What a fixed gas detector is
A fixed gas detector is installed at a point where a gas would accumulate and then left running, wired into a control system rather than carried by a person. Its purpose is to protect the plant: it watches the same place continuously, without anyone present, and it is expected to still be working months later.
The measurement technologies it uses are the same families as in a portable instrument, and that is where the similarity ends. Because a fixed detector sits in the atmosphere it is watching, continuously, rather than briefly during an entry, everything about where it is placed and how it survives becomes part of the specification. A sensor exposed to a plant for years has a different service life from one carried in a pocket, and the maintenance route decides whether the calibration that keeps it honest actually happens.
What fixes the rest of the specification is the gas itself, the classification of the area it sits in, and what the installation has to do when a limit is exceeded. This page covers those decisions, and the boundary between a point detector protecting a plant and the portable instruments and wider systems that are often confused with it.
How a fixed gas detector measures
Gas density relative to air decides mounting height before any instrument is chosen. A gas lighter than air rises and collects at the highest point of the space, so a detector for it belongs high up. A gas heavier than air settles, which means it will be found in pits, trenches, sumps and at floor level first. A detector placed at breathing height for a heavy gas reads air and reports it as safe, and no accuracy figure rescues a point that is in the wrong place. Where the gas is released from a specific fitting, the release point competes with the accumulation point as the place to watch.
The detector is itself inside the classified area, so its own approval forms part of the specification rather than being an accessory to it. The type of protection describes how the enclosure prevents the instrument from becoming an ignition source, the gas group describes the gases that protection is valid for, and the temperature class describes the highest surface temperature the equipment is allowed to reach, which follows from the ignition temperature of the gas present. A detector that does not carry the corresponding approval cannot be installed there, whatever it measures.
A fixed installation is judged over time rather than at a moment. A catalytic sensor needs oxygen to work and is permanently poisoned by silicones, leaded compounds and some solvents; an electrochemical cell is consumed by the gas it measures and has a finite life whether or not it is used. In a fixed point both of those become scheduled events rather than surprises, and the accessibility of the detector decides whether the schedule is kept. The alarm output matters in the same way: a reading that only appears on a display is noticed when someone walks past it, while a contact that starts a fan or closes a valve acts while the event still matters.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Gas and the sensor technology for it | Catalytic, infrared or electrochemical, chosen per gas | Whether the sensor responds to the hazard at all, and whether it survives being exposed to that atmosphere continuously. |
| Siting height and location | High level for a light gas, low level for a heavy one, or adjacent to the release point | Whether the detector sees the gas. A point at the wrong height measures clean air and reports it as a normal reading. |
| Mounting and sampling arrangement | Wall, pipe, duct or flow-through mounting, with or without a sample line | Where the detector can physically be fitted, and whether the sample has to be carried to it from a point that cannot be reached. |
| Area classification and approval | The type of protection, the gas group and the temperature class | Whether the instrument may be installed in the classified area, and the surface temperature it is permitted to reach. |
| Enclosure and weather protection | The ingress protection, and whether the housing is intended for indoor or outdoor mounting | Whether the installation survives rain, washdown, dust and sun, which for an outdoor point decides how long it lasts. |
| Number of points and the coverage they give | How many detectors the space needs, and where each one goes | Whether the installation covers the credible release points or only the one that was considered first. |
| Controller, outputs and interlock | Controller channel capacity, relay contacts, and what those contacts operate | Whether a breach only raises an alarm for someone to act on, or also starts ventilation, shuts a valve or trips the process. |
| Communication and power arrangement | Point to point wiring, or a bus back to a panel, with or without standby power | How many points one cable route can serve, and whether monitoring continues through a mains failure. |
| Calibration and maintenance access | How the detector is reached, and how the sensor cell is replaced | Whether the instrument can be serviced without a shutdown or scaffolding, which is what decides whether the calibration regime survives contact with the plant. |
How to choose a fixed gas detector
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Where the gas would collect in the space | Height and location decide whether the detector ever sees the gas, whatever its specification | Walk the space and say where the gas would go: ceiling, floor, pit, or a specific fitting. |
| The gases present, and what else is in the atmosphere | The technology is chosen per gas, and substances that poison or disable a sensor limit where one can be used | List the gases, and separately list anything else that may be present which could affect a sensor. |
| The classification of the area | The instrument itself must carry the approval valid for that area, group and temperature class | State the zone or division, the gas group and the temperature class the installation has to meet. |
| Indoor or outdoor, and what gets on the housing | Rain, washdown, dust and sun decide the enclosure and the mounting hardware | Describe the location and anything the housing will be exposed to during normal operation and cleaning. |
| What has to happen on a breach | A relay that interlocks ventilation or a valve changes the controller and the wiring needed | State the action required, and whether it is automatic or left to an operator. |
| How many points, and how far apart | Channel capacity, cable route and controller position all follow from the point list | Draw the points on a plan before enquiring, rather than sizing the controller after the fact. |
| Power availability and continuity | Standby power and the wiring arrangement follow from whether monitoring must survive a supply failure | State what happens to the process during a power failure, and whether the detection has to keep running. |
| How the record is kept | Some regimes require readings and alarm events to be retrievable after the fact | State what has to be recorded and for how long, and who has to be able to produce it. |
Where any of these are still open, send the plan and the gas list. A covering quotation can be issued from that and refined as the siting is agreed.
The nearest alternatives
These are the types an enquiry for a fixed gas detector usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a fixed gas detector |
| Portable gas detectors | Carried or worn by a person to warn them of the atmosphere where they are standing. It protects the individual and goes where they go, but it is present only while somebody is, and it cannot be left to watch a plant between shifts. |
| Gas detection systems | A plant-wide arrangement of many points reporting to a central panel with its own zoning, records and interfaces. A handful of detectors on a controller is a fixed installation; it becomes a system when the point count, the annunciation and the interface requirements are designed as one. |
| Gas leak detectors | Used to find the source of a leak by moving the instrument along a run and reading the gradient towards it. That is a survey, not continuous protection, and it answers where the leak is rather than whether the area is safe now. |
| Combustible gas detectors | Concerned only with flammable gas and vapour, reported as a fraction of the lower explosive limit. Where that is the sole credible hazard, the specification turns on the gas group and the temperature class of the area rather than on covering several toxic channels as well. |
| Toxic gas detectors | Dedicated to one toxic gas at the low concentrations that matter for exposure, where the reading is in parts per million rather than a percentage of an explosive limit. The sensor and its calibration gas are specific to that gas. |
| Oxygen gas detectors | Watch for deficiency or enrichment where oxygen is the hazard, such as an inerted vessel or an area served by a cryogenic line. Where oxygen is only one of several hazards, it becomes one channel of a multi-gas point rather than an instrument in its own right. |
The mistake that costs the most is a detector mounted where it is convenient to install rather than where the gas goes. It reports a normal reading, it is calibrated on schedule, and it is in the wrong place - which is harder to notice than a detector that is plainly not working.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The gases to be measured, and the sensor technology preferred for each if one has been decided
- A plan or description showing where each detector is to be mounted and at what height
- The classification of each area: zone or division, gas group and temperature class
- What else may be present that could affect a sensor
- Whether the housing is indoors or outdoors, and what it will be exposed to
- The number of points, and whether they report point to point or on a bus
- What each relay output has to operate, and whether the action is automatic
- Whether standby power is required, and whether monitoring must survive a supply failure
- Whether readings and alarm events must be retained, and for how long
- Quantity, destination port and required delivery date
