Gas Detection (Fixed)

What makes this type

Fixed gas detection differs from portable detectors in coverage and purpose: it monitors defined spaces continuously and automatically, without a crew member having to walk in with an instrument. It is installed where a gas hazard can build up unattended, engine rooms, pump rooms, cargo holds carrying certain cargoes, and enclosed cargo spaces on gas and chemical carriers, and it reports to a central control panel on the bridge or in the engine control room that can trigger alarms and, on some systems, automatic shutdowns or ventilation changes.

Main components

Sensor heads

Catalytic bead sensors for flammable gas, electrochemical cells for toxic gases such as H2S or CO, and infrared point or open-path detectors for hydrocarbons and CO2, chosen by the gas hazard specific to the space. Open-path (line-of-sight) detectors cover large areas like a cargo deck without needing dozens of point sensors.

Control panel and cabling

A central panel aggregates readings, sets alarm and pre-alarm thresholds, and logs events. Cabling runs to hazardous-area sensor junction boxes must be intrinsically safe or explosion-proof rated, matching the zone classification of the space.

Calibration and test gas system

Fixed sensors require periodic exposure to known-concentration test gas to confirm response, either manually with a test gas bottle and adapter or via a built-in calibration gas manifold on larger installations.

Selection and sizing

• Gas hazard specific to the space: flammable hydrocarbon vapour, H2S, CO2 from fixed fire-fighting systems, or oxygen depletion
• Sensor placement based on gas density (heavier-than-air gases need low-level sensors, lighter gases need high-level placement) and airflow patterns
• Response time and alarm threshold set against the actual lower explosive limit or toxic exposure limit for the gas in question
• Zone classification of the installation area, determining whether intrinsically safe or explosion-proof housings are required

Regulations and class

SOLAS Chapter II-2 requires fixed gas detection in specified spaces on tankers and gas carriers, and the IGC Code sets detailed requirements for gas carriers covering cargo hold and interbarrier space monitoring. Class rules require periodic function testing and calibration, typically annually, with records kept for survey. On tankers, fixed hydrocarbon gas detection in pump rooms is a specific SOLAS requirement with defined alarm and automatic ventilation trip functions.

Typical faults

• Sensor drift over time, giving falsely low readings that pass a casual glance but fail a proper calibration check
• Catalytic bead sensor poisoning from silicone vapours or leaded compounds, permanently reducing sensitivity without an obvious symptom
• Water ingress into junction boxes in exposed deck locations, causing intermittent faults or false alarms
• Sensors left isolated or in maintenance bypass after hot work and never reactivated, defeating the entire system

What to look for in a supplier

• Sensor technology matched to the actual gas hazard, not a generic multi-gas unit applied everywhere
• Hazardous area certification appropriate to the zone the sensor will be mounted in
• Integration with the ship's existing alarm and ventilation shutdown logic rather than a standalone panel needing separate monitoring
• Calibration gas and spare sensor availability through a supply chain the vessel's trading pattern can actually reach

Log every sensor bypass during hot work or maintenance in the deck log with a return-to-service check; a bypassed sensor left off the watch schedule is the single most common finding in fixed gas detection audits.