Professional Interpretation of Analog Alarm Thresholds for Smoke Sensors

Modern fire alarm systems have evolved from simple switch-based triggers to continuous analog monitoring. In industrial plants, data centers, rail transit, and high-value asset protection, whether thresholds are scientifically set directly determines whether a fire can be "detected early" instead of "confirmed after the fact." There is no universal fixed analog alarm threshold. It must be set considering sensor principles, signal type, installation environment, and application goals.

1. Basic Understanding of Analog Smoke Sensor Output

Nature of Analog Output

Analog smoke sensors output continuous smoke concentration information, not a simple "fire/no-fire" result. Common outputs include:

• 4–20mA current signal

• 0–5V / 0–10V voltage signal

• RS485 (Modbus RTU) continuous digital data

Among these, 4–20mA is the most widely used in industrial and fire systems, offering strong anti-interference capability and long-distance transmission suitability.

4–20mA Mapping to Smoke Concentration

• 4mA: clean air (zero smoke)

• 20mA: sensor full scale, maximum smoke concentration

• Alarm point: set at a percentage of full scale

• In practice, thresholds are often 15%–30% of full scale, not near full scale.

2. Threshold Differences Among Smoke Sensor Types

Different sensors perceive smoke particles differently, determining threshold logic.

Photoelectric Smoke Sensors

• Based on light obscuration or scattering changes, suitable for smoldering fires

• Full scale: typically 0.1–0.5 dB/m

• Level 1 warning: 0.02–0.05 dB/m

• Significant fire risk: above 0.08 dB/m

• Common in residential and public spaces

  
  

Ionization Smoke Sensors

• Respond quickly to fine combustion particles, suitable for fast-flaming fires

• Concentration unit: %obs/m

• Warning: 0.05–0.1 %obs/m

• High-risk fire: >0.15 %obs/m

• Advantage: fast response, more sensitive to environmental changes

Laser Scattering Smoke Sensors

• Detect very small particle sizes, core of high-end early warning

• Full scale: 0.05–0.2 dB/m

• Warning threshold: 0.01–0.02 dB/m

• Fire risk threshold: ≤0.03 dB/m

• Used in data centers, substations, cleanrooms with low tolerance for false or late alarms

3. Engineering Principles for Analog Alarm Thresholds

Multi-Level Alarms

• Level 1 (Pre-warning):

• Analog 15%–20% of full scale

• Corresponding current ~6.4–7.2mA

• Level 2 (Fire):

• Analog ≥30% full scale for ≥3 seconds

• Current ≥10.4mA

Environmental Impacts

• Dusty industrial areas → increase threshold 10%–20%

• High humidity or condensation → adjust using humidity compensation

• Strong ventilation or airflow → decrease threshold to compensate smoke dilution

4. Nexisense Intelligent Smoke Alarm Strategy

Focuses on judgment logic rather than single values.

• Multi-dimensional detection:

• Threshold concentration check

• Rate of smoke change (>5% FS/sec)

• Exceedance duration verification

• Trend analysis for abnormal growth

• False alarm mitigation:

• Differentiate smoke vs. dust particle sizes

• Reduce steam or water interference

• Enhance EMC design for industrial environments

5. Typical Threshold References by Application

• Residential & offices → lower thresholds, early warning, control false alarms

• Industrial & warehouses → tolerate background interference, moderate thresholds, emphasize duration

• Data centers & substations → earliest detection, laser scattering + low threshold + time confirmation

FAQ

• Can analog thresholds be unified? No, calibrate separately per environment even for same model.

• Must 10.4mA always indicate fire? It’s a reference, adjust according to range and risk level.

• Can one alarm point be used? Yes, but reduces pre-warning value.

Conclusion

Setting analog alarm thresholds is a system engineering task, balancing sensitivity, stability, false alarms, and response time. In shifting from passive fire response to proactive early warning, proper threshold design is often more important than expensive equipment. Nexisense continues to provide validated, practical, high-reliability smoke monitoring solutions for complex scenarios.