Sub-hundred device with particulate, temperature, humidity, an eCO2 number and a TVOC bar. The TVOC goes red about twenty minutes after cooking or cleaning, and the eCO2 tracks it almost exactly, which makes me suspect they are the same sensor wearing two hats. Should I be acting on either of those, or is the particulate reading the only part of this thing worth trusting?
I ended up buying a proper infrared CO2 unit to sit next to the cheap one, mostly out of curiosity. Particulate readings tracked each other reasonably. The CO2 lines had almost no relationship - the estimated one responded to cooking and cleaning products, the real one responded to how many people were in the room and whether a window was open, which is what you actually want to know.
One more thing about those gas sensors: most of them auto-baseline, meaning they take the cleanest thing they have seen recently as zero and report relative to that. So a monitor that has been in a stuffy room for a week is calibrated to stuffy, and you can get a green reading in a room that would horrify you. They also need a warm-up and they drift over months. Useful as a change detector, useless as an absolute measurement.
The auto-baseline behaviour explains the thing that confused me for ages - my numbers were always worst in the first hour after moving the device to a new room and then quietly settled.
Partial disagreement with writing off the VOC bar. As an absolute figure it is nonsense, agreed. As a relative alarm it has been useful to me twice - a new mattress that quietly outgassed for three weeks, and a cleaning product I did not realise I was breathing for an hour every Saturday. Use it as a hey, something changed signal and ignore whatever units it thinks it is reporting.
I binned a monitor that was behaving exactly like yours, assuming it was faulty, and bought another one that did the same thing because that is simply how they work at that price. Save yourself the second purchase.
@porosity_nerd · 3mo ago · 2 replies
Your suspicion is correct. On units at that price the eCO2 figure is estimated from the same metal oxide gas sensor that produces the TVOC number - it is not measuring carbon dioxide at all, it is inferring it from a correlation that holds in some rooms and not others. If you want a real CO2 number you need a non-dispersive infrared sensor, which is the single biggest reason some monitors cost several times what yours did. The particulate sensor is the part worth acting on, with the caveat that it drifts with humidity.
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@zigbee_zoe · 3mo ago
This explains why mine reported implausible CO2 every time I opened a bottle of solvent and none at all when the room was full of people. It was reporting my varnish.
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