What Is Gas Measurement and Why Is It Carried Out?
Gas measurement is the process of determining, with purpose-built and calibrated instruments, the concentration of flammable, explosive, toxic or asphyxiating gases present in enclosed or hazardous spaces aboard ships, in shipyards and on watercraft.
In Türkiye, this field is comprehensively regulated by the Regulation on Gas-Freeing of Ships and Watercraft, published in the Official Gazette No. 31737 dated 1 February 2022. The regulation covers all Turkish- and foreign-flagged vessels undergoing construction, conversion, maintenance, repair or dismantling operations within Turkish maritime jurisdiction or inland waters.
So why is this so critical? A vessel’s enclosed spaces — such as the bilge, cargo tanks or engine room — can accumulate hazardous gases such as residual fuel gases, hydrogen sulphide (H₂S), carbon monoxide (CO) and nitrogen that has displaced oxygen. Every moment spent entering these spaces without measurement carries a potential risk of death.
Why Is Enclosed-Space Gas Measurement Vitally Important?
A large proportion of the occupational accidents in the maritime sector originate in enclosed spaces. Enclosed compartments aboard a ship combine three fundamental hazards:
1. Explosive Atmosphere: When flammable gases and vapours reach a certain concentration, a single spark can trigger a devastating explosion.
2. Toxic/Asphyxiating Atmosphere: Gases such as H₂S give a warning at 10 ppm; however, at high concentrations they can instantly paralyse the sense of smell and cause death. Carbon monoxide, by contrast, is colourless and odourless, and accumulates undetected.
3. Oxygen Deficiency: In a normal atmosphere the oxygen level is approximately 20.9%. When this level falls below 16%, impaired consciousness sets in; below 6%, death is inevitable.
Under Turkish legislation, enclosed or hazardous compartments are classified as follows:
- Containing explosive gas at 10% or more of the LEL value
- An oxygen concentration below 16% or above 22%
- Toxic gas concentrations exceeding the defined threshold values
If any of these conditions is met, the space is deemed a “hazardous compartment” and no work permit is issued.
What Is the LEL? What You Need to Know to Prevent Explosion Risk
The LEL (Lower Explosive Limit) is the minimum concentration of a gas in air required for it to become explosive.
Gas detectors express this value as %LEL. If the instrument reads 0% LEL, there is no flammable gas in the atmosphere; 100% LEL means the full explosion threshold has been reached.
| LEL Value | Status | Permitted Action |
|---|---|---|
| 0% LEL | Clean atmosphere | All operations, including hot work |
| 0%–10% LEL | Caution zone | Ventilation must be increased |
| 10%–25% LEL | Hazardous | Evacuate personnel; no operations permitted |
| >25% LEL | Imminent danger | Immediate evacuation |
Alongside the LEL, the UEL (Upper Explosive Limit) is also important. If the gas concentration is above the UEL, there is insufficient oxygen for combustion; however, this does not mean the space is “safe” — sudden ventilation can bring the concentration back down into the explosive range.
For example:
- The LEL value of hydrocarbon gases is generally between 1% and 5%.
- The LEL value of natural gas (methane) is approximately 4.5%.
The Difference Between Safe for Entry and Safe for Hot Work
These two concepts are cornerstones of maritime safety and are frequently confused with one another.
Safe for Entry
This is the status certifying that a space is safe for personnel to enter. For this certificate, the atmosphere must meet the following conditions:
- Oxygen concentration: between 19.5% and 22%
- LEL value: 0% (preferably) or a maximum of 10%
- Toxic gases (H₂S, CO, etc.): below the defined threshold values
Safe for Entry status does not mean that work can be carried out inside; it only indicates that entering the space poses no fatal risk to a person.
Safe for Hot Work
For operations that create a spark or source of ignition — such as welding, grinding and cutting — far stricter criteria are required:
- LEL value: must be 0% (no flammable gas residue whatsoever is accepted)
- Oxygen levels must be within the normal range
- Continuous gas monitoring must be maintained throughout the work
- A written Hot Work Permit must be obtained from the Harbour Master
In other words: every “Safe for Hot Work” space is also “Safe for Entry,” but the reverse does not hold.
