What is the difference between fixed and portable gas detection systems?
Fixed and portable gas detection systems serve different but complementary roles on board a ship. Fixed systems are permanently installed in specific locations to provide continuous, around-the-clock monitoring of hazardous gases in defined spaces. Portable detectors are handheld devices that crew members carry with them to measure atmospheric conditions before and during entry into enclosed or confined spaces. Most vessels need both types to meet safety and regulatory requirements.
Where are fixed and portable gas detectors each used on ships?
Fixed gas detectors are installed in spaces where a continuous gas hazard is present or possible, such as engine rooms, pump rooms, battery rooms, and cargo holds. Portable gas detectors are used wherever crew members need to verify atmospheric safety before entering a space, particularly enclosed or confined areas that are not continuously monitored.
On a typical vessel, fixed detectors are wired into a central alarm panel that alerts the bridge or control room the moment a threshold is exceeded. This makes them well suited for machinery spaces, fuel handling areas, and any location where a gas leak could develop unnoticed during normal operations.
Portable detectors fill the gaps. A pump room may have fixed sensors at a fixed height, but gas can stratify at different levels depending on its density. A crew member entering that space with a portable unit can sample the atmosphere at multiple heights and locations before committing to entry. Portable detectors are also the standard tool for pre-entry testing in cargo holds, void spaces, ballast tanks, and any other enclosed space that is not continuously manned.
How does a fixed gas detection system work?
A fixed gas detection system works by using permanently mounted sensors connected to a central control panel. Each sensor continuously samples the surrounding atmosphere and transmits a signal to the panel. When gas concentrations exceed a preset threshold, the panel triggers an alarm, which can activate ventilation, close valves, or alert the bridge depending on how the system is configured.
Most fixed systems use either catalytic bead sensors for flammable gases or electrochemical sensors for toxic gases such as carbon monoxide or hydrogen sulphide. Some systems use infrared technology, which is particularly stable over long periods and less prone to sensor poisoning. The choice of sensor technology depends on the type of gas being monitored and the environment in which the sensor is installed.
Fixed systems are designed for reliability over time. They require regular calibration and maintenance to ensure readings remain accurate, but they provide something a portable unit cannot: continuous, unattended monitoring. If a gas leak develops at 03:00 while the ship is underway, a fixed system will detect it and raise the alarm without any crew member needing to be present.
Our fire and gas detection systems cover a wide range of fixed configurations for maritime applications, including panels, sensors, and complete system assemblies from more than 100 brands.
How does a portable gas detector work?
A portable gas detector works by drawing a sample of the surrounding air through one or more internal sensors, which measure the concentration of specific gases and display the readings on a screen. Most handheld units produce audible and visual alarms when readings exceed safe limits, alerting the user to evacuate or stop work immediately.
Portable detectors are battery-powered and designed to be carried on the body or held in the hand. They typically use electrochemical sensors for toxic gases and catalytic bead or infrared sensors for flammable gases. Modern units can measure multiple gases simultaneously, displaying each reading in real time.
Before entering any enclosed space, a crew member should test the atmosphere from outside the entry point using the detector’s sample draw hose, if available, before physically entering. This allows a preliminary check without exposing the person to a potentially dangerous atmosphere. Once inside, the detector continues to monitor conditions and will alarm if the situation changes during the work period.
Calibration is a critical maintenance step for portable detectors. A detector that has not been calibrated recently may give a false sense of security by displaying readings that do not reflect actual gas concentrations. Regular bump testing and full calibration are part of responsible portable detector management, and our service and repair team supports this with in-house calibration and maintenance.
What gases can each type of system detect?
Both fixed and portable gas detection systems can detect a wide range of gases, but the specific gases monitored depend on the sensor configuration of each unit. Common gases detected on board ships include oxygen (O₂), flammable gases expressed as a percentage of the Lower Explosive Limit (LEL), carbon monoxide (CO), hydrogen sulphide (H₂S), and, increasingly, carbon dioxide (CO₂).
Fixed systems are often configured to monitor the specific gas hazard relevant to a particular space. An engine room system might focus on flammable gases from fuel leaks, while a battery room system would prioritise hydrogen detection. This targeted approach allows the fixed system to be optimised for the most likely hazard in each location.
