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CO2 Sensor ABC Calibration Explained

Automatic Background Calibration algorithm (ABC) allows the CO2 sensor to dynamically shift its CO2 reading by a constant. It works via storing the lowest CO2 sample taken over the ABC Period and assuming that this low value is equal to a known value (the target value). It then adjusts the output of the CO2 reading by the delta between these values. This algorithm does not affect the linearization of the output signal.

For example, by default ABC is enabled with an ABC Period of 180 hours, a target value of 400ppm, and a maximum delta of 30ppm. This operates under the principle that ambient (fresh) outdoor air contains 400ppm of CO2.

The sensor will keep track of the lowest CO2 reading recorded over a period of 180 hours and then apply the following logic to it:

  • ABC Offset = 400ppm - Lowest Reading Recorded in Memory
  • New CO2 Reading = Actual CO2 Reading + ABC Offset

For example, suppose your CO2 meter display shows 500ppm, but the lowest recorded CO2 level is 420ppm

  • -20ppm = 400ppm - 420ppm
  • 480ppm = 500ppm + (-20 ppm)

In other words, because the sensor has drifted "up" the CO2 level reported will be lowered to account for it.

When using the CO2 sensor in applications where the CO2 level has a chance to record ambient air CO2 levels, this ensures that the value will counteract drift in sensor readings as components age.

When to enable:

  • Outdoor CO2 measurement.
  • Indoor CO2 measurement in an environment with a known low value.

When to disable:

  • In green-houses or consistently elevated CO2 environments.
  • In dynamic environments with unpredictable CO2 concentrations.

If you choose to disable this algorithm we recommend performing periodic CO2 calibrations. These calibrations can be done via a known test gas, by returning it to CO2meter for recalibration, or by taking your sensor outside and calibrating against 400ppm. Read the manual for your product to see which option is best for you.

Click here to learn more about ABC Calibration from SenseAir.

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