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Comment by amluto | Hacker News Reader

VOC index means literally nothing. (I've read the datasheet, implemented some kludges to log raw data, and contemplated the formulas. The data is high-pass filtered. You literally cannot conclude anything about a single measurement if you assume the sensor is somehow magically perfect. You might conclude that there is more VOC (or less -- not all VOCs have the same sign in their effect) when cooking than when not cooking.)

I target 0 PM2.5, but this only necessarily makes sense as a target if you have the ability to filter your home's supply air. And a good vent hood should be able to avoid any detectable bump while cooking (although a vent hood without filtered makeup air may cause a bump due to increased infiltration of exterior air).

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High pass or low pass? Many instruments have a low pass filter. Very few have a high pass filter.

High pass. Probably actually band pass.

The instrument is pretty crappy - it has basically no calibration or, AIUI, long term stability. So you can’t really use it to characterize the chemical content of the air. What you can do is observe changes. I’m guessing the manufacturer wanted to avoid people getting confused by the absurd raw reasons, so they came up with a formula called “VOC Index” that HPFs the data. It will average 100 IIRC. You’re supposed to use it to notice events that cause changes - if it hits 170 every day at 2 PM them you can guess that something’s happening at 2 PM.

None of this is in the chip. It’s implemented by the driver. There’s an implementation in esphome, for example.

>you can’t really use it to characterize the chemical content of the air

correct, that's not VOC sensors do. you need specific sensors for different gases for that. All VOC sensors are metal-oxide (MOx) sensors, i suggest reading on their supposed applications and uses.

What I mean is: the MOx sensors, in theory, measure some function of the concentrations of various gasses in the air. The datasheets even give you some flavor of this function for some of the sensors.

But the actual behavior, at least for the SGP4x series, is so outrageously unstable and path dependent that you still can’t learn the value if that function of the gas concentrations. At best you get changes on timescales of minutes to a few hours but not on timescales of days.

>you get changes on timescales of minutes to a few hours but not on timescales of days.

again, correct. That's how they're supposed to work. If the air is very slowly getting worse, MOx sensors might not catch it.

That being said, what "very slowly" means depends on the sensor. For example, for Bosch BME680 (very good with 'organics'), you can set the tracking interval between 4 and 28 days (i use 28). Sensirion SEN66 (very good with 'chemicals') can have a tracking interval of up to 1000 hours.

What sensor, exactly, are you describing here? Different VOC sensors have different gas sensibilities, as well as different algorithms to interpret the gas sensor raw data.

I’m thinking of the SGP40, which is a popular model for air quality sensors that is specifically intended to report “VOC Index”:

https://sensirion.com/media/documents/296373BB/6203C5DF/Sens...

I’ve personally played with the SGP41, but I’m guessing the poster I replied to has the SGP40 because they didn’t mention NOX Index.

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