Qingping Air Detector (AKA ClearGrass Air Detector) is one of the best portable devices reviewed yet.
Impressive tech specs and design. Can be used both: as portable and not portable. And in a mixed way too. Good sensors for tvoc, t/rh, co2. High-resolution touch screen, Wi-Fi (that remember all Wi-Fi you were connected to), ambient light sensors, android and ios mobile app. And all that for a bit more than 100$ (incl. Delivery). For this price tag, it is a kind of flawless device. So the minor issues are not critical at all. Like it might have a different CO2 sensor (SenseAir S8 instead of Sensirion SDC30 for example) inside. But since S8 is not bad. It is still ok.
- 1800 mAh battery (~4 hours of battery life)
- Compact (9x9x7cm, 218g)
- Calibration (tvoc, CO2) may be done manually
- Correct t/RH and CO2 measurements
- High resolution display with good viewangles
- Touchscreen and good UI usablility
- Keep multiple Wi-Fi AP settings
- Weather and time sync from Internet
- Ambient light sensor
- Good thermal design and active airflow
- Mobile App and 3-rd party app support
- Wireless (OTA) update w/o app needed
- CO2 high level is starting 2000ppm.
- Disassemble is not trivial.
- There might be other sensors inside.
- No alarm clock (but should it be there?)
Device inside: https://wiki.liutyi.info/display/CO2/Qingping+CGS1+inside
Device Thermal: https://wiki.liutyi.info/display/CO2/Qingping+CGS1+Thermal
Manufacturer link: https://www.qingping.co/air-monitor/overview
AliExpress price ~110$