How the sensor sees: optical CO₂ vs MOX

The two gas-sensing technologies inside the Terrestream sensor work on completely different principles, and understanding the difference helps explain why the readings behave the way they do.

Photoacoustic infrared spectroscopy is the technology the Sensirion SEN66 uses for its CO₂ channel. It is an optical, infrared-absorption method, but distinct from the transmission-cell NDIR sensors common in other CO₂ monitors. A small infrared source pulses at the wavelength CO₂ absorbs (4.26 µm); the absorbing molecules heat a small puff of gas, which produces a pressure wave a sensitive microphone picks up. The signal is proportional to CO₂ concentration via the Beer-Lambert law. The reading is absolute (no baseline needed), the physics is well-understood, and the calibration is stable for years. The cost: infrared gas sensors are bigger, slower, and warmer than gas sensors of the next type.

MOX, Metal-Oxide, is the technology behind the SEN66's VOC and NO_x indexes. A heated tin-dioxide ceramic surface (kept around 300 °C) has its electrical conductance modulated by adsorbed gases, reducing gases (most VOCs) push conductance one way, oxidizing gases (NO_x, ozone) push it the other. The sensor doesn't identify which gas; it sees the class.

Two consequences. First, MOX readings need a baseline, Sensirion's VOC index algorithm builds a rolling 24-hour baseline of "your normal air" and reports relative deviations on a 0–500 scale. Second, MOX surfaces drift slowly over months as the ceramic ages; Sensirion compensates algorithmically but absolute ppb-level VOC measurements are not possible from this class of sensor.

In practice: read the CO₂ ppm as an absolute value (the published indoor guidelines apply directly). Read the VOC and NO_x indexes as "compared to your normal." When the dashboard says "VOC elevated above baseline," that statement is precise; when it says "VOC at 240 ppb," that's a derived estimate with substantial uncertainty.