limit
- 5% is the applied UK / EU condemnation threshold
- Both figures are correct and serve different purposes and countries
For CO₂ cylinders in Europe and Britain, 5% is the correct figure to apply in routine inspection practice.
