Have you noticed this interesting phenomenon? When using gas detectors, different gases have different concentration units. For example, the unit for hydrogen sulfide gas detector is usually ppm, while for oxygen gas detector, it is %VOL, and for methane gas detector, it is LEL. Why are there different detection units for gas detection?
This is because some gases are toxic even at extremely low concentrations, requiring smaller units for detection, leading to the use of ppm and ppb. On the other hand, gases like oxygen, which are generally safe, are typically measured at higher concentrations, so percentage units are used. Additionally, mg/m³ is commonly used in environmental monitoring and industrial hygiene to indicate mass concentration, as it relates to health standards. Next, let me introduce you to five commonly used gas detection units in detail.
Top Five Units in Gas Detection
ppm (parts per million)
The ppm is the fundamental unit of gas volume concentration, representing the volume of the target gas per million volumes of air (1 ppm = 1×10⁻⁶). For example, if the air contains 1 ppm of sulfur dioxide (SO₂) gas, it means that one cubic meter of air is mixed with one cubic centimeter of SO₂. This unit originated from the need for toxic gas monitoring after the Industrial Revolution. In early industrial environments, workers were often exposed to low concentrations of highly hazardous gases (such as carbon monoxide and chlorine), and traditional percentage units (such as %VOL) could not accurately describe the risks of trace pollutants. The introduction of ppm filled this gap, becoming a core unit in occupational hygiene and environmental protection.
Representative gases
Carbon monoxide (CO) is a typical toxic gas measured in ppm. Its IDLH (Immediately Dangerous to Life or Health) concentration is 1200 ppm, while the long-term exposure limit (such as OSHA PEL) is only 50 ppm. Using ppm directly reflects the volume ratio of CO inhaled by the human body, which is directly related to the gas exchange mechanism of the respiratory system (such as alveolar absorption efficiency). In addition, gases such as hydrogen sulfide, ammonia, and volatile organic compounds (VOCs) are measured in ppm due to their wide variety and low safety thresholds.
%VOL (volume percentage concentration)
%VOL represents the percentage of a gas’s volume in the total volume of air (1%VOL = 10,000 ppm) and is primarily used for high-concentration gas detection. Its origin is closely related to industrial safety needs: in the 19th century, frequent coal mine gas explosions prompted engineers to develop combustible gas detection technologies. %VOL provides an intuitive way to quantify the volume fraction of combustible gases, helping to prevent explosion risks. Additionally, oxygen (O₂), as a life-supporting gas, has concentration variations that directly affect human physiological functions, making %VOL the key unit for monitoring hypoxic or oxygen-enriched environments.
Representative gases
Oxygen (O₂) is a typical gas measured in %VOL. The normal oxygen content in the air is 20.9%VOL, while concentrations below 19.5%VOL are defined as oxygen-deficient environments, which may cause confusion and impaired consciousness. On the other hand, concentrations above 23.5%VOL increase the risk of combustion and explosion. Using %VOL allows for a quick assessment of breathing environment safety. Additionally, in industrial hydrogen fuel systems, hydrogen purity is often expressed in %VOL (e.g., 99.99%VOL high-purity hydrogen), as its volume fraction directly affects combustion efficiency and equipment lifespan.
mg/m³ (milligrams per cubic meter)
mg/m³ represents the mass of a gas (in milligrams) per cubic meter of air and is a core unit in environmental science and occupational hygiene. Its origin dates back to the mid-20th century with the need for detecting particulate matter (such as PM2.5) and gaseous heavy metals (such as mercury). Unlike volume-based units (ppm, %VOL), mg/m³ directly quantifies the absolute mass of pollutants, making it suitable for establishing air quality standards (e.g., the WHO’s annual PM2.5 limit of 10 μg/m³) and assessing human inhalation exposure doses.
Representative gases
PM2.5 (particulate matter with an aerodynamic diameter ≤2.5μm) is a typical example of a substance measured in mg/m³. Its harm is related to lung deposition mass, so global Air Quality Index (AQI) standards use μg/m³ as the unit. Additionally, substances like asbestos fibers and lead vapor, which have densities far higher than air, are measured in mg/m³ to more accurately assess their settling rates and exposure risks in workplaces.
