A fixed gas detector is a permanently installed transmitter that continuously measures a target gas concentration at one sample point and outputs a signal to a controller, PLC or shutdown system [S2][S3].
Industrial catalogues segment fixed-point instruments by hazard class: combustible-gas detectors for LEL monitoring, toxic-gas detectors for ppm-level exposure, oxygen deficiency/enrichment detectors, and ultrasonic leak detectors for pressurised gas escapes [S3]. Sensor principle, output protocol and explosion protection method are the three engineering axes that drive model selection [S2][S3].
Catalytic Bead Sensors for Combustible Gases
Catalytic bead (pellistor) sensors use a catalytically active detector bead plus an inert reference bead on a Wheatstone bridge; combustible gas oxidises on the active bead, the resistance unbalance is proportional to gas concentration, and the double-detector compensation method is the cited reason for long-term stability [S6]. Dräger lists its catalytic-bead DrägerSensor as a dedicated flammable-gas-and-vapour detector in the fixed-portfolio, separating it from IR and flame alternatives [S6].
Catalytic bead is the lowest-cost LEL technology and works in a 0–100 % LEL window, but it requires oxygen to operate, can be poisoned by silicones, lead and sulphur compounds, and is typically specified in the lower-cost end of refinery, utility and boiler-house detection loops [S6].
Electrochemical Sensors for Toxic Gas and Oxygen
Electrochemical (amperometric) cells drive a current proportional to the partial pressure of the target gas diffusing through a membrane; the Dräger Polytron 5100 EC is positioned in the fixed line-up for toxic-gas and oxygen monitoring in standard applications using a plug-and-play electrochemical DrägerSensor, in an IP66 housing [S3]. The GRI-9106 toxic detector applies a Membrapor electrochemical cell from Switzerland, again pairing a low-power microprocessor with a dedicated toxic-gas cartridge.
Typical target species include H₂S, CO, NO, NO₂, SO₂, Cl₂, NH₃ and O₂; cells are 2- or 3-electrode, room-temperature devices, suited to personnel-exposure and battery-room applications where ppb-to-ppm resolution is needed [S3]. Cross-sensitivity, electrolyte drying and 1–3 year service life are the dominant failure modes that drive a fixed-cell sparing policy on a plant.
NDIR and PID Optical Sensors for Hydrocarbons and VOCs
Non-dispersive infrared (NDIR) sensors measure hydrocarbon gases by detecting the characteristic mid-IR absorption band of the C–H bond; the GRI-9105/9107 fixed detector with display is built around an NDIR or PID module plus a low-power microprocessor, outputting the concentration in real time over RS-485/4-20 mA style interfaces [S2][S4]. The Dräger Polytron 5700 IR uses the PIR 7000 infrared sensor inside an explosion-proof transmitter to detect most common hydrocarbon gases [S3].
NDIR is immune to catalytic-bead poisons, gives fail-safe response in oxygen-free backgrounds and is the default for methane, propane, butane and gasoline-vapour monitoring on offshore platforms, FPSOs and riser decks where exposure to silicone and sulphur is routine [S3][S6]. Photoionisation detector (PID) modules extend the same optical principle to volatile organic compounds down to low-ppm range, the same module family the GRI-9105/9107 offers for VOC monitoring in printing, painting and chemical-process rooms [S2][S4].
Explosion Protection, Output and Certification Architecture
Fixed detectors in hazardous process areas are typically ATEX/IECEx-certified; the K800-series explosion-proof fixed detector is offered with UL and ATEX approval, 2-, 3- or 4-wire connection, and 4-20 mA plus RS-485 Modbus outputs for continuous detection and monitoring. Wireless infrared options exist for retrofit sites where cable tray is impractical—the GasSecure GS01 is listed in the Dräger fixed line as a wireless IR transmitter with completely wireless signal transmission and power supply for flammable hydrocarbon gases and vapours [S3].
IP66 aluminium or stainless housings, Ex d flameproof or Ex e increased-safety enclosures, and dual 4-20 mA + Modbus RTU output are the current industry default; newer builds increasingly add HART 7 or Ethernet-APL on top of the same 4-20 mA channel to give field-device diagnostics without disturbing the analog loop.
Comparison of the Main Sensor Families
Across the four dominant principles, the trade-off matrix is consistent: catalytic bead gives the lowest unit cost and broad LEL coverage, but is poison-sensitive and oxygen-dependent; NDIR is poison-immune and oxygen-tolerant but costs 3–5× more and ignores hydrogen; electrochemical gives ppb-ppm resolution for a narrow gas list but has a 2–3 year cell life; PID covers VOCs below the LEL of the same chemical but requires a UV lamp and frequent cleaning [S2][S3][S6]. On a 200-loop refinery, plant engineers typically use catalytic bead or NDIR for hydrocarbon LEL, electrochemical for H₂S/CO at the fence line, and PID only in tank-farm and loading-rack VOC service.
Typical Industrial Applications and Plant Fit
Fixed detectors are specified for continuous area coverage where a portable cart cannot reach 24/7: offshore FPSOs and riser decks, petrochemical and refinery process units, metallurgical furnace rooms, mining motor rooms, fire pump houses, LNG regasification terminals, and battery storage rooms [S1][S2][S3]. Crowcon's fixed-gas portfolio is built specifically for the toxic-flammable-oxygen detection envelope in offshore platforms, while Dräger's line-up explicitly covers indoor and outdoor demanding environments from pharmaceutical cleanrooms to upstream wellheads.
For toxic gas detector selection, the governing choices are the target gas and its TLV, the sensor cross-sensitivity profile, the required response time T90, and the combustible gas detector zoning map drawn from the area classification drawing [S3].
Failure Modes, Calibration Cycle and Sourcing Constraints
Fixed-point detectors drift, poison, condensate-flood, and suffer desiccant exhaustion, which is why plant procedures enforce a 30–90 day bump-test cycle and an annual span calibration against certified gas [S6]. Specification writers should size detector populations to allow one transmitter in calibration while the redundant unit covers the loop, a configuration that aligns with a multi-channel multi gas detector controller such as the Dräger REGARD 7000 or REGARD 1000 [S3].
Procurement risk to watch: lead times for ATEX/IECEx-certified IR cartridges and the price gap between Chinese-origin K800-style units and European OEM detectors with PIR 7000-class sensors have widened since 2024 [S3]. Buyers are also cross-checking portable gas detector fleets against the same fixed-point gas library, so a single gas list simplifies cross-training and calibration gas inventory [S3]. Two trackable signals: the next revision of IEC 60079-29-3 governing fixed-gas-detector functional safety, and the spread of Ethernet-APL / HART-IP output on new ATEX Category 2 fixed transmitters [S3].
For related coverage, see Gas Alarm Controller TCO: How to Stop Underestimating the 10-Year Bill.