Gas detection is a critical part of safety in many industrial and property environments. When alarm thresholds are set according to gas-specific recommendations, they help ensure timely reactions to hazardous situations without triggering unnecessary alarms.
Different units are used depending on the type of gas and the measurement purpose. The right unit is selected based on the gas’s characteristics and associated risks:
- %LEL (Lower Explosive Limit) for flammable gases
- ppm (parts per million) for toxic gases
- %VOL (volume percentage) for oxygen and carbon dioxide
Flammable Gases (%LEL)
%LEL tells how close a gas is to its ignition point. Alarm thresholds are typically set at two levels:
- Warning: 10% LEL
- Danger: 20% LEL (evacuation or suppression actions)
For example, hydrogen has an LEL of 4% in air. Therefore, 10% LEL corresponds to just 0.4% hydrogen—enough to warrant action. Propane, being heavier than air, behaves differently, making detector placement critical: hydrogen rises, while propane settles near the floor.
Toxic Gases (ppm)
Toxic gases are measured in ppm. Alarm levels are often based on occupational exposure limits (e.g. HTP values in Finland). Limits vary by gas type and application.
Typical thresholds:
- Carbon monoxide (CO):
- Warning: 25 ppm
- Danger: 50 ppm
- Hydrogen sulfide (H₂S):
- Warning: 5 ppm
- Danger: 10 ppm
- Ammonia (NH₃):
- Warning: 25 ppm
- Danger: 50 ppm
- Chlorine (Cl₂):
- Warning: 0.5 ppm
- Danger: 1 ppm
Quick and reliable alarms are especially crucial, as even low concentrations can cause serious health effects rapidly.
Oxygen and Carbon Dioxide (%VOL)
Oxygen and CO₂ levels are measured in volume percentage. Both low and high levels can be dangerous.
Oxygen (O₂):
- Normal: 20.9%
- Warning: 19%
- Danger: 17% or lower
- Upper limit: >23.5% (increased fire risk)
Carbon Dioxide (CO₂):
- Warning: 0.5%
- Danger: 1.5–2%
- Critical: >5%
These gases can accumulate quickly in enclosed or poorly ventilated areas.
Planning is Key
Properly set alarm thresholds are the heart of any gas detection system. Too high, and you may miss a real hazard. Too low, and you’ll face unnecessary shutdowns and costs.
Detector’s experts help define site-specific alarm thresholds and calibrate your equipment to meet current standards and operational needs.
Want to ensure your workplace alarm thresholds are accurate and up to date?
Contact us and we’ll help build a safe and reliable solution tailored to your needs.