Environment

Temperature, humidity and pressure — the sensors that tell you what the air in a room, a cabinet or a greenhouse is doing.

Most measure several things at once, which is usually what you want: humidity without temperature is nearly meaningless, and both together cost less than apart.

Choosing

You need	Reach for
temperature and humidity, indoors	`SHT31`, `SHT40`, `SHTC3`, `AHT20`, `HDC1080`
temperature, humidity and pressure	`BME280` — the classic three-in-one
the same plus air quality	`BME680`
the cheapest usable option	`DHT12`, `AM2320`
an industrial humidity probe	`HYT271`
air flow	`FS3000`
ionising radiation	`GDK101`

BME280 is on more boards than anything else here, and deservedly: three measurements, one small part, an address that rarely collides. Start there unless something says otherwise.

The traps

Self-heating

A sensor next to a warm component reads the component, not the room. Humidity suffers worse than temperature: two degrees of self-heating throws relative humidity out by several percent.

Keep it away from regulators, displays and anything that dissipates power — ideally on a little tongue of board with slots cut around it.

Condensation lies

A sensor that has been below the dew point reads high for a long time afterwards, sometimes permanently if the cell has been contaminated. Where condensation is likely, choose a sensor with a protective membrane and expect to replace it.

A pressure sensor needs to breathe

A sealed enclosure with a pressure sensor inside measures the enclosure, not the weather. Give it a vent, and put a filter in the vent.

The chips

Environmental sensors

Chip	Bus	What it is
`AHT20`	I2C	I2C temperature + humidity sensor (Aosong) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x38; 80 ms conversion, CRC checked.
`AM2320`	I2C	I2C temperature + humidity sensor (Aosong, the wired DHT successor) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x5C; wake + read command, CRC16 checked; the first transaction after sleep is a wake-up NAK by design.
`BME280`	I2C	I2C environmental sensor — channels: 0 = temperature [°C], 1 = humidity [%RH], 2 = pressure [hPa]. Bosch datasheet compensation (calibration constants are read from the chip at startup). Normal mode, one sample every 250 ms.
`BME680`	I2C	I2C environmental sensor — channels: 0 = temperature [°C], 1 = humidity [%RH], 2 = pressure [hPa], 3 = gas resistance [Ω] (heater 300 °C / 100 ms, forced mode, 1 s poll). Datasheet float compensation. The gas channel is a RELATIVE air-quality signal that needs minutes after power-up — not a calibrated ppm. Address 0x76–0x77.
`BMP280`	I2C	I2C pressure + temperature sensor (BME280 without humidity) — channels: 0 = temperature [°C], 1 = pressure [hPa]. Bosch datasheet compensation, calibration read from the chip at startup.
`BMP388`	I2C	I2C barometer (also the BMP390 — chip id 0x50/0x60) — channels: 0 = pressure [hPa], 1 = temperature [°C]. Datasheet float compensation from the NVM coefficients. Address 0x76–0x77.
`DHT12`	I2C	I2C temperature + humidity sensor (the wired DHT11 heir) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x5C; 5-byte register read with a sum checksum.
`ENS210`	I2C	I2C temperature + humidity sensor (ScioSense) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Continuous run, valid bits checked. Fixed address 0x43.
`FS3000`	I2C	I2C air velocity sensor — channel 0 = raw 12-bit counts (the m/s curve is nonlinear and ordering-specific; approximate via variable gain/offset). Fixed address 0x28. VERIFY ON THE BENCH.
`GDK101`	I2C	I2C gamma radiation sensor — channel 0 = dose rate [µSv/h] (10-minute average). Address 0x18–0x1B. VERIFY ON THE BENCH.
`HDC1080`	I2C	I2C temperature + humidity sensor (TI) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x40; both values in one sequential acquisition.
`HDC2010`	I2C	I2C temperature + humidity sensor (TI) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Triggered measurement, little-endian data. Address 0x40–0x41.
`HTU21D`	I2C	I2C temperature + humidity sensor (also Si7021/SHT21 compatible) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x40; no-hold measurements alternate T/RH.
`HTU31D`	I2C	I2C temperature + humidity sensor (TE) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Conversion command + 6-byte CRC frame. Address 0x40–0x41.
`HYT271`	I2C	I2C temperature + humidity sensor (B+B/IST industrial) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x28; measurement request + 4-byte data fetch.
`SHT31`	I2C	I2C temperature + humidity sensor (Sensirion) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Single-shot high repeatability, CRC checked; a new measurement every cycle pass (~15 ms conversion).
`SHT40`	I2C	I2C temperature + humidity sensor (Sensirion, 4th gen) — channels: 0 = temperature [°C], 1 = humidity [%RH]. High precision command, CRC checked.
`SHTC3`	I2C	I2C temperature + humidity sensor (Sensirion SHTC3) — channels: 0 = temperature [°C], 1 = humidity [%RH]. Fixed address 0x70; wakeup + normal-mode measurement, CRC checked.

Water quality

Chip	Bus	What it is
`EZO`	I2C	Atlas Scientific EZO probe circuit (pH/EC/DO/ORP/RTD...) — channel 0 = the probe value as the circuit reports it. ASCII R command with a ~900 ms conversion; one value a second. The I2C address is set per probe (default 0x61–0x67 by type).

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.