What to check on a Fixed Gas Detection System.
A fixed gas detection system monitors defined spaces continuously and automatically, without anyone carrying an instrument into the space first. Sensor heads are hard-wired (or increasingly wireless) to a central control panel that raises an alarm the moment a gas concentration crosses a set threshold, whereas a portable detector only reports what it measures at the moment someone switches it on and carries it in. The two are complementary rather than interchangeable: fixed detection covers a space around the clock and can trigger interlocks such as shutting a ventilation fan…
What sets a fixed system apart from portable gas detection
A fixed gas detection system monitors defined spaces continuously and automatically, without anyone carrying an instrument into the space first. Sensor heads are hard-wired (or increasingly wireless) to a central control panel that raises an alarm the moment a gas concentration crosses a set threshold, whereas a portable detector only reports what it measures at the moment someone switches it on and carries it in. The two are complementary rather than interchangeable: fixed detection covers a space around the clock and can trigger interlocks such as shutting a ventilation fan or closing a fuel valve, while portable detection is what confirms a space is actually safe before entry, sensor by sensor, at head height and floor level.
Main components
Sensor heads
Catalytic bead sensors for flammable gas, electrochemical cells for toxic gas such as H2S or CO, and infrared point or open-path sensors increasingly used for flammable gas where catalytic sensors would be poisoned by silicones or lead compounds.
Control panel
Receives signals from every sensor loop, displays gas concentration per point, and drives the visual and audible alarm outputs at pre-alarm and high-alarm thresholds.
Cabling and junction boxes
Intrinsically safe wiring runs through hazardous areas, terminated in explosion-protected junction boxes, since the wiring itself must not become an ignition source.
Interlock outputs
Relay contacts wired to ventilation fans, fuel valves, or electrical equipment shutdown, so a confirmed high-gas alarm can automatically remove an ignition source or stop the leak's source.
Selection and sizing
- Gas types actually present in the space — flammable hydrocarbon vapour in a pump room is a different sensor technology from H2S in a sewage tank or CO2 in a fixed firefighting system space.
- Number and location of sensor points, following the gas dispersion pattern of the specific space (heavier-than-air gas collects low, lighter-than-air gas collects at deckhead) rather than an arbitrary grid.
- Response time and sensor drift characteristics, since catalytic sensors typically need more frequent calibration than electrochemical or infrared types.
- Certification for the hazardous area zone the sensor sits in (Zone 0, 1, or 2), matched against the ship's hazardous area plan.
Regulations and class
SOLAS Chapter II-2 requires fixed gas detection in specific spaces on certain ship types, most notably in cargo areas of gas carriers and tankers and in enclosed spaces where flammable or toxic gas can accumulate. The IGC Code and IGF Code set out detailed detection requirements for gas carriers and gas-fuelled ships respectively, including sensor placement, alarm set points, and required interlocks with ventilation and fuel systems. Class surveys check calibration records, functional testing of the alarm and interlock chain, and physical condition of sensor heads at each periodic survey.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Sensor reads zero permanently | Catalytic bead poisoning from silicone vapour, common near paint or sealant use | Space appears safe on the panel while actually being unmonitored — the most dangerous failure mode of this equipment |
| Frequent nuisance alarms | Sensor drift beyond calibration tolerance, or condensation on an electrochemical cell | Crew begins to distrust and eventually ignore alarms, defeating the purpose of the system |
| Loss of signal from one zone | Cable damage or a failed junction box seal allowing moisture ingress | Loop fault alarm, and that zone effectively has no fixed detection until repaired |
| Interlock fails to activate on alarm | Relay contact fault or a bypass switch left in the test position after maintenance | Ventilation does not stop or fuel valve does not close automatically, leaving manual response as the only safeguard |
What to look for in a supplier
- Sensor technology matched to the actual gas hazard, not a generic flammable-gas-only package sold for every space.
- Hazardous area certification documentation for every sensor and junction box in the loop.
- Calibration gas availability and interval, since catalytic sensors in particular need routine bump testing and calibration to stay trustworthy.
- Panel compatibility with the ship's existing alarm and monitoring system, to avoid a standalone panel nobody on watch actually looks at.
A gas detector reading zero is not proof of a safe space until someone has confirmed the sensor itself is still alive — bump-test fixed sensors on the calibration schedule, not only when something already seems wrong.