LEL (lower explosive limit)
LEL represents the lowest volume percentage of a flammable gas in the air that can cause an explosion (e.g., the LEL of propane is 2.1%VOL). This gas detection unit emerged from the explosion protection needs of the petrochemical industry. In the early 20th century, frequent flammable gas explosions in refineries led engineers to discover significant differences in explosion thresholds among gases (e.g., hydrogen LEL = 4%VOL, gasoline vapor LEL ≈ 1.4%VOL). To standardize and simplify safety monitoring, the LEL gas detection unit was introduced.
From the above content, you can see that both LEL and %VOL are used for flammable gas detection. Essentially, LEL is a derived unit of %VOL, normalized based on the lower explosive limit. For example, if a gas detector displays 50%LEL methane, it means the current concentration has reached half of its explosive threshold (5%VOL), i.e., 2.5%VOL. Compared to %VOL, LEL focuses more on the relative level of explosion risk rather than absolute concentration, making it easier to set standardized alarm thresholds across different gas types (e.g., 20%LEL for low alarms, 50%LEL for high alarms).
Representative gases
Methane (the main component of natural gas) is a typical application of the LEL unit. In coal mines and natural gas pipelines, methane concentrations exceeding 5%VOL enter the explosive range, while everyday leakage concentrations are usually below 1%VOL. Using the %LEL unit standardizes the detector’s measurement range to 0-100%LEL, enhancing operators’ ability to assess risk levels. Additionally, the LEL gas detection unit is compatible with catalytic combustion sensors, which have a broad response to flammable gases. This eliminates the need for individual calibration for each gas, significantly reducing the cost of multi-gas detection.
ppb (parts per billion)
The ppb represents the volume of a target gas per billion volumes of air (1 ppb = 1×10⁻⁹), which is one-thousandth of ppm. The widespread use of this gas detection unit is related to advancements in environmental monitoring technology in the 21st century: trace pollutants (such as ozone and benzene compounds) can cause cancer or chronic diseases even at very low concentrations (in the ppb range). Traditional ppm units cannot meet the precision requirements. For example, the World Health Organization (WHO) specifies that the 8-hour average ozone concentration limit is 50 ppb, which requires ppb-level detection technology to support.
The difference between ppb and ppm is not only in magnitude (1 ppm = 1000 ppb) but also marks a turning point in gas detection technology. ppm-level gas detection often uses electrochemical or semiconductor gas sensors, while ppb-level detection requires high-sensitivity technologies such as Photoionization Detection (PID) or Tunable Diode Laser Absorption Spectroscopy (TDLAS). Compared to mg/m³, ppb is still a volumetric unit, making it more sensitive to low molecular weight gases (e.g., ozone, M = 48), while mg/m³ is more suitable for higher molecular weight substances (e.g., polycyclic aromatic hydrocarbons).
Representative gases
Ozone (O₃) is a typical example of a gas measured in ppb. Its harmful effects on the respiratory system and plants become evident at the ppb level, with ozone concentrations in urban photochemical smog often reaching 100-200 ppb. Using the ppb unit allows for precise quantification of regional pollution differences (e.g., a difference of several tens of ppb between urban and suburban areas) and provides data support for implementing traffic control measures. Additionally, in the semiconductor manufacturing industry, extremely low concentrations of corrosive gases (e.g., HF, with a limit of 1 ppb) require ppb-level real-time monitoring to prevent wafer contamination.
