Introduction
Pyroelectric sensors are widely used in flame detectors for industrial safety systems. Unlike motion detection, flame detection relies on sensing the characteristic flicker of flames and their spectral signature.
Flame Characteristics
Flames from hydrocarbon fires have two key properties useful for detection:
- Spectral emission: Strong emission bands from CO2 at 4.3 µm and from water vapor at various wavelengths.
- Flicker frequency: Flames flicker at characteristic frequencies, typically 1-20 Hz, depending on fuel and conditions.
How Pyroelectric Sensors Detect Flames
A flame detector using a pyroelectric sensor typically includes:
- An optical filter that passes only the flame-specific wavelength (e.g., 4.3 µm for CO2).
- A pyroelectric detector that responds to modulated IR (the flame flicker).
- Signal processing that looks for the characteristic flicker frequency.
Multi-Spectral Flame Detection
To reduce false alarms from other IR sources (sunlight, heaters, hot machinery), professional flame detectors use multiple sensors with different filters:
- Channel A: Flame wavelength (e.g., 4.3 µm).
- Channel B: Reference wavelength (e.g., 5.0 µm) where flames don’t emit but hot objects do.
- Channel C (optional): UV sensor for additional confirmation.
Detection is declared only when Channel A sees a signal, Channel B sees little or no signal, and the flicker frequency is correct.
Pyroelectric Detectors for Flame Sensing
Specialized dual-channel pyroelectric detectors are available with integrated filters:
- InfraTec LIE-332f: Dual-channel with integrated filters for flame detection.
- Pyreos PYS3228: Dual-channel with custom filter options.
- Excelitas PY1420: Pyroelectric detector with integral filter.
Flicker Frequency Analysis
The flame flicker frequency is extracted using analog or digital filtering. A bandpass filter centered on the expected flicker frequency (e.g., 5 Hz) passes the flame signal while rejecting DC and high-frequency noise.
Comparison with Other Flame Detection Technologies
| Technology | Pros | Cons |
|---|---|---|
| Pyroelectric IR | Low cost, room temp operation, good for hydrocarbon fires | Can be fooled by modulated IR sources |
| UV sensors | Fast response, detects all fires | False alarms from lightning, welding; expensive |
| IR cameras | Visual confirmation, wide area | Very expensive, complex |
| Thermocouple | Simple, reliable | Slow, requires contact |
Applications
- Industrial plants: Petrochemical, oil & gas, power generation.
- Aircraft hangars: Detection of fuel fires.
- Mining: Conveyor belt fire detection.
- Data centers: Early fire warning.
- Residential: High-end smoke/heat detectors.
Design Considerations
- Lens material: Must be transparent to 4.3 µm (germanium, sapphire, or special IR-transparent materials).
- Window contamination: Oil, dust, and water can block IR. Regular cleaning or self-test features needed.
- Response time: Industrial standards often require detection within 3-5 seconds.
- False alarm immunity: Must reject sunlight, arc welding, hot machinery.
Standards and Certifications
- EN 54-10: Fire detection and fire alarm systems – Flame detectors.
- UL 268: Smoke detectors (some include flame sensing).
- FM 3260: Radiant energy-sensing fire detectors.
Conclusion
Pyroelectric sensors are a proven technology for flame detection. By combining spectral filtering and flicker analysis, they provide reliable fire detection at a fraction of the cost of imaging systems.