Aranet4 is what you might call the "gold standard" of consumer-grade portable indoor CO2 monitoring devices. Rarely discounted in Australia, this is a time-limited deal for the next few hours that I spotted today on Amazon AU to purchase one via Amazon US. It happens to be the cheapest price ever according to the Camels, also approx ~15% discount compared with the standard price over at CO2Radical.
Why might you want one?
Well as exposure to high levels of CO2 indoors can be linked to poor health - including risk of airborne pathogens - and may even affect concentration/tiredness/cognitive function, knowing what you are breathing indoors could be beneficial by informing you on whether appropriate action (e.g. opening a window, installing an air purifier, masking-up, or just getting the hell out of there!) could be taken to improve it. Cheaper NDIR-sensor devices are available, but most have a much shorter battery life and don't do data-logging.
Here's the blurb from the Amazon listing:
* The perfect companion for indoor environments, allowing you to monitor CO2 levels, temperature, relative humidity, and atmospheric pressure in real-time
* Easy for anyone to use – the color coding and the optional buzzer warns you when the CO2 concentration gets too high
* Power-efficient e-ink display ensures super long battery life for this wireless device (up to 4 years)
* Smartphone app for viewing and analyzing historical data up to 14 days history (supports iPhone and iPad with iOS 12 or later, Android devices with Oreo 8.0 or later, compatible with Bluetooth 4.1 or later)
* Uses nondispersive infrared sensor (NDIR) technology for most precise and reliable CO2 measurements
Is there a budget brand for this kind of thing?