What distinguishes a number from data is its unit. Units enable us to understand what we are dealing with when collecting data, conducting measurements or analyses, and generating or interpreting graphs. Consequently, understanding units frequently used during gas measurement and analysis, such as %VOL, PPM, and %LEL, enables us to better interpret collected data.
Gas is a state of matter in which particles are positioned far apart. In the gaseous state, matter has no fixed shape, only volume. Because the volume of a gas is very large when particles are dispersed, its density is correspondingly low. Consequently, different units of measurement may be required to define gas concentrations.
What Do the %LEL and %UEL Measurement Units Mean?
Gases exhibit explosive properties within a specific concentration range. This concentration range varies for each gas; however, common to all gases is the requirement of oxygen and an ignition source (spark) for an explosion to occur.
The lower flammability threshold is termed the LEL (Lower Explosive Limit). Concentrations below the LEL are too lean, meaning there is insufficient flammable gas present. Concentrations up to the LEL value are expressed as %LEL. An environment entirely free of the gas is expressed as 0% LEL, whilst an environment at the exact LEL concentration is expressed as 100% LEL.
Above the flammable range, the gas concentration is so rich that insufficient oxygen is present to support combustion; therefore, an explosion cannot occur. The upper boundary of the explosive range is defined as the UEL (Upper Explosive Limit).
This concept can be visualised as follows:
As can be seen from this graphical representation, an explosion occurs within the concentration band between the LEL and UEL levels; no explosion occurs at concentration levels below the LEL or above the UEL. In other words, at the baseline both gas volume (% vol) and Lower Explosive Limit (%LEL) are zero. As volume increases, the LEL percentage rises accordingly; when the gas concentration reaches a specific volume, the explosive threshold—namely 100% LEL—is reached. The objective of gas detection instruments is to measure gas at lower concentrations prior to reaching 100% LEL, allowing control measures to be taken to eliminate explosion risks. As noted above, LEL and UEL levels differ for every gas. Some gases reach their LEL and UEL values at low volumetric concentrations (% vol), whereas others reach these values at higher volumetric concentrations. To provide examples of LEL and UEL concentration levels for common industrial and everyday gases:
Lower and Upper Explosive Limits
As illustrated in this table, when Methane reaches 5% concentration by volume, it has reached the explosive threshold; once it exceeds 15% concentration, it has surpassed the explosive threshold (explosion risk is no longer present). For Hydrogen, however, this explosive band is considerably wider, reaching the explosive threshold at 4% concentration by volume and exceeding it at 75% concentration by volume.
What Does the PPM Measurement Unit Mean?
We previously discussed using LEL-UEL units for measuring gases at explosive levels and their correlation with volumetric concentration. However, in certain applications gases cannot be measured with these units, necessitating much smaller measurement units. For toxic gases, for instance, presence in the environment even at extremely low concentrations is far more critical than reaching explosive limits. These low concentrations are expressed in the unit ppm.
Frequently encountered, ppm (parts per million) denotes one part in a million. Imagine a room containing one million balls, one of which is red. To describe this single ball, we can state that "there is 1 ppm of red balls in the room". The ppm unit can be used interchangeably with mL/m³ or mg/kg when describing a substance present in water or soil.
Ppm is utilised to denote toxic gases whose presence in minute quantities can be highly detrimental or even fatal. For example, exposure to H2S hydrogen sulphide gas—frequently used or generated during processes in the petrochemical industry—at concentrations above 10 ppm causes severe respiratory complications.
In facilities operating with natural gas, leaks of flammable methane gas can have catastrophic consequences. Consequently, field detection surveys are carried out using methane detectors. Detecting even the smallest quantities is critical to risk mitigation. Therefore, when methane detection is conducted for leak checking purposes, detectors measuring in ppm may be employed.
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A comparable unit used to express even lower concentrations is ppb (parts per billion), representing one part in a billion. Analogous to the example above, this unit can represent a single red ball amongst one billion balls in a room. You may see ppb used when referring to CO (carbon monoxide) levels in ambient air; for instance, the Earth's troposphere contains an average of approximately 100 ppb of CO gas.
Below are typical ppm-level occupational exposure limit values (action levels / TLVs) for select gases:
| Chemical Substance | ppm Threshold Limit Value |
|---|---|
| Nitric Acid | 2 |
| Ammonia | 25 |
| CO2 | 5000 |
| NO2 | 3 |
| SO2 | 2 |
| Chlorine | 0,5 |
| Fluorine | 1 |
| H2S | 10 |
| NO | 25 |
| CO | 25 |
What Does the %VOL Measurement Unit Mean?
This unit is used to express volume as a percentage. For example, ambient air comprises approximately 21% oxygen and 78% nitrogen by volume. In other words, if we sample and analyse 100 mL of air from your room, 21 mL will be oxygen, 78 mL will be nitrogen, and the remaining ~1 mL will consist of trace gases such as neon, carbon dioxide, and hydrogen.
This unit can be converted into ppm. As stated, 1 ppm represents 1/1,000,000; expressing this as a percentage yields 0.0001/100, or 1 ppm = 0.0001%. Alternatively, 1% vol = 10,000 ppm.
The conversions between the PPM, LEL, and VOL units detailed above can be summarised as follows:
100 ppm gas > is equivalent to 0.01 %vol gas
1 %vol gas > is equivalent to 10,000 ppm gas
1 %LEL Methane gas > is equivalent to 0.05 %vol Methane gas, > and is equivalent to 500 ppm Methane gas.
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Please click here for the AII GPR series fixed oxygen analysers.
Understanding the units discussed in this article will facilitate a better understanding of gas-related operations and aid during analytical assessments.
Frequently Asked Questions
Why is the topic 'Gas Concentration Units: Which One, Why, and Where Used?' important?
This article summarises the fundamental concepts, intended use, and practical considerations regarding gas concentration units and their selection, adopting the NOVEL technical content approach.
How should accurate measurement or product selection be conducted?
Measurement range, environmental conditions, target parameters, and application objectives must be evaluated collectively. Product or methodology selection should not be based solely on a single technical parameter.
Whom may I contact for technical support?
The NOVEL Technical Team provides engineering assistance with measurement methodologies, instrument selection, and application-specific equipment evaluation.