A gas detector is only as good as its match to the hazard. The same instrument that protects one site can give a false sense of safety on another, because it is monitoring the wrong gas, using the wrong sensor technology, or is not rated for the area it sits in. This guide covers the four decisions that matter when specifying gas detection for oil & gas and industrial sites.
1. Which gases — and at what levels
Start with a gas hazard assessment. On most oil & gas sites the priority gases are:
- Combustible gas — measured as a percentage of the Lower Explosive Limit (% LEL). Common alarm thresholds are around 20% LEL (low) and 40% LEL (high), per site policy.
- Hydrogen sulphide (H₂S) — highly toxic, measured in ppm. Typical alarms are around 10 ppm (low) and 15–20 ppm (high).
- Carbon monoxide (CO) — toxic, measured in ppm.
- Oxygen (O₂) — for deficiency (below ~19.5%) or enrichment (above ~23.5%).
Alarm set-points should follow your occupational exposure limits and site safety case, not generic defaults.
2. Fixed or portable
| Fixed system | Portable monitor | |
|---|---|---|
| Purpose | Continuous protection of a defined area or asset | Personal protection, confined-space entry, surveys |
| Mounting | Permanently installed, wired to a controller | Carried or worn by personnel |
| Best for | Compressor houses, wellheads, process areas | Maintenance, inspection, confined-space work |
| Typical output | Alarms, relays, plant shutdown signals | Local audible/visual alarm, data logging |
Most sites need both: fixed detection guarding the plant, and portable monitors for people working in and around it.
3. Sensor technology
The sensor is the heart of the decision. Each technology has a sweet spot:
- Catalytic bead — detects combustible gases by catalytic oxidation on a heated bead. Inexpensive and proven, but requires oxygen and can be poisoned by silicones, sulphur compounds and lead. Suited to general hydrocarbon detection where poisons are absent.
- Infrared (IR) — measures hydrocarbon absorption of infrared light. Immune to catalytic poisons, fails safe, and works in inert or oxygen-free atmospheres. Excellent for methane and hydrocarbon vapours and for high-reliability fixed points. Note: IR does not detect hydrogen.
- Electrochemical — the standard for toxic gases and oxygen (H₂S, CO, O₂). Gas reacts at an electrode to produce a measurable current. Accurate at low ppm; sensors have a finite life and need periodic replacement.
- Photoionisation detector (PID) — measures volatile organic compounds (VOCs) at low concentrations, useful for benzene and solvent vapours that other sensors miss.
A practical fixed installation often pairs IR for combustibles with electrochemical sensors for H₂S, CO and O₂.
4. Hazardous-area certification
Any electrical instrument in a classified area must carry explosion-protection certification appropriate to the zone and gas group:
- Zone 0 / 1 / 2 describe how likely an explosive atmosphere is present.
- Protection concepts include Ex d (flameproof enclosure) and Ex ia (intrinsic safety).
- Certification schemes are typically ATEX (European) and IECEx (international), referencing the IEC 60079 series.
Match the detector's Ex marking to your hazardous-area classification drawings. The performance of combustible gas detectors is also assessed against standards such as EN/IEC 60079-29-1.
5. Keep them honest: bump test and calibration
Detection equipment must be maintained to be trusted:
- Bump test — a brief exposure to a known gas to confirm the sensor responds and the alarms activate. Done frequently, often before each use for portables.
- Calibration — adjusts the instrument's reading against a certified gas standard, on a defined interval.
Budget for calibration gas, regulators and docking/test stations as part of the system, not as an afterthought.
Sourcing in Nigeria
When you buy gas detection, specify the gases and ranges, fixed vs portable, sensor technologies, and the required Ex rating for each location. FirstSupply.ng supplies fixed and portable gas and flame detection, calibration gases and accessories — certified and priced in Naira, with delivery from Lagos and Port Harcourt. Share your area classification and gas list with a quote request and we will specify a compliant solution.