LEL & Flammable Gas Detection

Flammable gases and vapors can explode when they mix with air in the right amount and meet a spark. A combustible gas detector measures how close the air is to that point. It’s a key tool before hot work, in confined spaces, around fuel and in process areas. But it only protects you if you understand the readings, test the meter and know its limits.

• General Industry
• Oil & Gas
• Hot Work

What LEL and UEL mean

• The lower explosive limit (LEL), also called lower flammable limit, is the lowest concentration of a gas or vapor in air that can burn or explode. Below the LEL, the mixture is too lean to burn.
• The upper explosive limit (UEL) is the highest concentration that can burn. Above it, the mixture is too rich, though it can become flammable if it mixes with more air.
• Between the LEL and UEL is the flammable range. Each gas has its own range. Methane’s LEL is about 5 percent by volume in air, propane about 2 percent and gasoline vapor around 1.4 percent.
• Gas meters usually show readings as a percent of the LEL. 100 percent LEL means the air has reached the lower explosive limit and can explode. It does not mean 100 percent gas.
• You can find the LEL and UEL of a material on the Safety Data Sheet, in Section 9. See Safety Data Sheets and Flammable Liquids.

Alarm levels and action limits

These are common practices, not universal rules. Your employer’s program, the entry permit and the standards that apply set the exact numbers. See Hot Work Permits, Drums & Tanks: Hot Work and Confined Spaces.

Types of sensors

• Catalytic bead: burns the gas on a heated bead and measures the change. Common and inexpensive, but needs enough oxygen to work (often at least about 10 percent), and can be “poisoned” by silicones, lead, sulfur compounds and some other chemicals.
• Infrared (IR): measures gas by how it absorbs infrared light. Works in low oxygen and isn’t poisoned by silicones, but may not detect hydrogen and may be gas-specific.
• Other sensors: thermal conductivity, photoionization and electrochemical sensors are used for specific gases and toxic gases. A flammable gas meter often combines several sensors, such as oxygen, LEL, carbon monoxide and hydrogen sulfide. See Gas Detectors.

Using a meter correctly

• Know your meter. Read the manual. Know its sensors, gases, alarm settings and limits.
• Bump test before each day of use (or as the maker and your program require). Expose the meter to a known gas and confirm it responds and alarms. If it fails, calibrate or take it out of service.
• Calibrate on the schedule the maker gives, with certified calibration gas, and keep records. Calibrate sooner if the meter is dropped, exposed to poisons or fails a bump test.
• Let it warm up and zero it in clean air as directed.
• Check the gas the meter is calibrated for. Meters are often calibrated to methane or pentane, and the reading for other gases needs a correction (response) factor. Use the manufacturer’s chart, since a reading may understate the true hazard.
• Test oxygen first, then flammable gas, then toxic gases. A low oxygen level can make a catalytic LEL reading wrong. See Oxygen Atmospheres.
• Sample at different levels. Gases lighter than air, like methane and hydrogen, rise. Heavier ones, like propane, gasoline vapors and butane, settle low. Test at the top, middle and bottom of a space, and in corners and low spots.
• Use a sampling pump and hose for confined spaces and tanks so you can test before entering, and let the reading settle.
• Monitor continuously during hot work and entry, since conditions can change when you disturb sludge, weld or open piping. Keep the meter where the reading reflects the work area.
• Keep the sensor inlet clean and dry. Replace filters and don’t block ports.

Limits and mistakes

• Don’t trust a meter that hasn’t been bump-tested or that is overdue for calibration.
• Don’t read zero as “safe.” The meter may not detect the gas that’s present (for example hydrogen on some sensors), or the sensor may be poisoned or the oxygen too low.
• Don’t test only at one spot or only at the opening of a tank.
• Don’t ignore alarms or silence them to keep working.
• Don’t use a meter in an atmosphere above 100 percent LEL, which can damage sensors, unless it’s designed for that.
• Don’t use a meter not approved for the hazardous area (intrinsically safe or rated for the classified location). See Hazardous (Classified) Locations.

When the alarm sounds

1. Stop work and shut off ignition sources if it is safe to do so.
2. Leave the area upwind, and warn others. Don’t use phones, radios or vehicles that could cause a spark until you are clear.
3. Call for help and follow the emergency plan. See Emergency Action Plans.
4. Return only after a qualified person finds and fixes the source, ventilates the area and gives the all clear.

Meter checklist

• Meter is the right type for the gases on site, and its alarms are set correctly
• Bump test passed today; calibration is current
• Oxygen tested first, then LEL; response factors considered
• Tested at top, middle and bottom of the space, with continuous monitoring during work
• Hot work only below the permitted LEL level (many sites use 10 percent)
• Everyone knows the alarm response and the evacuation route

Last updated: October 4, 2026