Hti HT-401 is the most portable CO2 meter I have. It is small and simple. It is only 75 mm in diameter and 25 mm in depth. Weight is 62 g. That is including 14250 (1/2AA) 300 mAh battery. Its only button is power. It
Four-digits LED display only shows CO2 level and 3 emoticons ?<600ppm ?600-1200ppm ?>1200. That is it. No settings for emoticons limits. No setting for the power-off timer. Not display temperature and humidity, nor time and date. Just one sensor inside and that is well known SenseAir S8 LP. That provides correct readings. Changing the environments not impact accuracy. The device is relatively cheap (I bought it for about 75$ without battery on Aliexpress)
- 300 mAh charging time: 2h 30m
- 300 mAh battery life: 5 hours
Links:
https://wiki.liutyi.info/display/CO2/HT-401+power+consumption