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Guide

How to Understand PPM, %LEL and %Volume in Gas Detection

Gas instruments show concentration in ppm, %LEL or %volume depending on what they are for. Reading one against the other, and knowing which scale a limit is written in, is a routine part of gas work. This guide gives the conversions, a worked table, and the mistakes the scales cause.

By the Akermin Editorial Team · 2 min read

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The three scales describe the same thing at different magnitudes. Parts per million counts molecules per million of air; %volume is the same fraction expressed as a percentage (1 %vol = 10,000 ppm); %LEL expresses concentration as a fraction of the lower explosive limit, which differs for each gas. To convert, you need the gas’s LEL: %LEL = (ppm ÷ LEL in ppm) × 100. Leak detectors read ppm because leaks are found at trace levels; safety monitors read %LEL because fire risk is what they judge; purge and process meters read %volume at high concentrations.

The conversions

  • ppm ÷ 10,000 = %volume
  • %volume × 10,000 = ppm
  • %LEL = (ppm ÷ LEL_ppm) × 100 = (%vol ÷ LEL_%vol) × 100
  • ppm = %LEL × LEL_ppm ÷ 100

Conversion table

Gas LEL (%vol) LEL (ppm) 10 %LEL 20 %LEL 50 %LEL
Methane 5.0 50,000 5,000 ppm (0.5%) 10,000 ppm (1%) 25,000 ppm (2.5%)
Propane 2.1 21,000 2,100 ppm 4,200 ppm 10,500 ppm
Butane 1.8 18,000 1,800 ppm 3,600 ppm 9,000 ppm
Hydrogen 4.0 40,000 4,000 ppm 8,000 ppm 20,000 ppm
Gasoline vapor About 1.4 About 14,000 1,400 ppm 2,800 ppm 7,000 ppm
Acetylene 2.5 25,000 2,500 ppm 5,000 ppm 12,500 ppm
Ammonia 15 150,000 15,000 ppm 30,000 ppm 75,000 ppm

LEL values vary slightly between references; instruments use the value in their firmware.

Which instrument shows which

Instrument Scale Reason
Leak sniffer ppm or relative Leaks are found at tens to hundreds of ppm at the fitting
Personal and area combustible monitor %LEL Explosion risk starts at 10 to 20 %LEL
Toxic gas monitor (CO, H₂S) ppm Exposure limits are in ppm
Oxygen sensor %volume Normal is 20.9 %vol; alarms at 19.5 and 23.5
Purge and process monitor %volume Concentrations above the explosive range
Domestic gas alarm Alarms at a fixed %LEL; no display Certified setpoint

Worked examples

  • A sniffer reads 400 ppm at a propane fitting. That is 400 ÷ 21,000 = 1.9 %LEL at the fitting; a real leak, far from explosive at that point, but gas accumulates.
  • A monitor reads 15 %LEL in a basement with natural gas. That is 7,500 ppm, 0.75 %vol; the high alarm is near; evacuate and ventilate.
  • A methane-calibrated monitor reads 10 %LEL in propane. The true propane %LEL is higher, since the catalytic sensor responds less to propane; apply the correction factor (often 1.5 to 2) or calibrate to propane.
  • A CO monitor reads 50 ppm. Flammability is irrelevant (LEL is 125,000 ppm); this is a toxic exposure reading against a 35 ppm 8-hour limit; act.

Mistakes the scales cause

  • Reading a sniffer’s ppm as if it described the room; it describes the fitting.
  • Assuming 100 %LEL is 100 percent gas; it is 5 percent gas for methane.
  • Comparing %LEL readings across different gases without the correction factor.
  • Treating a toxic gas as safe because it is far below its LEL.

Background in PPM vs %LEL in gas detection; instruments in how to choose a multi-gas detector and 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 %LEL to ppm?

Multiply the %LEL by the gas's LEL in ppm and divide by 100. For methane (LEL 50,000 ppm), 10 %LEL is 5,000 ppm. For propane (LEL 21,000 ppm), 10 %LEL is 2,100 ppm.

What is 1 %volume in ppm?

10,000 ppm. 100 %volume is 1,000,000 ppm. So 5 %volume methane is 50,000 ppm, which is methane's LEL.

Why do some gases have toxic limits far below their LEL?

Because toxicity and flammability are unrelated. Hydrogen sulfide is dangerous at 10 to 100 ppm and flammable at 40,000 ppm; carbon monoxide harms at 35 to 100 ppm and burns at 125,000 ppm. Toxic gases are monitored in ppm; flammable ones in %LEL.

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