Non-dispersive infrared (NDIR) is the dominant approach for quantitative CO₂ monitoring in buildings, greenhouses, fermentation, and medical anesthesia. Unlike chemical absorption or MOS inference, NDIR targets the 4.26 µm absorption band directly, delivering strong long-term stability. Senseiot integrates NDIR cores across IAQ modules and fixed monitors; this article explains how they work and how to deploy them reliably.
Fundamental NDIR Physics
CO₂ strongly absorbs near 4.26 µm. NDIR sensors filter that band, measure attenuation, and apply the Beer–Lambert law. Sources are MEMS IR emitters or LEDs; detectors are pyroelectric or photodiode elements.
Unlike scanning spectrometers, NDIR avoids full-spectrum dispersion—compact and fast for 400–5000 ppm and higher continuous monitoring.
High CO₂ selectivity with minimal VOC or humidity interference makes NDIR the default for demand-controlled ventilation. See Industry Applications.
Single vs Dual-Wavelength Optics
Single-channel NDIR is simple and low cost, but emitter aging and detector drift affect zero and span—often relying on ABC auto-calibration.
Dual-wavelength designs add a reference channel (~3.9 µm) not absorbed by CO₂, compensating source fluctuation and dirty optics in real time.
Premium modules use gold-plated cavities and folded paths for longer effective path length. Compare path length and noise in the Product Catalog.
Range, Accuracy, and Response
IAQ ranges 0–2000 or 0–5000 ppm at 1 ppm resolution; ±(50 ppm + 3% reading) is common. Process control may need 0–10% VOL with different gain settings.
T90 is often 30–120 s, limited by chamber exchange. Fast DCV may need small cells or external pumps.
Distinguish display resolution from measurement uncertainty. Request test curves via Request a Quote for bulk projects.
ABC Auto-Calibration and Field Span
Automatic Baseline Correction assumes periodic ~400 ppm fresh air to trim zero drift—effective in ventilated offices, risky in always-elevated or sealed industrial rooms.
Field calibration uses 400 and 1000 ppm reference gas after 30+ minutes warm-up at stable temperature and humidity.
Senseiot recommends manual calibration APIs and logs in BMS integrations to avoid silent ABC drift under abnormal conditions.
Humidity, Pressure, and Mounting
NDIR cross-sensitivity to humidity is low, but condensation can foul windows. Avoid sprays, direct diffusers, and sun; use breathable membranes or heaters if needed.
Barometric pressure affects density—apply compensation above 1500 m altitude or for high-precision labs.
Diffusion mounts need gentle exchange; pumped sampling must respect flow limits to avoid low readings. See Industry Applications.
NDIR vs Alternative Methods
Chemical absorption is accurate but offline; photoacoustic is sensitive but costly; MOS CO₂ inference drifts and is unsuitable for compliant IAQ.
NDIR balances cost, size, and stability—recognized by WELL and LEED for CO₂ monitoring.
Multi-parameter stations combine NDIR CO₂ with laser PM sensors on UART/I2C—see the Product Catalog.
Integration and Maintenance
Firmware should expose warm-up status, ABC enable, manual cal commands, and filtered alarms (>5000 ppm sustained) decoupled from PID loops to prevent hunting.
Inspect optical windows every 6–12 months in dusty plants; avoid organic solvents on coated surfaces.
Senseiot supplies NDIR modules, IAQ terminals, and reference firmware—submit needs via Request a Quote.