AirGradient Forum

Abnormally low CO2 readings

Components
#1

I have an I-9PSL. In the last week I’ve noticed abnormally low CO2 readings when opening the window of the room where the device is.

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O-1PST for reference, on a balcony, 5 meters away from the indoor unit.

Lowest value in the last week was 103.

The sensor was working fine until recently. Only thing that might have thrown it off-course was the fact that I left the apartment empty for about a month (from mid-December to mid-January). During this time, the value stayed around 400, but I don’t see how that might have affected it.

Below is the month of December (on the 15th I left):

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Running 3.6.0 with following config:

{
  "country": "AT",
  "pmStandard": "ugm3",
  "ledBarMode": "co2",
  "abcDays": 8,
  "tvocLearningOffset": 12,
  "noxLearningOffset": 12,
  "mqttBrokerUrl": "",
  "temperatureUnit": "c",
  "configurationControl": "cloud",
  "postDataToAirGradient": true,
  "ledBarBrightness": 10,
  "displayBrightness": 0,
  "offlineMode": false,
  "monitorDisplayCompensatedValues": false,
  "model": "I-9PSL",
  "disableCloudConnection": false,
  "corrections": {
    "atmp": {
      "correctionAlgorithm": "none",
      "slr": null
    },
    "rhum": {
      "correctionAlgorithm": "none",
      "slr": null
    }
  },
  "httpDomain": "",
  "extendedPmMeasures": false
}

Any idea what might cause the value to drop so low now?

#2

Mine did something like this with values down to 160 ppm and I think it was due to the ABC and insufficient ventilation of the room. I solved the problem with a calibration request leaving the window open long enough.

#3

Based on my understanding of how calibration works, when calibration is triggered, it looks at the readings history for the last 8 days and picks up the lowest value and calibrates against that. In that case, it shouldn’t matter if you open the window during calibration as long as your lowest reading from the previous 8 days was around 400.

However, if that’s indeed how it works, it also creates a problem with abnormally low readings like mine because they will be treated as the new “normal” (400 ppm) and will skew the future measurements.

#4

The automatic calibration, indeed is based on the last 8 days (unless you change the settings), so I requested a calibration from the advanced settings, because I thought the CO2 values in my room were too high in the days before that 160 ppm reading.

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#5

That is my understanding of how the calibration works and I run into the same issue, as my home never gets down to 400 due to being nearly always occupied and well sealed.

Around every 3-4 months I take my devices outside for 10 minutes, do a calibration to ensure it is around 400, then wait 5 minutes and bring them inside and turn off the Automatic Baseline Calibration. This works pretty well as long as I’m not moving the sensors around in between as moving can throw it off until the next calibration, along with standard drift over time.