In industrial gas detection systems, four measurement units are commonly used:
- %LEL (Lower Explosive Limit)
- %VOL (Volume Percent)
- ppm (parts per million)
- mg/m³ (milligrams per cubic meter)
Although all of them describe gas concentration, they serve different safety purposes and engineering logic. Misunderstanding these units can lead to incorrect alarm settings, misjudged risk levels, and unsafe field decisions.
This guide explains their differences, conversions, and real industrial applications.
1. What Do These Units Mean?
1.1 %VOL — Volume Concentration
%VOL represents the volume percentage of a gas in air.
For example:
- Oxygen in air ≈ 20.9% VOL
- Methane in air can be expressed as volume percentage at higher concentrations
👉 It means:
“How much space the gas occupies in a given volume of air.”
Typical use:
- Oxygen (O₂)
- Carbon dioxide (CO₂)
- High-concentration hydrocarbons
1.2 ppm — Parts Per Million
ppm stands for parts per million (10⁻⁶).
1 ppm means:
1 unit of gas per 1,000,000 units of air
Typical use:
- Toxic gases: H₂S, CO, NH₃
- Volatile organic compounds (VOCs)
👉 ppm is used for low concentration gas monitoring, especially for health-related exposure limits.
1.3 %LEL — Explosion Risk Indicator
LEL (Lower Explosive Limit) is the minimum concentration at which a gas can ignite.
%LEL represents:
Current concentration as a percentage of the explosive limit
For methane:
- LEL = 5% VOL
- UEL = 17% VOL
So:
- 100% LEL = fully reached explosion threshold
- 50% LEL = half of explosion risk limit
⚠️ Important:
%LEL is not a concentration unit, but a safety risk indicator.
1.4 mg/m³ — Mass Concentration
mg/m³ represents:
milligrams of gas per cubic meter of air
It is widely used in:
- Occupational health standards
- Environmental monitoring
- Regulatory exposure limits
For gases, mg/m³ can be converted from ppm using molecular weight.
2. Key Conversions You Must Know
2.1 %LEL ↔ %VOL ↔ ppm
If:
- X = measured %LEL
- LEL = gas lower explosive limit (%VOL)
Then:
%VOL:
ppm:
Example 1 — Methane (CH₄)
- LEL = 5% VOL
- 25% LEL =
→ %VOL = 1.25%
→ ppm = 12,500 ppm
Example 2 — Hydrogen Sulfide (H₂S)
- LEL ≈ 4% VOL
- 10% LEL =
→ %VOL = 0.4%
→ ppm ≈ 4,000 ppm
Key Insight:
Toxic gases often become dangerous long before reaching explosive levels.
2.2 ppm ↔ mg/m³ Conversion
At 25°C and 1 atm:
Example — H₂S (MW = 34.08):
10 mg/m³ ≈ 7.2 ppm
3. Typical Gas Reference Values (Industrial Data)
| Gas | LEL (%VOL) | 10% LEL | 25% LEL | 100% LEL |
|---|---|---|---|---|
| Methane (CH₄) | 5.0 | 5,000 ppm | 12,500 ppm | 50,000 ppm |
| Hydrogen (H₂) | 4.0 | 4,000 ppm | 10,000 ppm | 40,000 ppm |
| Propane (C₃H₈) | 2.1 | 2,100 ppm | 5,250 ppm | 21,000 ppm |
| H₂S | 4.0 | 4,000 ppm | 10,000 ppm | 40,000 ppm |
| CO | 12.5 | 12,500 ppm | 31,250 ppm | 125,000 ppm |
👉 Notice:
- Highly toxic gases (H₂S, CO) are dangerous at very low ppm levels
- Flammable risk and toxic risk are not the same thing
4. Industrial Safety Applications
4.1 Combustible Gas Detection (%LEL)
Typical alarm settings:
- Low alarm: ≤ 25% LEL
- High alarm: ≤ 50% LEL
Used in:
- Oil & gas facilities
- Chemical plants
- Refineries
- Storage tanks
4.2 Toxic Gas Monitoring (ppm / mg/m³)
Used for:
- H₂S (hydrogen sulfide)
- CO (carbon monoxide)
- NH₃ (ammonia)
- VOC gases
Alarm strategy:
- Based on occupational exposure limits (OEL)
- Often far below explosive thresholds
4.3 Oxygen Monitoring (%VOL)
Normal air oxygen:
- 20.9% VOL
Safety limits:
- Low oxygen alarm: 19.5%
- High oxygen alarm: 23.5%
Risks:
- Oxygen deficiency → suffocation
- Oxygen enrichment → fire acceleration
4.4 Confined Space Entry
Before entry, measure:
- Oxygen (%VOL)
- Toxic gases (ppm)
- Flammable gases (%LEL)
Continuous monitoring is required during operation.
4.5 Hot Work (Welding / Cutting)
Gas must be below:
- 0.5% VOL (if LEL ≥ 4%)
- 0.2% VOL (if LEL < 4%)
Equivalent to approximately:
- ≤ 10% LEL for many gases
5. Common Misunderstandings in Gas Detection
❌ Mistake 1: “ppm and mg/m³ are interchangeable anytime”
They are not. Conversion depends on molecular weight and temperature.
❌ Mistake 2: “Low %LEL means safe”
Not true. Toxic gases can be deadly at ppm levels far below explosion limits.
❌ Mistake 3: “Alarm = safe limit”
Alarm means action required, not safe exposure.
❌ Mistake 4: One detector fits all applications
Different sensing technologies matter:
- Catalytic sensors → combustible gases
- Electrochemical → toxic gases
- IR sensors → hydrocarbons (not hydrogen)
❌ Mistake 5: Smell can replace detection
Many dangerous gases are odorless (e.g., CO), and odor fatigue is common (H₂S).
6. Engineering Summary
Each unit serves a different safety purpose:
- %LEL → explosion risk
- %VOL → volume concentration (O₂ / CO₂ / high gas levels)
- ppm → low-level toxic gas monitoring
- mg/m³ → regulatory mass concentration
In real industrial environments, gas safety must combine:
- Gas type
- Toxicity
- Explosion limits
- Oxygen level
- Ventilation conditions
- Proper instrumentation strategy
7. Industrial Gas Detection Solutions
At Zero Instrument, we provide industrial gas detection solutions for:
- Combustible gas (%LEL) monitoring
- Toxic gas (ppm) detection
- Oxygen (%VOL) measurement
- 4–20mA / RS485 / Modbus output systems
- Fixed and portable gas detection systems
👉 If you need a reliable gas detection solution for your project, feel free to contact us for technical support and quotation.