Mismatched CO₂ readings are common in BMS, classrooms, cold storage, and greenhouses. Often the issue is not a "bad sensor" but combined effects of technology, location, air mixing, and maintenance.
How does sensing technology affect readings?
Most CO₂ sensors use NDIR absorption at specific wavelengths. Optical path length, filter bandwidth, detector gain, and temperature/barometric algorithms vary by vendor. Without harmonized calibration, ±30–50 ppm spread at 400–2000 ppm indoors is possible.
Pumped sampling vs. diffusion changes response time and spatial representativeness. A probe at a return grille vs. a dead corner can differ by hundreds of ppm. Senseiot NDIR modules offer multiple ranges and compensation options in our product catalog.
Why does mounting location matter?
CO₂ is slightly denser than air and can accumulate near the floor or breathing zone when ventilation is poor. Guidelines often recommend 1.1–1.8 m height, away from doors, diffusers, and exhausts. Ceiling return vs. desk height installs show systematic bias.
Direct sun, heat sources, and cold bridges add drift. For comparisons, co-locate units under identical micro-climate conditions. For layout review, contact us.
Are swings normal when ventilation changes?
Outdoor air fraction, return ratio, damper position, and occupancy change indoor CO₂ balance. VAV systems may cut fresh air at part load; door openings cause short dips. Snap comparisons are unreliable—use trends and steady-state windows.
For control, use zone averages with deadbands (e.g., 50 ppm). Senseiot transmitters offer analog and Modbus for DDC/BMS filtering and sensor fusion.
Do calibration, altitude, and pressure compensation matter?
Factory calibration assumes standard barometric pressure. At altitude, missing pressure compensation causes proportional error. Some low-cost modules compensate temperature only—not ideal for high elevation or sealed volumes.
Single-point fresh-air zero helps baseline; span checks need certified 1000 ppm or higher gas. One recently calibrated unit vs. one neglected unit will diverge. Standardize procedures and cross-check with a reference analyzer.
How to accept and troubleshoot fairly
Steps: ① match model, range, filtering, and units (ppm vs. %vol); ② co-locate probes 20–30 minutes before reading; ③ inspect optics and firmware; ④ verify zero/span in fresh air or certified gas.
If co-located error still exceeds spec, consider service or sensor replacement. Senseiot supports module, transmitter, and system solutions with batch consistency tests. Request a quote for project support.