SCD4x

Connects over

I²C

Address

0x62

Made by

Sensirion

Datasheet

sensirion.com

Use it

hardware {
    device scd4x: SCD4x
}

every 1s {
    let value: int = scd4x.co2()
    log("scd4x", "co2={value}")
}

On its own, the declaration puts the SCD4x on the first I²C bus (SDA 21, SCL 22 on the ESP32) at 0x62. To choose the pins or the address, declare the bus and attach the device: device scd4x: SCD4x on i2c0 at 0x62 with i2c i2c0(sda: 21, scl: 22).

Methods

• scd4x.calibrate(target_ppm: int) -> int

  Forced recalibration to a known CO2 level in ppm (e.g. 420 outdoors) after 3 min of stable readings; returns the correction applied in ppm, or -32768 on failure.

• scd4x.co2() -> int

  CO2 concentration in ppm.

• scd4x.humidity() -> float

  Relative humidity %.

• scd4x.ready() -> bool

  True once the first measurement has been read.

• scd4x.set_auto_calibration(on: bool)

  Turn automatic self-calibration on or off (on by default).

• scd4x.set_pressure(hpa: int)

  Ambient pressure in hPa for pressure compensation (e.g. from a BME280); can be called at any time.

• scd4x.set_temperature_offset(offset: float)

  Self-heating offset in °C subtracted from the temperature reading (default 4).

• scd4x.temperature() -> float

  Temperature in °C.

Wiring

Specifications

Co2

SCD40: 400 - 2000 ppm ±(50 ppm + 5%); SCD41: 400 - 5000 ppm ±(40 ppm + 5%)

Current

~15 mA average in periodic mode (peaks 175 mA)

Humidity

0 - 100% RH ±6%

Sample rate

one measurement every 5 s

Temp

-10 to 60 °C ±0.8 °C

Voltage

2.4V - 5.5V

Tips

• A new value arrives every 5 s; reading more often returns the last value. ready() is false until the first one (about 5 s after boot).
• Automatic self-calibration assumes it sees fresh outdoor air (~420 ppm) at least once a week; turn it off with set_auto_calibration(false) in rooms that never get that, and use calibrate(420) outdoors instead.
• calibrate(), set_temperature_offset() and set_auto_calibration() pause measurements for about 1 s and are not kept across power loss.
• Self-heating makes it read 2-4 °C warm in an enclosure; correct it with set_temperature_offset().

Alternatives worth knowing:

• SGP30: Cheaper, but eCO2 is estimated from VOCs, not measured.
• ENS160: VOC-based AQI / eCO2.