Combustible Gas Detection: Methane / LPG Leak Alarms and Sensor Selection

Why one gas class means two deployments

The Lower Explosive Limit (LEL) is the minimum gas concentration in air that can ignite. Below the LEL there is no explosion risk, but action should begin at 25–50% of the LEL — the point of an alarm is not 'about to explode' but 'concentration has left the safe zone'. Methane's LEL is about 5% vol (~50,000 ppm), propane about 2.1% vol (~21,000 ppm), and isobutane about 1.8% vol.

What actually fixes detector placement is the density difference. Methane (MW 16) is lighter than air (~29) and collects in the upper room; LPG (propane MW 44, butane MW 58) is heavier, flowing along the floor and pooling in trenches and sumps. In the same kitchen, a natural-gas detector goes near the ceiling while an LPG detector must sit close to the floor — mount them swapped and you have effectively no alarm.

Two sensing principles: catalytic bead vs NDIR

Catalytic bead (catalytic pellistor / Wheatstone bridge) is the oldest combustible-gas method: the gas burns flamelessly on a platinum-coated bead, the bead heats, its resistance changes and the bridge unbalances to produce a signal. It is cheap and linear across most combustible gases, but its shortcomings are equally clear: it needs oxygen to burn, so it drifts or goes blind in oxygen-deficient or nitrogen-purged spaces; silicon, lead, sulphur and halogens poison the catalyst permanently; and its life is spent by cumulative burn-off.

NDIR (Non-Dispersive Infrared) takes a different path: it measures concentration directly from the gas's infrared absorption, with no combustion. It therefore does not need oxygen, cannot be poisoned, lasts long and is maintenance-free, with good selectivity and low cross-interference for hydrocarbons like methane and propane. The trade-off is a higher optical cost than a pellistor, and distinguishing isomers and background gases needs dual-wavelength algorithms. For long-duty, unattended wireless nodes and portable instruments, NDIR's total cost of ownership is usually lower.

• First question: can the site go oxygen-deficient or get nitrogen-purged? If yes, go NDIR — don't bet a pellistor stays accurate after a purge.
• Second question: attended or battery/wireless long-duty? For wireless nodes and portables, favour low-power NDIR to skip frequent calibration and battery swaps.

How to set the two-stage alarm points

The common design basis in China is GB 50493-2019 (Design standard for combustible and toxic gas detection and alarm): the Stage 1 alarm set-point should be ≤ 25% LEL and Stage 2 ≤ 50% LEL. Stage 1 triggers ventilation and inspection; Stage 2 triggers evacuation readiness. These are the most-cited figures for industrial sites.

Standalone residential and commercial standards differ slightly: EN 50194 (European residential gas detectors) requires alarm action at far lower concentrations than the industrial tier; UL 2075 (North American gas-detection standard) sets response ranges by detector type. For multi-market export equipment, design to the strictest target-market tier and configure per zone in software.

Mounting height: put the detector where the gas pools

Methane is lighter than air: mount the detector 0.3–0.6 m below the ceiling (or above a false ceiling / near the return air inlet) to cover the upper pooling zone. Typical sites: natural-gas boiler rooms, commercial kitchens, CNG stations, upper pipe galleries.

LPG is heavier than air: mount the detector 0.3–0.6 m above the floor, with priority on trenches, pipe bottoms, low spots and equipment bases — propane flows along the ground and settles in depressions. Typical sites: LPG cylinder rooms, bottled-gas kitchens, filling workshops, underground pressure-regulating stations.

Do not hug the detector to the source directly above or below it — transient puffs cause nuisance alarms. Place it on the diffusion path between the likely source and where people stay, balancing response speed with stability. In open, well-ventilated outdoor areas, densify by airflow and equipment layout.

• Explosion protection must match the hazard zone: in explosive atmospheres choose Ex d (flameproof) or Ex i (intrinsic safety) with valid IECEx / ATEX or Chinese Ex certification; prefer 4–20 mA / Modbus output to wire into PLCs and ventilation interlocks.
• A 0–100% LEL range covers the alarm need; if you also want concentration trending, pick a model with digital (UART) output to log concentration and temperature directly.

Frequently asked questions