The principle of Particulate Matter (PM) sensors primarily relies on optical scattering (light scattering), though other methods exist. Here's a breakdown of the key principles:
Core Principle: Light Scattering (Most Common):
Light Source: An infrared (IR) or laser diode emits a beam of light into a sensing chamber through which air containing particles is drawn (often by a small fan or pump).
Particle Interaction: As airborne particles (dust, smoke, pollen, etc.) pass through this light beam, they scatter the light in various directions. The amount and pattern of scattering depend on the particle's size, shape, composition, and concentration.
Photodetector: A sensitive photodetector (like a photodiode or phototransistor), positioned at a specific angle (often 90° or less commonly forward/backward scatter), detects the scattered light.
Signal Conversion: The photodetector converts the scattered light intensity into an electrical signal.
Correlation to PM Concentration: The intensity of this scattered light signal is correlated with the concentration (mass per volume, typically µg/m³) of particles in the air. Higher particle concentration leads to more scattered light and a stronger signal.
Size Differentiation (PM2.5/PM10): Some sensors can estimate particle size distribution using:
Optical Models: Algorithms analyzing the scattering pattern/intensity differences.
Size-Selective Inlets: Physically separating particles above a certain size (e.g., >10µm for PM10) before they enter the optical chamber.
Calibration: Calibration against reference instruments for specific size fractions (like PM2.5).
Alternative Principle: Beta Attenuation (Used in Reference/Regulatory Monitors):
Radioactive Source: A weak radioactive source (like Carbon-14) emits beta particles (electrons).
Filter Tape: A filter tape collects airborne particles drawn through the instrument.
Attenuation Measurement: Beta particles pass through a clean section of the filter tape and are detected by a sensor, establishing a baseline. Then, beta particles pass through the particle-laden section of the tape.
Mass Calculation: Particulate matter mass on the filter absorbs/scatters beta particles, reducing the number reaching the detector. The attenuation (reduction) in beta particle count is directly proportional to the mass of particles collected on the filter. Combined with the sampled air volume, it gives PM mass concentration (e.g., µg/m³). This method is highly accurate for mass measurement but more complex and expensive.
Other Less Common Principles:
Resonant Microbalance (TEOM - Tapered Element Oscillating Microbalance): Particles are collected on a vibrating filter tip. The mass change alters the tip's resonant frequency, which is measured to determine mass concentration.
Electrostatic Detection: Measures the charge acquired by particles passing through a charging section or the charge naturally present on particles.
Key Considerations for Optical (Scattering) Sensors (Most Common Type):
Calibration: Requires calibration against reference instruments (like beta attenuation monitors) due to variations in particle properties affecting scattering. Factory calibration is common, but environmental factors (humidity, particle type) can cause drift.
Humidity Sensitivity: Water vapor can condense on particles or scatter light itself, leading to overestimation, especially at high humidity. Advanced sensors incorporate humidity sensors and compensation algorithms.
Particle Composition Sensitivity: Different particle types (e.g., soot vs. dust) scatter light differently. Calibration is often optimized for typical ambient mixtures.
Size Range Limits: Very small particles (<~0.3µm) and very large particles may scatter insufficient light or bypass the detection chamber, limiting the effective size range.
Resolution/Lower Detection Limit: There is a minimum concentration below which the sensor cannot reliably distinguish signal from electronic noise.
Applications:
Optical PM sensors are widely used due to their relatively low cost, small size, and real-time output in:
Consumer air purifiers
Indoor air quality monitors
Wearable pollution trackers
Industrial process monitoring
Smart HVAC systems
Urban air quality sensor networks (though with calibration/quality control considerations)
In summary, while various principles exist, the dominant technology in consumer and many industrial PM sensors is optical light scattering, where the amount of light scattered by airborne particles passing through a beam is measured to estimate particle mass concentration, often calibrated for specific size fractions like PM2.5 or PM10.
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