Recent amendments to IMO guidance place greater emphasis on carbon dioxide (CO₂) monitoring during atmospheric testing for enclosed and confined spaces. This reflects the growing recognition that hazardous atmospheres are not limited to oxygen deficiency or toxic gases alone; CO₂ can accumulate to dangerous levels without obvious warning signs and must therefore be monitored appropriately.
For most operators, this does not mean existing 4-gas monitors need to be replaced. Instead, it highlights the importance of selecting the right combination of sensor technologies and understanding their capabilities and limitations in different operating environments.
Meeting the New CO₂ Monitoring Requirement
A practical and cost-effective way of meeting the updated requirement is to supplement an existing 4-gas monitor with a dedicated personal CO₂ monitor.
Suitable instruments include:
These compact, lightweight devices provide continuous CO₂ monitoring while personnel are working and can be used alongside an existing multi-gas detector without replacing current equipment.
Some models offer Bluetooth connectivity, allowing gas readings and alarm status to be viewed on compatible mobile devices or remote monitoring platforms. This can improve supervisory oversight, support electronic record keeping, and enhance situational awareness during confined-space operations.
Most personal CO₂ monitors are diffusion-based rather than pumped instruments, so appropriate procedures should be adopted when using them for pre-entry atmospheric assessment.
Understanding Sensor Performance in Inert Atmospheres
The performance of gas sensors in oxygen-deficient or inert atmospheres is often misunderstood. Different sensor technologies behave very differently under these conditions, making it essential to understand their limitations when interpreting gas readings.
Catalytic (Pellistor) LEL Sensors
Catalytic bead (pellistor) sensors measure combustible gases by oxidising the gas on a heated catalyst bead. Because this reaction requires oxygen, reliable operation typically requires oxygen concentrations of around 10–12% or higher.
In inert or oxygen-deficient atmospheres, pellistor sensors may:
- Under-read combustible gas concentrations
- Fail to respond completely
- Indicate a falsely safe atmosphere despite the presence of flammable gas
For this reason, catalytic LEL sensors should not be relied upon for combustible gas detection in inerted spaces.
Electrochemical Toxic Gas Sensors
Electrochemical sensors, commonly used for carbon monoxide (CO) and hydrogen sulphide (H₂S) detection, operate using a different principle than LEL pellistor sensors. Gas diffuses into the sensor where an electrochemical reaction produces an electrical current proportional to the gas concentration.
A minimum concentration of oxygen is required for the correct operation of all electrochemical cells, making them unsuitable for certain process monitoring applications. Although the electrolyte contains a certain amount of dissolved oxygen, enabling short-term detection (minutes) of the target gas in an oxygen-free environment.
Low oxygen concentrations can influence their performance, potentially resulting in:
- Slower response times
- Reduced sensitivity
- Measurement drift
- Reduced overall accuracy
Readings obtained under these conditions should therefore be interpreted with caution.
Interpreting Gas Readings Safely
No gas reading should ever be considered in isolation. Atmospheric assessment should always consider the relationship between:
- Oxygen concentration
- Combustible gas concentration
- Carbon dioxide concentration
- Toxic gas concentrations such as CO and H₂S
Where oxygen concentrations fall below approximately 10%, electrochemical sensor readings should be regarded as indicative rather than absolute, and confined space entry should not proceed until the atmosphere has been made safe in accordance with IMO guidance and company procedures.
Choosing the Correct Sensor Combination
For operations involving inert or oxygen-deficient spaces, it is recommended that the instrument is initially operated in a standard oxygen environment to ensure all the gas sensors are reading correctly (bump test the instrument to confirm), and then readings can be taken from the confined space. The most common sensor configuration is:
- Infrared (IR) LEL sensor for combustible gases*
- Infrared CO₂ sensor
- Electrochemical oxygen sensor
- Electrochemical toxic gas sensors for CO, H₂S and other toxic gases as required
Unlike catalytic sensors, infrared combustible gas sensors do not rely on oxygen and therefore provide reliable %LEL measurement in inert atmospheres.
* IR sensors can’t measure Hydrogen or Acetylene.
A Practical Path to Compliance
For many operators, compliance with the updated IMO requirements can be achieved without replacing existing gas detection equipment.
Adding a dedicated CO₂ monitor alongside an existing 4-gas detector provides a practical and cost-effective solution while improving personnel protection. Equally important is understanding the limitations of catalytic sensors and the behaviour of electrochemical sensors in oxygen-deficient atmospheres, ensuring gas readings are interpreted correctly and hazards are not underestimated.
Ultimately, effective confined space monitoring is not about having more sensors; it is about using the right sensor technology for the environment being assessed and understanding the limitations of each measurement before making decisions on safe entry.
Need Advice on the Right Solution?
Every vessel and operating environment presents different gas detection challenges. Selecting the appropriate combination of sensors and instruments is essential for achieving regulatory compliance and protecting personnel.
If you would like guidance on the most suitable solution for your vessel type or operational requirements, contact Shawcity. Our gas detection specialists can recommend equipment that meets the latest IMO expectations while making the best use of your existing monitoring systems.
01367 899419
solutions@shawcity.co.uk
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01367 899553
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