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Comment by brendangregg | Hacker News Reader

I've had a few CO2 meters, and learned not to trust any <$100 as they don't really work. I look for meters that use non-dispersive infrared diffusion sensors (NDIRs), like the Aranet4. I've found the TIM10 desktop model from co2meter.com to be accurate (AFAICT), uses NDIRs, and only US$139.

I also have other air quality meters. (I collect measuring devices.) I wish there was a do-it-all air meter.

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I have a TIM10 that was once reporting negative CO2 levels one spring with the windows open. I'm not sure what I think about it. It's not a scam product by any means, it can definitely tell when someone is in the room or a window is opened or closed. I needed the space and unplugged it awhile ago and just don't worry about CO2 levels currently.

My understanding is that some co2 sensors assume the minimum value seen over the last 30 days is 400ppm as a calibration. This could possibly explain a negative value if the sensor has been in higher co2 environment for some time, and is suddenly exposed to the outside.

You're right. The instructions say to place the sensor outside for a time to calibrate it, which I never did.

Does it require callibration?

Yes. All consumer CO2 detectors I’m aware of take periodic calibration.

Not Aranet4

Yes you do. Once a year: https://aranet.com/faq/

Their data sheet also advises calibration if used at “high elevation.”

Do you know if the Aranet4 can be used over bluetooth/wifi without internet access? I've been looking at a bunch of devices and haven't seen aranet yet.

Also, have you ever put all of your devices near each other to compare the variance between them all?

I got my Aranet4 because it had very good reviews and allowed local read out of sensor data without any "cloud" involvement. I've been very happy with it, the readings are integrated into my Home Assistant setup so automations can respond to high CO2 (e.g. turn on a fan directed at a window, when temperatures are high enough to have a window open).

I wrote a bridge to get readings in to Homekit and prometheus as well. Useful to track multiple sensors and process historic data easier than the app

https://github.com/ryansouza/aranet4-exporter-go

AraNet4 does Bluetooth, yes. Mine is doing that right now.

You can get wifi bridges as well, see https://devel.aranet.com/products/

ESPHome on a ESP32 makes a nice Bluetooth2Ethernet Bridge.

I've seen Aranet4's on eBay for under $150 pretty regularly, so I'd look there.

Yes the base model not supports Bluetooth

Sensirion SCD4x CO2 Gadget is $60

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