The following is a review of a budget level CO2 monitor purchased from Bay for $25
Portable CO2 Air Monitor Quality Temperature Humidity Home Detector (White) ❤ | eBay
I note the price is now $30.

I looked at purchasing a CO2 mete about 10years ago but back then they cost about $200 and just the sensor along cost $70. With the indetification of COVID19 as an an airborne virus, and increased interest in using CO2 monitors to assess general ventilation in inhabited spaces there are now hundreds of these types units available on ebay range from $20 to thousands of dollars. The higher end units usually have data logging capabilities and other more advanced features.

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Unless you have a well sealed up shed, a CO2 monitor will have little relevance to woodworkers, and it's certainly no substitute for a particle detector in terms of assessing shed ventilation for wood dust control purposes. However, for general fresh air/ventilation measurement purposes this unit should be able to provide useful info for likelihood of transmission of air borne viruses.
The unit comes in an attractive 89 x 70 x 35mm plastic box and clearly displays CO2 levels in ppm, plus temperature (C or F) and humidity.The CO2 range is from ambient ~400 ppm up to 6000ppm, supposedly to within +/- 1ppm - although somehow I doubt the latter.
Over 1000 ppm of CO2 is considered “stuffy” and requiring attention eg opening more room doors/windows etc though I would be doing something well before it got to that level.
It takes a few minutes after turning it on it to reach the 415 ppm, which is a nominal/ambient atmospheric level, but once up to speed it seems to respond quickly
Breathing over the sensor showed it reacted within a few seconds and shot up to over 2000ppm.


Power is via a supplied USB (5V) cable but you have to provide the actual adapter.
There is also provision for a (18650) LI-ion battery (not supplied) inside the case that can be charged in situ.
Claim is one 18650 battery will last for about 7 hours of use, dependent of course on the actual mAh rating of the battery used.


The manual refers to the unit being able to measure two other air quality parameters (TVOC - Totally Volatile Organic Compounds, and HCOH - formaldehyde).
When I first read the blurb I thought these were able to be displayed separately but it appears not to be the case for this unit - the display just show the sum of these two plus the CO2 which is not all that helpful if you are specifically interested in each component.
My understanding is the sensor in this unit cannot discriminate between CO2, TVOC and HOCH and just measures the sum of all 3.
Either way, the main outcome of measuring the “stuffiness” of a room and thus needing to ventilate is met.
If you need to, for about $60, you can purchase a unit that displays each component independently.


The unit also includes a temperature/Humidity sensor and display.
I’m not sure what sensors are being used but the temperature read out appears 2ºC higher than an adjacent precision thermometer, and about 20% higher in humidity.
The temperature is just within spec, with temperature reading claim in the manual being +/- 2ºC but the humidity reading appears a long way out at +/- 3%.



Despite its short comings. in terms of value for money I’m not complaining.
The battery lifetime is certainly sufficient for you to bring the unit along to say a restaurant or movie theatre and monitor what is going on although I’m not sure how practical that is.
Are you really going to walk out half way through proceedings if the CO2 levels go too high?

I'm going to be taking it along to my mums nursing home - it always seems WAAAAAY too stuffy in there, although I think its because they keep the central heating up too high for my liking.
Have also ordered a CO2 Sensor ($11 - same as the one inside this unit) and will build my own Arduino based monitor with data logging etc.