Complete Measurement Units in Gas Detection
| Abbreviation | Full Name | Unit Definition | Application |
|---|---|---|---|
| ppm | Parts Per Million | 1 ppm = 10⁻⁶ (volume ratio) | Toxic gas detection (CO, H₂S, NH₃) |
| ppb | Parts Per Billion | 1 ppb = 10⁻⁹ (volume ratio) | Trace gas monitoring (O₃, VOCs) |
| ppt | Parts Per Trillion | 1 ppt = 10⁻¹² (volume ratio) | Ultra-trace pollutant monitoring (e.g., dioxins) |
| %VOL | Percentage by Volume | 1% VOL = 10,000 ppm | Oxygen (O₂), carbon dioxide (CO₂), methane (CH₄) |
| %LEL | Lower Explosive Limit Percentage | Normalized to 100% at the lower explosive limit | Combustible gas detection (CH₄, H₂) |
| mg/m³ | Milligrams per Cubic Meter | Mass concentration unit | Air quality monitoring, occupational hygiene |
| μg/m³ | Micrograms per Cubic Meter | Mass concentration unit | Particulate matter (PM2.5, PM10) monitoring |
| g/m³ | Grams per Cubic Meter | Mass concentration unit | High-concentration gas monitoring |
| mg/L | Milligrams per Liter | 1 mg/L = 1 g/m³ | Water and gas monitoring |
| μg/L | Micrograms per Liter | 1 μg/L = 1 mg/m³ | Ultra-trace gas analysis |
| TLV-TWA | Threshold Limit Value – Time Weighted Average | 8-hour average exposure limit | Occupational safety (OSHA, ACGIH) |
| TLV-STEL | Threshold Limit Value – Short-Term Exposure Limit | 15-minute short-term exposure limit | Toxic gas short-term exposure safety |
| IDLH | Immediately Dangerous to Life or Health | Concentration causing immediate severe health effects or death | Emergency response, hazardous environments |
| OEL | Occupational Exposure Limit | Legally defined workplace exposure concentration | Workplace air quality |
| MAK | Maximale Arbeitsplatz-Konzentration (Germany) | European occupational exposure limit | European industrial safety standards |
| REL | Recommended Exposure Limit (NIOSH) | Limit by the National Institute for Occupational Safety and Health | Industrial hygiene |
| PEL | Permissible Exposure Limit (OSHA) | Legally defined occupational exposure limit | Occupational safety (USA) |
| STEL | Short-Term Exposure Limit | 15-minute permissible short-term exposure | Hazardous gas short-term exposure |
| TVOC | Total Volatile Organic Compounds | Total VOC concentration | Indoor air quality monitoring |
| AQI | Air Quality Index | Standardized pollution level indicator | Environmental air monitoring |
| MAC | Max. Allowable Concentration | Max. acceptable workplace concentration | Industrial hygiene |
| LEL | Lower Explosive Limit | Min. combustible gas concentration | Flammable gas detection |
| UEL | Upper Explosive Limit | Max. combustible gas concentration | Combustible gas monitoring |
| V/V | Volume Ratio | Gas volume as a fraction of total air volume | Gas mixture measurement |
| W/W | Weight Ratio | Gas content expressed by weight | Chemical industry |
| OD | Optical Density | Measure of light transmission or absorption | Optical gas detection (NDIR, TDLAS) |
Conversion Between VOL%, LEL%, and ppm
First, the conversion between VOL and PPM is relatively simple, as %VOL represents the volume percentage, while PPM is the volume per million. Therefore, 1% (VOL) = 10,000 PPM.
Next, the conversion between VOL and LEL requires identifying the lower explosive limit (LEL) of the flammable gas. When the concentration of a flammable gas in air reaches its LEL, the environment is considered to be at 100% explosion risk. For example, hydrogen has an LEL of 4% VOL, meaning when its volume percentage in the air reaches 4% VOL, it will explode upon contact with a flame. Therefore, 4% VOL is regarded as 100% risk, i.e., 4% VOL = 100% LEL, and thus 1% VOL = 25% LEL.
Third, the conversion between PPM and LEL cannot be directly calculated. First, LEL must be converted to VOL, and then VOL is converted to PPM. Here is a formula: PPM = %LEL × LEL (vol%) × 100. For example, to calculate the PPM of methane at 20% LEL, the formula would be: 20 (%LEL) × 5 (%VOL) × 100 = 10,000 PPM.
In addition to LEL%, VOL%, and PPM, gas detectors may also use other units to represent gas concentrations, depending on the type of gas sensor and the measurement range.
Measurement Units help in Selecting Gas Detectors
Units in gas detection (such as ppm, ppb, %VOL, mg/m³, etc.) play a key role in the selection of gas detectors, directly affecting the instrument’s applicability, detection accuracy, safety, and compliance. The following are specific impacts:
1. Matching detection requirements
Different gas detections require different concentration units. For example, in the industrial safety field, gas detectors that use %VOL or LEL as units are commonly selected. For environments with toxic gases, detectors using ppm or mg/m³ as units are typically chosen. For environmental monitoring (e.g., air pollutants), equipment using ppb or μg/m³ as units is selected.