Why Is a Gas Free Certificate Required for Ships?
The Gas Free Certificate, issued by a gas measurement expert under Turkish legislation, is the official document confirming that spaces are safe for defined periods of time.
This document is mandatory in the following cases:
- Shipyard entry: Before vessels that have carried flammable/explosive materials are admitted to a shipyard for maintenance or repair
- Dry-docking: Measurement and documentation of all hazardous tanks before the vessel is dry-docked
- Ship dismantling: Even where a Gas Free certificate has been obtained for tankers in particular, additional half-daily or every-other-day measurements are mandatory under the ship recycling regulation
- Insurance and P&I claims: Enabling surveyors and adjusters to work safely on damaged vessels
Following the measurements carried out by the relevant gas measurement expert, the Gas Free certificate must be uploaded to the web-based GAS System operated by the General Directorate of Shipyards and Coastal Structures of the Turkish Ministry of Transport and Infrastructure (UAB). Documents must be retained for at least five years.
How Is Gas Measurement Carried Out Before Hot Work?
Gas measurement before hot work requires a more rigorous protocol than a standard enclosed-space measurement. The process follows these steps:
1. Preparation and ventilation Before measurement, the space is cleaned by means of mechanical ventilation. Clean air, not pure oxygen, must be used for ventilation; an oxygen-enriched atmosphere dramatically increases the risk of explosion.
2. Multi-point, multi-level measurement Depending on their density, gases accumulate either at the bottom (heavy gases such as LPG and H₂S) or at the top (light gases such as methane) of a space. Measurements must therefore be taken separately at the upper, middle and lower levels.
3. Verification of instrument calibration Before measurement, it must be confirmed that the instrument’s calibration is up to date. Turkish legislation requires that all measurement instruments be calibrated by accredited bodies.
4. Continuous monitoring Throughout the hot work, measurements are repeated periodically. Measurement continues even during pauses in the work (breaks, equipment changes).
5. Emergency response plan When a sudden rise in the LEL value is detected, the work is stopped immediately, personnel are evacuated, and the Harbour Master is notified.
The Importance of Gas Measurement in Ship Recycling
Ship dismantling processes are among the highest-risk maritime operations in terms of gas measurement. There are several fundamental reasons for this:
Persistent hydrocarbon residues: On a tanker that has carried petroleum products for decades, hydrocarbon vapours that have penetrated the structural steel may remain even after the tanks have been emptied and cleaned. This explains cases of fire and explosion triggered by cutting torches even in spaces for which a Gas Free certificate has been issued.
EU SRR and Hong Kong Convention requirements: For recycling to take place at facilities covered by the EU Ship Recycling Regulation (EU SRR) and the Hong Kong Convention, comprehensive documentation is mandatory, including the Gas Free certificate, the IHM (Inventory of Hazardous Materials) and a radiation analysis certificate. In addition to the Gas Free certificate, periodic half-daily or every-other-day measurements must be carried out under the ship recycling regulation and submitted for inspection by the Harbour Master.
With specific reference to the Aliağa Shipyards Zone: Aliağa in İzmir is Türkiye’s largest ship recycling centre. The gas measurement processes of the facilities operating in this zone are subject to both Turkish legislation and international standards (the Hong Kong Convention), and non-conformities can be subject to serious sanctions.
How to Read a Gas Measurement Report
The report issued by the gas measurement expert (the Gas-Freeing Certificate) contains both technical data and operational decisions. To read a report correctly, attention should be paid to the following sections:
Spaces measured: The measurement points listed separately for each compartment. Even for a single space, measurements should have been taken at more than one level.
Parameters measured and their values:
- %LEL: Percentage of flammable gas (0 is safest)
- %O₂: Oxygen concentration (ideal: 20.9% ± 1)
- H₂S (ppm): Hydrogen sulphide concentration (working limit: 10 ppm)
- CO (ppm): Carbon monoxide concentration (working limit: 50 ppm)
Certificate type: The conclusion section of the report states the status under which the space is accepted:
- “Safe for Entry”
- “Safe for Cold Work”
- “Safe for Hot Work”
- “Unsafe”
Validity period: The measurement certificate is valid for a defined period. This period varies according to the nature of the work and the assessment of the measurement expert; when it expires or when environmental conditions change, measurement must be repeated.
