Terrestream uses three sensor packages. 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. An integrated module means fewer failure modes, fewer separate calibrations to track, and one supply chain to vet. The Bosch BMP390L (Germany) measures barometric pressure. The TI OPT3001 (United States) reads ambient light in lux through a filter matched to the human eye's response, so the reading reflects how bright the room looks rather than a raw photon count.
Inside the SEN66, four physical mechanisms run in parallel. The particle channel is a small fan pulling air through a chamber where an infrared laser (850 nm) crosses the stream; particles scatter the light onto a photodiode, and size and count are inferred from the scattering. It sizes particles from 0.3 µm up, and it calculates PM4 and PM10 from the measured size distribution rather than counting coarse particles directly. The CO2 channel is photoacoustic: pulses of infrared light at the wavelength CO2 absorbs make the gas expand slightly, and a tiny microphone measures the resulting pressure wave, whose strength tracks the CO2 concentration. That design fits in a fraction of the volume of the transmission-cell NDIR sensors used in many other CO2 monitors.
The VOC and NOx channels are metal-oxide sensing. A heated surface changes its electrical conductance as gases adsorb: reducing gases (most VOCs, alcohols, aldehydes, hydrocarbons) push it one way, oxidizing gases (NOx, ozone) the other. The sensor cannot identify which gas; it sees the class. The Sensirion gas-index algorithm turns the raw signal into an index against the room's own recent history. On the VOC index, 100 means typical for this room; on the NOx index, 1 is the normal baseline and higher values mean an oxidizing-gas event. Humidity and temperature in the same module come from standard capacitive humidity and temperature elements, factory-calibrated by Sensirion.
The two supporting chips do narrower jobs. The BMP390L pressure sensor is a MEMS element that Bosch specifies at ±0.50 hPa absolute accuracy; pressure is used for weather context and trend detection, and to understand infiltration. The OPT3001 light sensor covers 0.01 to 83,000 lux and gives context: daylight, lamps and screens, and the daily rhythm of a room. Across the three packages that is six sensing elements producing eleven measured signals. Terrestream calculates a twelfth, the IAQ index: the higher of the EPA AQI sub-indices for PM2.5 and PM10.
Those signals are different kinds of evidence, and the difference matters. Direct measurements report a physical quantity with vendor-published units and limits: particle mass, CO2, temperature, humidity, and pressure. Index outputs, the VOC and NOx indexes, show relative change and events, not laboratory concentrations. Context such as light helps explain what is happening without proving anything on its own. Interpretation is the software layer on top: baselines, patterns, recovery, and likely sources. That layer has to stay honest about what the hardware actually measured. The Measurement Evidence page shows the hardware, the calculations, and the limits.
What the device cannot do, with the physical reasons. No CO: there is no electrochemical cell and no carbon-monoxide-specific optical channel, so Terrestream cannot read CO and never claims to. Use a UL 2034 listed CO alarm; see CO vs CO2. No smoke alarm: there is no ionization chamber and no dedicated photoelectric smoke chamber, so keep your code-required smoke alarms; see smoke detector vs IAQ monitor. No specific VOC quantification: metal-oxide sensing is non-specific, so "240 ppb of benzene" is not a number this sensor can produce; the index says "different from this room's normal," and that is what it is for. No outdoor measurement: the device is indoor-only by design, so outdoor readings come from Open-Meteo and the Google Air Quality API, not from a sensor on your wall.
