A combustible gas detector senses flammable gases and vapors, such as methane, propane, butane, hydrogen and fuel vapors, and reports their concentration, usually as a percentage of the lower explosive limit (%LEL) or in parts per million. Handheld leak detectors (“sniffers”) have a flexible probe and a sensitive tick-rate or bar display for tracing a leak to its source; area monitors and multi-gas instruments watch a whole space and alarm at 10 to 20 percent LEL. Neither replaces a fixed gas alarm in a home with gas appliances.
What it measures
Every flammable gas has a lower explosive limit (LEL), the concentration in air below which it will not burn, and an upper limit above which the mixture is too rich. Methane’s LEL is about 5 percent by volume (50,000 ppm); propane’s about 2.1 percent. Detectors report against these limits: 10 percent LEL of methane is 0.5 percent by volume, or 5,000 ppm. Leak sniffers work far below that, down to tens of ppm, because a leak that is tiny at the fitting can accumulate elsewhere. See PPM vs %LEL in gas detection.
Sensor types
| Sensor | How it works | Strengths | Limits |
|---|---|---|---|
| Catalytic bead (pellistor) | Gas burns on a heated catalyst bead, raising its temperature and resistance | Responds to nearly all combustibles; the industrial standard for %LEL | Needs oxygen; poisoned by silicones, lead, sulfur; drifts; must be bump-tested |
| Semiconductor (metal oxide) | Gas changes the resistance of a heated tin-oxide film | Very sensitive at low ppm; cheap; the usual sniffer sensor | Non-specific; affected by humidity, alcohol, cleaners; not for accurate %LEL |
| Infrared (NDIR) | Hydrocarbon absorbs infrared at a set wavelength | No oxygen needed; immune to poisoning; stable | Does not detect hydrogen; costlier |
| Thermal conductivity | Gas changes the thermal conductivity of the sample | Works at high concentrations, %volume | Insensitive at low levels |
Sniffers versus monitors
- Leak sniffers (Klein ET120, TOPTES PT199, UEi CD100A) use semiconductor sensors on a gooseneck probe. They respond within seconds to a few ppm and increase their tick rate or bar graph as the probe nears the source. They locate; they do not quantify hazard.
- Personal and area monitors use catalytic or infrared sensors calibrated in %LEL, with alarms at fixed setpoints (typically 10 and 20 percent LEL). Multi-gas versions add oxygen, CO and hydrogen sulfide for confined-space work. See what a multi-gas detector is.
- Fixed domestic gas alarms are certified to a standard, mounted near appliances, and sound at a set fraction of LEL. They are the only appropriate protection for a home.
Using a leak detector
- Switch on in clean air and let it warm up and zero; semiconductor sensors take 30 to 60 seconds.
- Move the probe slowly along joints, fittings, valves and appliance connections, a centimeter or so from the surface. Gas is lighter (methane) or heavier (propane) than air; check above or below accordingly.
- Follow the increasing indication to the source; confirm with leak-detection fluid.
- Be aware of false responses: cleaning products, sealants, alcohol, exhaust and steam all trigger semiconductor sensors.
- If you smell gas strongly or the detector alarms in open air, leave and call the gas emergency line; do not switch anything electrical.
Maintenance
All combustible sensors drift and age. Catalytic sensors need regular bump tests against calibration gas; see why gas detectors need bump tests and calibration. Sensor lifespan is finite; see sensor lifespan. Detectors are covered in the gas detection category.
Recommended Tools
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Frequently Asked Questions
What gases does a combustible gas detector detect?
Methane (natural gas), propane, butane, hydrogen, gasoline vapor, alcohol vapor, acetylene and most other flammable hydrocarbons. Sensitivity differs by gas; detectors are calibrated to one reference gas.
Does it detect carbon monoxide?
Generally no, and a semiconductor sensor may respond weakly to CO without being reliable for it. CO needs an electrochemical sensor or a CO alarm.
What is the difference between a gas sniffer and a gas alarm?
A sniffer is a handheld with a probe, tuned for sensitivity to locate leaks. An alarm or monitor watches a space and sounds at a set percentage of the explosive limit.
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