Fixed Gas Detector

What sets fixed detection apart

Portable detectors answer one question at a time: is this space safe to enter right now. Fixed gas detection answers a different question continuously: has anything changed in a space that is normally occupied or normally sealed, such as an engine room, a cargo pump room, a battery locker or, on gas carriers, the cargo compressor room and hold spaces around independent tanks. The sensor heads are hard-wired to a central control panel that logs readings, triggers local and bridge alarms, and in many installations interlocks ventilation fans or shuts down electrical equipment automatically once a set threshold is crossed.

Main components

Sensor heads

Catalytic bead sensors detect flammable gas as a percentage of the lower explosive limit; infrared sensors do the same without consuming the target gas, which makes them tolerant of oxygen-deficient atmospheres; electrochemical cells target specific toxic gases such as H2S or CO; separate cells cover oxygen depletion or enrichment.

Control panel and wiring

The panel powers the sensors, processes the signal, drives audible and visual alarms at two thresholds (typically a pre-alarm and a high alarm), and often provides relay outputs to the fire and safety system. Cable runs into hazardous zones must use intrinsically safe barriers or explosion-proof glands.

Calibration points

Each sensor location has an accessible calibration gas port so the head can be bump-tested and calibrated without removal.

Selection and sizing

Sensor placement is driven by the gas properties and the space geometry: lighter-than-air gases are monitored near deckhead level, heavier vapours near the bilge or tank bottom. The number of points depends on space volume, ventilation pattern and the presence of dead pockets where gas could accumulate undetected. Response time (T90), cross-sensitivity to interfering gases, and the certified explosion-proof rating (commonly IIA, IIB or IIC depending on the gas group) all factor into the choice of sensor type for a given hazardous zone.

Regulations and class requirements

SOLAS Chapter II-2 requires fixed gas detection in specific spaces on tankers, including cargo pump rooms, and class rules extend similar requirements to enclosed spaces where flammable or toxic atmospheres are a known risk. Gas carriers under the IGC Code carry additional fixed detection for cargo hold spaces, interbarrier spaces and compressor rooms, monitoring for both flammable cargo vapour and, where relevant, toxicity or refrigerant leakage. Type approval and periodic testing intervals are set by class and flag administration; records of calibration and function tests are checked at survey.

Typical faults

• Sensor drift over time, giving readings that creep away from true concentration until a routine calibration catches it, or fails to.
• Expired or depleted calibration gas cylinders, leaving the crew unable to verify sensor response when it matters.
• Cable or connector faults in the harsh environment of a pump room or engine room, producing intermittent fault signals or a dead channel.
• Catalytic bead poisoning from silicone vapour or leaded compounds, which silently kills sensitivity while the sensor still appears to respond to bump tests with clean gas.
• Nuisance alarms from condensation or vibration, which crews sometimes respond to by disabling the point rather than fixing the fault, defeating the whole purpose of the installation.

What to look for in a supplier

• Explosion-proof certification appropriate to the zone (IECEx or ATEX, matched to the gas group present).
• Class type approval covering the specific vessel type and application, not a generic industrial certificate.
• Ready availability of matching replacement sensor cartridges, since panels are often kept in service far longer than any individual sensor.
• Calibration kits and bump-test gas suited to the exact sensor range fitted, not a close approximation.

A gas detector that has stopped nuisance-alarming is not necessarily fixed; check whether someone disabled the point before assuming the fault cleared itself.