# Alarm Thresholds #safety #system-design #alarm Alarm thresholds define the gas concentration levels at which different alarm levels are triggered, initiating appropriate response actions. ## Typical Alarm Levels ### Alarm Range 1 (Low Alarm) - Typically set at 20% LEL for flammable gases - Activates countermeasures (e.g., ventilation) - Early warning level - Allows time for preventive action ### Alarm Range 2 (High Alarm) - Typically set at 40% LEL for flammable gases - Activates emergency measures (e.g., shutdown) - Critical level requiring immediate action - Should rarely be reached in properly designed systems ## Alarm Response - **Alarm Range 1**: Countermeasures activated to reduce concentration - **Alarm Range 2**: Emergency measures activated (shutdown, evacuation) - Properly designed systems rarely reach Alarm Range 2 ## Threshold Selection - Based on gas properties (LEL, toxicity) - Considered response time requirements - Account for system response delays - Must allow time for safe response ## Safety Considerations - Thresholds must be set below dangerous levels - Account for sensor response time - Consider environmental conditions - Regular review and adjustment may be needed