Air quality

Fourteen sensors for CO₂, volatile compounds, particulates and gases.

These are the least straightforward parts in the catalogue, and it is worth knowing why before choosing: most of them do not measure what their label suggests. They measure something correlated with it, and the correlation is only as good as the calibration behind it.

Real CO₂ or estimated CO₂

True NDIR sensors — SCD30, SCD41 — measure carbon dioxide optically. They are bigger, slower and dearer, and their number means what it says.

eCO₂ sensors — CCS811, ENS160, SGP30, IAQCORE — measure volatile compounds and estimate CO₂ from them. Breathe on one and it rises; open a tin of solvent nearby and it rises just as much. For "is this room stuffy" that is fine. For ventilation that must hold CO₂ below a limit, it is not.

If somebody is going to read the number as parts per million of CO₂, pay for NDIR.

Choosing

You need	Reach for
CO₂ you can defend	`SCD41`, `SCD30`
a cheap "the air is stale" signal	`ENS160`, `SGP4X`, `CCS811`
particulates (PM2.5, PM10)	`PMSA003I`, `SPS30`, `SEN5X`, `HM3301`
formaldehyde	`SFA30`
ozone	`SEN0321`
a general air-quality index	`ENS160`, `SGP4X`

SEN5X is worth singling out: particulates, VOC, temperature and humidity in one module, which saves both board space and the cross-compensation work below.

The traps

They need burning in, then re-baselining

Most gas sensors read badly for the first hours or days of their life. Several also expect to see clean air regularly so they can re-baseline — a sensor that never sees fresh air drifts.

Put the sensor where fresh air reaches it sometimes, and do not trust the first day's readings.

Temperature and humidity change the answer

Every gas sensor is sensitive to both. Modules with their own temperature and humidity cell compensate themselves; the ones without need you to feed them values, or they drift with the weather.

None of these are safety instruments

A sensor saying the air is breathable is not a gas detector and must never be used as one. Life-safety detection uses certified instruments with their own alarms, tested to a standard — not a breakout board on an I2C bus.

The chips

Air quality

Chip	Bus	What it is
`AGS10`	I2C	I2C TVOC sensor (ASAIR) — channel 0 = TVOC [ppb]. The chip refreshes every ~1.5 s and the driver polls every 2 s; CRC checked. Fixed address 0x1A. VERIFY ON THE BENCH.
`CCS811`	I2C	I2C air quality sensor — channels: 0 = eCO2 [ppm], 1 = TVOC [ppb]; drive mode 1 s. Needs ~20 min burn-in after power-up before the values mean anything. Address 0x5A–0x5B.
`ENS160`	I2C	I2C air quality sensor (ScioSense) — channels: 0 = eCO2 [ppm], 1 = TVOC [ppb], 2 = AQI index 1–5. Standard operating mode; the first minutes after power-up are warm-up. Address 0x52–0x53.
`HM3301`	I2C	I2C particulate matter sensor (Seeed/Tomoto) — channels: 0 = PM1.0, 1 = PM2.5, 2 = PM10 [µg/m³] (atmospheric). 29-byte frame with a sum checksum. Fixed address 0x40.
`IAQCORE`	I2C	I2C VOC sensor (ams iAQ-core) — channels: 0 = eCO2 [ppm], 1 = TVOC [ppb]. Plain 9-byte read; samples with a non-zero status (warm-up, drift) are dropped. Fixed address 0x5A.
`PMSA003I`	I2C	I2C particulate matter sensor (Plantower) — channels: 0 = PM1.0, 1 = PM2.5, 2 = PM10 [µg/m³] (atmospheric values). Fixed address 0x12; 32-byte frame with checksum.
`SCD30`	I2C	I2C CO2 sensor (Sensirion NDIR, the SCD41 predecessor) — channels: 0 = CO2 [ppm], 1 = temperature [°C], 2 = humidity [%RH]. Fixed address 0x61; continuous measurement every 2 s, float values, CRC checked.
`SCD41`	I2C	I2C CO2 sensor (Sensirion NDIR) — channels: 0 = CO2 [ppm], 1 = temperature [°C], 2 = humidity [%RH]. Fixed address 0x62; periodic measurement, a fresh value every ~5 s, CRC checked.
`SEN0321`	I2C	I2C ozone sensor (DFRobot Gravity) — channel 0 = O3 [ppb], automatic mode, 1 s poll. Register map from the vendor wiki. Address 0x70–0x73 (DIP switches). VERIFY ON THE BENCH.
`SEN5X`	I2C	I2C environmental node (Sensirion SEN54/55) — channels: 0–3 = PM1.0/2.5/4.0/10 [µg/m³], 4 = humidity [%RH], 5 = temperature [°C], 6 = VOC index, 7 = NOx index (SEN55; 0 elsewhere). Fixed address 0x69.
`SFA30`	I2C	I2C formaldehyde sensor (Sensirion) — channels: 0 = HCHO [ppb], 1 = humidity [%RH], 2 = temperature [°C]. Continuous measurement, CRC checked. Fixed address 0x5D.
`SGP30`	I2C	I2C VOC air quality sensor — channels: 0 = eCO2 [ppm], 1 = TVOC [ppb]. Fixed address 0x58; 1 Hz measurement, CRC checked. The first ~15 s after power-up report the fixed 400/0 warm-up values; baseline persistence is not stored.
`SGP4X`	I2C	I2C VOC sensor (SGP40/41) — channel 0 = RAW resistance counts (the Sensirion VOC-index algorithm is not in the firmware; trends via variable gain/offset). Fixed address 0x59.
`SPS30`	I2C	I2C particulate matter sensor (Sensirion) — channels: 0 = PM1.0, 1 = PM2.5, 2 = PM4.0, 3 = PM10 [µg/m³]. Fixed address 0x69; integer output format, CRC checked. VERIFY ON THE BENCH.

Every entry above is in the editor: pick the chip, give it an address or a chip-select pin, and its channels become variables. See Adding a peripheral for the click-through.