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. The reading is a concentration in ppm, not an index, and the sensor's automatic self-calibration keeps it within Sensirion's accuracy specification as long as it sees fresh air (around 400 ppm) at least once a week. 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 NOx 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 (NOx, 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 learns a rolling baseline of "your normal air" (index 100) and reports deviations on a 1–500 scale; the NOx index works the same way but rests at 1 in clean air. 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 NOx indexes as "compared to your normal." A statement that VOC is elevated above the room's baseline is precise; a VOC figure given in ppb or µg/m³ is a derived estimate with substantial uncertainty.
