Terrestream uses three sensor chips. The Sensirion SEN66 (Switzerland) is the workhorse: a single calibrated module that delivers PM1, PM2.5, PM4, PM10, CO2, a VOC index from 1 to 500, an NOx index from 1 to 500, relative humidity, and temperature. Picking an integrated module instead of a rack of discrete sensors means fewer failure modes, fewer calibration drifts to chase, and one supply chain to vet. The Bosch BMP390L (Germany) handles barometric pressure: it is the lowest-noise consumer-grade pressure sensor on the market, with sub-Pascal sensitivity better than most weather-station MEMS. The TI OPT3001 (United States) reads ambient light in lux through a filter tuned to the human eye's photopic response, which is what makes a "is the room actually bright" reading rather than a raw photon count.
Inside the SEN66, four physical mechanisms run in parallel. The PM channel is a small fan pulling air through a chamber where a 660 nm laser shines across the stream; particles scatter the light onto a photodiode and the size + count distribution is inferred from the scattering pattern. Same physics as airport-grade optical particle counters, scaled down to a thumbnail-sized module. The CO2 channel uses Sensirion's PASens photoacoustic infrared spectroscopy, distinct from the transmission-cell NDIR used in most CO2 monitors: an IR LED pulses light at the 4.26 µm wavelength that CO2 absorbs, the absorbed pulses locally heat the gas, that produces a tiny pressure wave, and a microphone reads the wave amplitude. The amplitude is proportional to CO2 concentration. Photoacoustic detection lets the sensor fit in a fraction of the volume that a traditional NDIR transmission cell would need.
The VOC and NOx channels are metal-oxide-semiconductor films. A heated ceramic surface has its electrical conductance modulated by adsorbed gases: reducing gases (most VOCs, alcohols, aldehydes, hydrocarbons) push conductance one way, oxidizing gases (NOx, ozone) push it the other. The sensor cannot identify which gas, it sees the class. The Sensirion gas-index algorithm normalizes the raw signal against a slowly tracking baseline of the room's own typical air, producing the 1-to-500 index where 100 is the room's long-term median. The index is non-specific by chemistry but it tracks "is the air different from this room's normal" reliably, which is the question users actually want answered. RH and temperature in the same module use standard capacitive humidity and bandgap temperature elements, factory-calibrated against Sensirion reference instruments.
The two supporting chips do narrower jobs. The BMP390L pressure sensor is a piezo-resistive MEMS element with ±0.5 hPa typical accuracy; pressure is used for weather correlation, trend detection, and as a sanity input when computing dew point and absolute humidity. The OPT3001 lux sensor covers 0.01 to 83,000 lux with a photopic response curve, which means it weights wavelengths the way the human eye does instead of just counting photons. The dashboard uses lux to provide circadian context, to time notifications away from sleeping hours by default, and to disambiguate "the room is dark because nobody is home" from "the room is dark because it is 3 AM." Across the three packages that is six distinct sensing elements producing eleven physically measured signals; the dashboard derives a twelfth, the IAQ index, by combining several of them.
What the device cannot do, with the actual physical reasons. No CO: there is no electrochemical cell and no wavelength-specific NDIR for carbon monoxide, so the dashboard cannot read it and never claims to. Use a UL 2034 listed CO alarm instead, see CO vs CO2. No smoke detection: there is no ionization chamber and no dedicated photoelectric smoke chamber, so the device is not a smoke alarm. Keep your code-required smoke alarms. No specific VOC quantification: MOX is non-specific by chemistry, so "240 ppb of benzene" is not a number this sensor can produce; the index says "elevated relative to your baseline" with high confidence and that is what it is for. No outdoor measurement: the device is indoor-only by design (the PM fan and gas sensors are not weather-sealed), so outdoor readings come from Open-Meteo and the Google Air Quality API rather than from a sensor on your wall.
