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Comment by jeffreygoesto | Hacker News Reader

Relative VOC measurements at least are able to detect a no. 2 in nappies. Great to distinguish if you should go in and change them or would be a distraction from falling to sleep when checking. I bulit a Pi ZeroW that measures on the side of the bed and publishes a curve of the last hour of the values. You can see a pretty salient drop when "it" happens.

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I have an Awair sensor, and I can confirm that the VOC sensor is able to detect...well, pretty much anything from food to hair products to outgassing from some new thing that I opened up and accidentally left too close to the sensor.

Yeah, I don’t know how good it actually is, but the one time I painted indoors the thing cratered in a way it never has before or since.

And my daughter keeps trying to see if flatulence will actually drop the air quality depending on proximity.

I got 3 awairs and they were all over the place sitting right next to each other.

What sensor are you using? I bought a few trash ones from Microcenter. The readings were all over the place and I couldn’t get reliable numbers without smoothing numbers and throwing readings out. One of the spec sheets even said you would get crazy readings and would need to throw them out.

I'm a little surprised that no one has mentioned Aranet yet. From what I can tell, they appear to work quite well. However, it is quite pricey.

I did look into the AirGradient when geerlingguy put out a video about it but they were out of stock at the time. It looks like there might be a new version but that may be something worth looking at.

I tried the Vitalight CO2s but the readings were all over the place despite recalibration with 2 devices side by side. Ended up returning these. Do not recommend.

yeah I was looking for mentions of Aranet. I have two and they came with calibration info iirc.

I am using a Bosch BME680 and read it out with https://pypi.org/project/bme680/

This suffers from exactly what is mentioned in the article i assume. The temperature and humidity are thus unreliable.

True. You need to know that and see if you can live with it for your use case. Some information is available at least, like the temperature coefficient of the pressure sensor [0]. Bosch offers an "interpretation library" that you also can train yourself specifically, that can be used for the successor BME688 [1]...

[0] https://community.bosch-sensortec.com/t5/Knowledge-base/BME2...

[1] https://github.com/BoschSensortec/BME68x-Sensor-API

yah, i think a lot of these IAQ (indoor air quality) products are ok at relative differences (which is mostly what lay consumers care about). the article mainly points out that they're not good at absolute measurements, and for something like VOCs, it's likely using a CO₂ sensor and a lookup table that's static and only really valid for the test room it was created in.

> for something like VOCs, it's likely using a CO₂ sensor and a lookup table that's static and only really valid for the test room it was created in.

It's the other way around, usually. CO₂ is more typically the estimated value, since its direct detection involves a relatively expensive NDIR value. VOCs as a category need some sort of modeling and estimation, but sensors can go fairly far by directly measuring a few species (hydrogen, for example), and hoping.

oh, right, dunno where that brain fart came from... thanks for the correction!

I bought a couple of Figaro TGS2602 from Aliexpress and they were quite sensitive to "human gas emissions".

Could you use this to detect flatulence in adults?

If you were to have an array of such sensors, perhaps you'd be able to estimate direction of travel.

"I'm sorry, Frank. The sensors don't lie. I'm going to have to ask you to leave."

Now there is a budding seismologist's answer! ;-)

For sure. I have a couple of Winix air quality sensors / purifiers and they power up (with red LEDs and rushing fans) almost immediately if anyone has a little gas anywhere in the room.

My kids call our winix purifiers "fart detectors"

DIY: https://projects.raspberrypi.org/en/projects/fart-detector

Nice, I'd be interested in a write-up with plots!

Why drop? Less gas afterwards?

Maybe a voltage drop? Most sensors just return a voltage. And then you have to calibrate that value and convert it into some sort of measurement using an equation. So an increase in VOC might result in a drop in voltage on the sensor.

It’s a relative indicator, so it would spike and then (relative to that spike) drop steadily as the VOCs diffuse and vent.

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