SCD4x Library

The SCD4x Library provides an interface to interact with Sensirion SCD4x true CO2 sensors using the I2C protocol in Arduino.

Warning: Auto-Calibration Issue and Recommended Actions

The SCD4x CO2 sensors come with a default auto-calibrate mode that assumes the lowest CO2 level from the past week as 400ppm. However, it's crucial to be aware that this auto-calibration can result in inaccurate readings if the sensor is placed in an environment that doesn't reach 400ppm CO2 within a week. This issue is particularly important to note when using the sensor for scientific purposes.

To address this issue, it is recommended to permanently disable the auto-calibration mode by using the provided code snippet:

#include "scd4x.h"

SCD4X co2;

void setup() {
  // Initialize the SCD4x library
  co2.begin();

  // Disable auto-calibration
  co2.setCalibrationMode(false);

  // Save the settings to EEPROM
  co2.saveSettings();
}

void loop() {
  // Your code here
}

Please exercise caution when using the saveSettings() command as it writes to the EEPROM, which has a limited lifespan of approximately 2000 write cycles before potential failure.

Despite the auto-calibration issue, the Sensirion SCD4x CO2 Sensors remain an excellent choice for monitoring indoor air quality, as they have proven to be more accurate than other popular eCO2 sensors.

Factory Calibration with Auto-Calibration Off

Features

• Supports multiple I2C busses: scd4x.begin(Wire1);
• No extra dependencies required
• Implements only necessary functions
• Uses doubles (64-bit floating-point numbers) for accurate data calculations

Warnings

• not all functions are implemented
• not compatible with other SCD4x Arduino libraries
• only tested with the esp32

Setup

#include "scd4x.h"
SCD4X co2;
double CO2 = 0, temperature = 0, humidity = 0;

Wire.begin();
co2.begin(Wire);
co2.startPeriodicMeasurement();

Loop

while (co2.isDataReady() == false) {
	vTaskDelay(50 / portTICK_PERIOD_MS); //check every 50ms
}

if (co2.readMeasurement(CO2, temperature, humidity) == 0) {
	Serial.printf("%4.0f,%2.1f,%1.0f\n", CO2, temperature, humidity); //nice formatting of data
}
vTaskDelay(4750 / portTICK_PERIOD_MS); //new data available after approx 5 seconds

🖼️ Schematic

• The SCD4x sensor can draw up to 205mA peaks at 3.3V (only 18mA average), so ensure you have a robust power source (the above schematic has been tested to work).
• You only need to solder the 6 shown pins and the thermal pad on the underside to get it working.
• Unfortunately, soldering these sensors with a soldering iron is not easy; an oven or hotplate seems to be the only way.
• Watch out for a temperature offset if you place the sensor in a case or enclosure.

Function Refreence

Function	Description
begin()	Initializes the library. Must be called before any other library functions.
isConnected()	Checks if the device is correctly connected by verifying the response, manufacturer ID, and part ID.
startPeriodicMeasurement()	Starts periodic measurement, with new data available in approximately 5 seconds.
stopPeriodicMeasurement()	Stops periodic measurement. Wait at least 500ms before sending further commands.
readMeasurement()	Reads the sensor output, including CO₂ concentration in ppm, temperature in °C, and relative humidity in %RH. The measurement data can only be read out once per signal update interval as the buffer is emptied upon read-out.
isDataReady()	Checks whether new measurement data is available for read-out.
setCalibrationMode()	Sets the calibration mode and stores it in the EEPROM of the SCD4x. The automatic self-calibration algorithm assumes that the sensor is exposed to the atmospheric CO2 concentration of 400 ppm at least once per week. Use this function sparingly to avoid unnecessary wear of the EEPROM.
getCalibrationMode()	Gets the calibration mode. Returns true if auto calibration is enabled, false otherwise.
saveSettings()	Stores settings in the EEPROM of the SCD4x. Wait at least 800ms before sending further commands. EEPROM is guaranteed to endure at least 2000 write cycles before failure.
getErrorText()	Converts an error code into descriptive text. Returns a pointer to a constant character array containing the descriptive text of the error. If the error code is not recognized, "Unknown error" is returned.

Credits

Based on the work of Raphael Nestler and everyone at Sensirion AG. Originally created by Raphael Nestler in 2021. https://github.com/Sensirion/arduino-i2c-scd4x

To help support my work, check out my store: https://keastudios.co.nz/