Expert details and signature: The person issuing the report must be a Gas Measurement Expert authorised by the Ministry of Transport and Infrastructure. These experts hold a bachelor’s-level degree from an engineering or science faculty and have at least 2 years of shipyard or ship experience.
What to Consider When Procuring a Gas Measurement Service
Incorrect or inadequate gas measurement can lead to explosion, fire and death, as well as to legal and administrative sanctions. To choose the right service provider, bear the following criteria in mind:
Legal authorisation: To perform gas measurement in Türkiye, the service provider must employ a Gas Measurement Expert authorised by the Turkish Ministry of Transport and Infrastructure (UAB). The list of these experts can be queried through the GAS System of the General Directorate of Shipyards and Coastal Structures.
Equipment calibration: Confirm that the instruments used are regularly calibrated by accredited laboratories and that calibration records are kept. A measurement made with an instrument lacking a calibration certificate is legally invalid.
Multi-parameter measurement capability: A quality gas measurement firm uses multi-gas detectors capable of measuring LEL, O₂, H₂S and CO simultaneously. Firms measuring only a single parameter may produce an inadequate safety assessment.
Upload to the GAS System: Documents must be uploaded to UAB’s GAS System immediately after measurement. Firms that fail to meet this requirement carry a risk of legal non-compliance.
Sector experience: The shipyard environment, the ship recycling zone and offshore operations each carry their own specific risks. Ask about the service provider’s references and experience in these areas.
Response time and continuity: Gas measurement is tied to time-critical processes such as dry-docking, the start of maintenance and repair, or emergency repairs. A service provider that offers 24/7 availability and will not bring your operations to a standstill should be selected.
What Risks Do Businesses Face If Gas Measurement Is Not Carried Out?
Gas measurement is not only a legal obligation but also a safety measure that, when neglected, gives rise to serious consequences.
Loss of life and injuries Deaths and serious injuries caused by explosion, poisoning or oxygen deficiency in enclosed spaces entered without gas measurement are reported worldwide every year.
Criminal and legal liability Under Turkish legislation, operations begun without gas measurement — or with inadequate measurement — may be subject to heavy administrative fines and workplace closure sanctions under occupational safety legislation. In the event of an occupational accident, criminal proceedings may be brought against the responsible managers.
Operational disruption and cost of delays A fire or explosion that is not prevented can force a shipyard to halt operations for a long period, render the vessel scrap and give rise to serious insurance disputes. Compared with the cost of the gas measurement service, these costs can reach astronomical proportions.
Loss of certification and accreditation During international inspections (PSC — Port State Control), deficiencies in gas measurement documentation can lead to the vessel’s detention and serious reputational damage. For shipyards, it also carries the risk of removal from the EU SRR list.
Falling outside insurance cover P&I clubs and marine insurers may reject compensation claims where they determine that mandatory safety procedures have not been followed.
Looking for the Right Partner for Gas Measurement Services
At Orionis Shipping, our team of experts — authorised by the Turkish Ministry of Transport and Infrastructure (UAB) and working in line with IMO and SOLAS standards — provides gas measurement, gas-freeing and IHM services in Türkiye’s leading shipyard regions. From measurement to documentation, and from uploading to the GAS System to periodic follow-up, we manage the entire process on your behalf.
Contact us to learn more about our services and to request a quote.
This content has been prepared for informational purposes. For your legal obligations, rely on the current legislation of the Turkish Ministry of Transport and Infrastructure and on the advice of accredited experts.
Tags: gas measurement, gas free certificate, LEL, safe for entry, safe for hot work, enclosed space safety, ship safety, hot work permit, ship recycling, shipyard safety, IMO, SOLAS, Aliağa, gas-freeing, H2S, gas measurement expert
