The "TFA Dostmann AirCO2ntrol" devices are popular in Germany. Searching for "co2 meter" on Amazon.com one of the top results is an identical looking device.

It measures with an infrared spectroscopy-based sensor, which should be quite accurate. There are a few good software packages for recording data, e.g. https://github.com/vfilimonov/co2meter.

Repost from my comment above, but for a couple hundred bucks you can get an accurate CO2 sensor that reads out via USB-serial to a computer. I use them at work and we've shown they are quite good over the long term. Plus to recalibrate, you can just take them outside. https://www.co2meter.com/products/k-30-co2-sensor-module

MH-Z14