Designed for use during caving to verify that it is safe to go deeper, as CO2 can build up at the bottom of some caves due to various factors.

Building it

Total: ~50€

Schematics

Sensor schematic

Programmation

The Arduino communicates to the sensor via UART using the MH-Z-CO2-Sensors library

VSCode and Platformio are used for development, building and dependency management: Getting started  (Visual Studio Code est un éditeur de code source léger mais puissant qui s'exécute sur votre bureau et est disponible pour Windows, macOS et Linux. Il prend en charge JavaScript, TypeScript et Node.js et dispose d'un riche écosystème d'extensions pour d'autres langages (tels que C++, C#, Python, PHP, Go) et environnements d'exécution (tels que .NET et Unity)1.)

Working state

At boot a loading bar is displayed as the sensor need arround 20-30 seconds before we can read a meaningfull value from it.

Then a new value is read every 4 seconds. There is a small indicator at the top left to indicate that the device is still alive. It is best to leave the device on for a few minutes so that the value stabilizes.

A battery charge should provide at least 5 hours of reading

Limitations

This is not an accurate device it should be used for informational purposes only. Its reading value has not been verified against other accurate CO2 sensors

The sensor datasheet states an accuracy of 50 ppm + 5% of the reading value. Operating conditions range from -10°C to 50°C and 0 to 95% relative humidity without condensation.

The datasheet also states that it should be calibrated at least every 6 months by placing the sensor in a reference environment at 400ppm for 20 min and triggering the calibration function.

Improvements and ideas

CO2 and human physiology

450 ppm 0.04%: Normal concentration in air outside
1000 ppm 0.01%: Normal concentration in air inside
5000 ppm 0.5%: OSHA Standard limit for and averaged 8 hours workday
30 000 ppm 3%: Mild respiratory stimulation, headaches, dizziness, fatigue
50 000 ppm 5%: Exaggerated respiratory response, severe headache and fatigue. Prolonged exposure results in irreversible effects to health.

The duration of exposure plays an important role. The level of tolerance can also differ significantly between cavers.

More information:

https://en.wikipedia.org/wiki/Carbon_dioxide#Human_physiology
https://en.wikipedia.org/wiki/Hypercapnia

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