Mismatched gas detection units may lead to misjudgment: for instance, if a gas detector only supports %VOL but needs to detect low concentrations of toxic gases (in the ppm range), it will not meet the required needs.
2. Range and accuracy
The range design and accuracy requirements of a gas detector are highly dependent on the selected units. Different sensor technologies significantly affect the range and accuracy. Electrochemical sensors are suitable for ppm-level gas detection, but they may become saturated in high-concentration environments. Infrared sensors (NDIR) perform more stably in %VOL ranges but are more expensive and insensitive to low concentrations (ppb levels). Therefore, high-precision scenarios require instruments that support native unit output and have dynamic calibration functions to reduce secondary calculation errors.
3. Regulations and standards compliance
Compliance with gas detection units is one of the core considerations when selecting instruments. Various national regulations and industry standards have specific requirements for units, such as:
Occupational Safety: The U.S. OSHA standard requires CO exposure limits to be in ppm (PEL = 50 ppm).
Environmental Monitoring: The EU’s Air Quality Directive specifies that PM2.5 be measured in μg/m³, while the U.S. EPA requires ozone monitoring in both ppm and ppb.
If the instrument’s units do not align with regulatory requirements, the data may not be recognized by regulatory authorities. Additionally, international projects may involve multiple standards. For example, oil drilling platforms must meet both ISO 21489 (which requires flammable gases to be displayed in %LEL) and API RP 55 (which recommends using ppm for H₂S detection). Therefore, when selecting instruments, priority should be given to those that support the legal units of the target market and ensure they are certified to standards (such as ATEX, UL, or CE) to demonstrate compliance with regulatory data output.
4. Sensor technology limitations
Different sensor technologies have inherent limitations in adapting to specific measurement units. For example:
- Electrochemical sensors: generate an electrical current through a chemical reaction between the gas and the electrode. They have high sensitivity and are suitable for detecting ppm-level toxic gases (e.g., CO sensor and H₂S sensor). However, their range is typically limited to 0-100 ppm, and their lifespan is affected by cross-gas interference.
- Infrared sensors (NDIR): detect gas concentration based on the absorption of specific infrared wavelengths. They are ideal for %VOL-level detection (e.g., CO₂ detector) with a range of up to 0-100%VOL but cannot detect symmetric gas molecules.
- Photoionization detectors (PID): use UV light to ionize gas molecules, making them suitable for ppb-level VOC detection (e.g., benzene and xylene). However, they are ineffective for inorganic gases.
5. Safety threshold recognition
Instruments must support a direct association between measurement units and safety thresholds. For example, marking “TWA” (Time-Weighted Average) at 50 ppm and “STEL” (Short-Term Exposure Limit) at 200 ppm. When selecting a device, it is essential to confirm whether its alarm functions support threshold settings in the target unit and whether it provides visual risk alerts.
6. International unit differences
Regional preferences for measurement units require instruments to be adaptable. For example:
- North America: OSHA standards predominantly use ppm, while environmental monitoring commonly employs ppb.
- Europe: EU regulations (e.g., REACH) prioritize mg/m³ and μg/m³. Some industries (such as automotive manufacturing) follow the German TA Luft standard, which references ppb.
- Asia: Chinese national standards (GB) specify both ppm and mg/m³, while Japan partially adopts volume-based units (e.g., %VOL).
When conducting gas detection, it is essential to select instruments based on the target market or confirm with suppliers whether the firmware supports multi-unit configuration.
The core reason for the diversification of gas detection units lies in the differences in application scenarios:
- Health and Safety Requirements: ppm/ppb for toxic gases, mg/m³ for mass exposure, %VOL/LEL for explosion risk.
- Sensor Technology Limitations: High concentrations (%VOL) and trace levels (ppb) require different sensor principles.
- Regulatory and Standardization Drivers: International organizations (e.g., ISO, OSHA) establish unit standards based on different hazard types.
If you have any questions about gas detection, please contact us. Renke is an original supplier focusing on the intelligent development of sensors and Internet of Things technology. Its gas sensors and detectors serve various industries at home and abroad with excellent quality, high precision and high stability.