Portable detectors have traditionally used a four-gas configuration measuring O₂, LEL, CO, and H₂S. However, updated IMO guidance now requires that portable detectors used for enclosed space entry be capable of measuring at least five gases, with CO₂ added as a mandatory parameter. This matters because CO₂ cannot be reliably detected by standard electrochemical sensors at the required resolution. Non-Dispersive Infrared (NDIR) technology is now the benchmark for CO₂ measurement in portable detectors, providing the parts-per-million resolution needed to confirm that CO₂ is below the 0.5% (5,000 ppm) limit required before entry.
The table below summarises the typical gas detection capabilities of each system type:
- Fixed systems: O₂, LEL (flammable gases), CO, H₂S, CO₂ (where configured), and space-specific gases such as refrigerants or ammonia
- Portable 4-gas detectors (legacy): O₂, LEL, CO, H₂S
- Portable 5-gas detectors (current standard): O₂, LEL, CO, H₂S, CO₂ (NDIR sensor required for CO₂)
Which regulations apply to fixed and portable gas detection on vessels?
Fixed and portable gas detection on vessels is governed primarily by SOLAS, which requires ships to carry portable gas detectors under Regulation XI-1/7, and mandates enclosed space entry and rescue drills under Regulation III/19. Classification societies add further requirements for fixed detection in specific spaces, and IMO resolutions set the technical standards for how detectors must perform.
The most significant recent development is IMO Resolution MSC.581(110), which entered into force on 3 December 2025. This resolution replaces the previous Resolution A.1050(27) and raises the bar for portable gas detection used in enclosed space entry. For the first time, CO₂ is a mandatory measurement parameter, meaning that traditional four-gas detectors no longer satisfy the recommendations for pre-entry atmospheric testing. Ships must now use detectors capable of measuring at least five gases, with CO₂ confirmed below 0.5% before any enclosed space entry is permitted.
For fixed systems, classification society rules vary by society and vessel type, but most require fixed gas detection in engine rooms, pump rooms, and cargo spaces where flammable or toxic gas hazards exist. Port State Control inspectors check both the presence of appropriate equipment and whether it is properly maintained and calibrated.
Fleet engineers and technical superintendents should review their current portable detector inventory against the MSC.581(110) requirements. If existing units are 4-gas only, the recommended compliance path is to supplement them with a dedicated standalone CO₂ detector rather than replacing the entire fleet, which avoids unnecessary capital expenditure while closing the regulatory gap.
When should a ship carry both fixed and portable gas detection systems?
A ship should carry both fixed and portable gas detection systems whenever crew members work in or around spaces where hazardous atmospheres can develop. In practice, this applies to virtually every commercial vessel, since fixed systems provide continuous monitoring of high-risk spaces while portable detectors are needed for pre-entry testing and work in spaces that are not permanently monitored.
Fixed systems cannot replace portable detectors because they only sample at fixed points. A fixed sensor mounted at deck level will not detect a heavy gas pooling at the bottom of a tank. Portable detectors allow crew to sample the atmosphere at multiple locations and heights, providing a complete picture of conditions before entry.
Equally, portable detectors cannot replace fixed systems because they depend on a crew member being present to use them. A gas leak that develops overnight in an unattended machinery space requires a fixed system to detect it and raise an alarm before the situation becomes dangerous.
The two systems work together: fixed detection provides the continuous safety net, and portable detection provides the targeted, person-specific protection needed for confined space entry and maintenance work. Ships that rely on only one type are leaving a gap in their safety coverage that regulations and good seamanship both require to be closed.
How Lavastica helps with gas detection systems
We supply and support both fixed and portable gas detection systems for maritime vessels, with more than 25 years of experience in the maritime and offshore sector. Whether you need a replacement sensor that is compatible with your existing fixed panel, a compliant 5-gas portable detector for enclosed space entry, or a standalone CO₂ unit to supplement your current fleet, we can help you find the right solution without replacing what is already working.
- Fixed system components: Sensors, panels, and spare parts from more than 100 brands, including reconditioned and obsolete equipment
- Portable detectors: 4-gas and 5-gas units, including NDIR CO₂-capable models that comply with MSC.581(110)
- Calibration and maintenance: In-house calibration and repair to keep your detectors accurate and audit-ready
- Compliance advice: Practical guidance on which equipment meets current IMO and classification society requirements for your vessel type
- Fast worldwide delivery: Stock available in Rotterdam for quick dispatch when a ship is in port and time is short
Want to know which gas detection solution fits your existing installation? Learn more about us or get in touch directly via our contact page. You can also reach our team by phone at +31 (0) 10 265 5070 or by email at [email protected].
