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PPM vs %LEL in Gas Detection

Gas instruments use three scales: parts per million for trace levels, percent of the lower explosive limit for fire risk, and percent by volume for high concentrations. A methane reading of 5,000 ppm, 10 %LEL and 0.5 %vol is the same air. Knowing which scale is on the display is essential.

By the Akermin Editorial Team · 2 min read

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Parts per million (ppm) is a count: how many gas molecules per million molecules of air. Percent of lower explosive limit (%LEL) is a fraction of the concentration at which the gas can burn. Percent by volume (%vol) is the gas’s share of the air. For methane, whose LEL is 5 percent by volume: 5,000 ppm = 0.5 %vol = 10 %LEL. Leak detectors read in ppm because leaks are found at trace levels; safety monitors read in %LEL because fire risk is what they judge; %vol appears on instruments for high-concentration work such as purging.

The three scales

Scale Meaning Used for Typical instrument
ppm Molecules per million Trace detection, leak location, toxic gas exposure Leak sniffers, CO meters, toxic gas monitors
%LEL Percent of the lower explosive limit Fire and explosion risk Personal and area combustible monitors, confined-space meters
%vol Percent of the air by volume High concentrations, purging, oxygen Thermal conductivity and infrared %vol sensors, oxygen sensors

Converting

1 %vol = 10,000 ppm. %LEL = (concentration ÷ LEL) × 100. Each gas has its own LEL:

Gas LEL (%vol) LEL (ppm) 10 %LEL in ppm
Methane (natural gas) 5.0 50,000 5,000
Propane 2.1 21,000 2,100
Butane 1.8 18,000 1,800
Hydrogen 4.0 40,000 4,000
Gasoline vapor About 1.4 About 14,000 About 1,400
Acetylene 2.5 25,000 2,500

A monitor calibrated to methane reading 10 %LEL in propane is not reading 10 %LEL of propane; correction factors apply, and a monitor should be calibrated to the gas it will meet.

Why the scales are hundreds apart

A leak detector that alarms at 50 ppm of methane is responding at 0.1 %LEL. That sensitivity is right for finding a pinhole at a fitting, where the gas is concentrated but the total amount is tiny. A room monitor that alarmed at 50 ppm would sound constantly in a kitchen with a gas hob. It alarms at 10 %LEL, a hundred times higher, because that is where the hazard begins. Using a leak detector to judge whether a room is safe, or a %LEL monitor to find a small leak, are both the wrong tool.

Toxic versus flammable

Some gases are dangerous long before they are flammable. Hydrogen sulfide is lethal at hundreds of ppm and flammable at 40,000 ppm. Carbon monoxide harms at 100 ppm and is flammable at 125,000 ppm. For those, the ppm scale and toxic exposure limits are what matter, which is why multi-gas monitors carry separate electrochemical sensors in ppm alongside the %LEL sensor. See what a multi-gas detector is.

Alarm setpoints

  • Combustible: 10 %LEL low alarm, 20 %LEL high alarm are the common defaults.
  • Domestic natural gas alarms: certified to sound below a fixed fraction of LEL, typically 10 to 20 percent.
  • Leak detectors: adjustable sensitivity; no safety setpoint.

For how the sensors produce these readings, see what a combustible gas detector is. Instruments are in the gas detection category.

TopTes Guard-101 Gas Detector, 4 Gas Monitor for H2S, CO, LEL and O2, with Vibration, Visual and Audible Alarms, 14h Long Battery Life, Safety Explosion-Proof, for Work, Home - Orange

TopTes Guard-101 Gas Detector, 4 Gas Monitor for H2S, CO,

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Klein Tools ET120 Gas Leak Detector

Klein Tools ET120 Gas Leak Detector

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Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

How do I convert ppm to %LEL?

Divide the ppm by the gas's LEL in ppm and multiply by 100. Methane LEL is 50,000 ppm, so 5,000 ppm is 10 %LEL. Propane LEL is 21,000 ppm, so 5,000 ppm is about 24 %LEL.

What %LEL is dangerous?

Alarms are commonly set at 10 %LEL (low) and 20 %LEL (high). Above 100 %LEL the mixture can ignite. Toxic effects of some gases begin far below the explosive range.

Why does my leak detector show ppm but my confined-space monitor shows %LEL?

Different purposes. The leak detector is tuned to find tiny leaks at the fitting; the monitor is tuned to judge whether a space is safe to enter. The two scales differ by a factor of hundreds.

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