Introduction
While commonly used for motion detection, pyroelectric sensors are also the heart of Non-Dispersive Infrared (NDIR) gas sensors. This article explains the principles and applications.
NDIR Principle
NDIR gas sensing works on the principle that many gases absorb infrared at specific wavelengths. By measuring the amount of IR absorbed, the gas concentration can be determined.
A typical NDIR sensor consists of:
- An infrared source (often a pulsed incandescent lamp or MEMS emitter).
- A gas cell (optical path through the sample gas).
- An optical filter that selects the absorption wavelength of the target gas.
- A pyroelectric detector (PIR sensor) to measure the transmitted IR.
Why Pyroelectric Detectors?
Pyroelectric detectors are ideal for NDIR because:
- They respond to modulated IR (the source is typically pulsed).
- They have high sensitivity in the mid-IR region where gases absorb.
- They operate at room temperature (no cooling needed).
- They are low cost compared to cooled detectors.
- They can be made with specific spectral response via filters.
Gas-Specific Filters
The key to NDIR is the optical filter placed in front of the detector. Common gas absorption bands:
| Gas | Absorption Wavelength |
|---|---|
| CO2 | 4.26 µm |
| CO (carbon monoxide) | 4.67 µm |
| CH4 (methane) | 3.33 µm or 7.7 µm |
| NOx | 5.3 µm |
| SO2 | 7.3 µm |
| Hydrocarbons | 3.4 µm (C-H stretch) |
Dual-Channel NDIR
Many NDIR sensors use two detectors: one with the gas-specific filter (active channel) and one with a reference filter at a non-absorbing wavelength. This compensates for source aging, window contamination, and other common-mode effects.
Pyroelectric Detectors for NDIR
Specialized pyroelectric detectors for NDIR often feature:
- Dual elements with integrated filters (active and reference).
- Hermetic packaging.
- Low noise for high sensitivity.
- Wide dynamic range.
Examples: InfraTec LIE-332f, Pyreos PYS3228, Murata IRA-S series with custom filters.
Circuit Design for NDIR
The pyroelectric detector produces a small AC signal at the modulation frequency (typically 1-10 Hz). A lock-in amplifier or synchronous demodulation extracts the signal amplitude, which is proportional to the transmitted IR.
Applications
- CO2 sensors: HVAC control, indoor air quality, horticulture.
- Methane detectors: Natural gas leak detection, mining safety.
- Refrigerant leak detection: HVAC servicing.
- Automotive: Cabin air quality, exhaust sensing.
- Medical: Capnography (CO2 in breath).
Advantages over Chemical Sensors
- Long life (no consumables).
- No poisoning.
- Selective (with proper filter).
- Fast response.
- Can measure multiple gases with multiple detectors.
Limitations
- Higher cost than some chemical sensors.
- Requires optical path (can be affected by dust).
- Power consumption (source needs power).
- Water vapor interference (in some bands).
Conclusion
The same pyroelectric technology used in motion sensors is the foundation of NDIR gas detection. Understanding this application broadens your view of PIR sensor capabilities.