Announcing the co2mon crate

This is a couple of years too late, but I want to write a quick post announcing my first crate, co2mon. It's a small library for reading data from Holtek and similar CO₂ USB monitors. These are relatively popular, because the protocol was reverse-engineered and it's easy to communicate with them.

Linux prerequisites

If you're on Linux, you will need permissions to access the device node. The following udev rule will give permissions to every user on the system:

ACTION=="add|change", SUBSYSTEMS=="usb", ATTRS{idVendor}=="04d9", ATTRS{idProduct}=="a052", MODE:="0666"

See the crate documentation for details on how to install it. You can check the vendor and product id pair using the lsusb command.

If you get a linker error and you might need to install a libusb-1.0-0-dev or libusb package.

Getting started

Add the co2mon dependency to your Cargo.toml, then copy-paste the following code and try to run it:

use co2mon::{Result, Sensor};
use std::thread;
use std::time::Duration;

fn main() -> Result<()> {
    let sensor = Sensor::open_default()?;
    loop {
        match {
            Ok(reading) => println!("{:.4} °C, {} ppm CO₂", reading.temperature(), reading.co2()),
            Err(e) => eprintln!("{}", e),

The sensor sends various types of messages, in an arbitrary order. Sensor::read waits for a pair of temperature and CO₂ measurements and returns that. There's also a Sensor::read_one method, which returns a single message. This can be useful for versions of the sensor that also measure the relative humidity.

Device support

Tested using a TFA Dostmann AIRCO2NTROL MINI, but thanks to a kind contributor, the crate should also work for newer versions like the COACH.

What to do with it

Open the windows. The CO₂ levels you'll find in a closed room aren't exactly dangerous, but can still cause headaches, malaise and impair thinking skills. At one point I took mine to the office and it showed an error because it only goes up to 3000 ppm. Reading from USB still worked, giving concentrations around 3500 ppm.

Having a constant airflow in an office will also help against Covid, possibly even more than wearing masks.


My Grafana isn't in the best of shapes these days, but here's a screenshot: