The Senseair ABC-algorithm

Thanks to our self-calibration function, you can mount your sensor and then forget it for the next 15 years and it will still be accurate.

All Senseair sensors are maintenance-free in normal environments thanks to the built-in self-correcting ABC-algorithm. ABC stands for Automatic Baseline Correction, a Senseair self-calibration function for achieving maintenance-free gas sensors. That means that our sensors have a life expectancy of at least 15 years and do not require any further calibration when used in normal indoor air applications.

How does the ABC-algorithm work?

The ABC-algorithm constantly keeps track of the sensor’s lowest reading over a preconfigured time interval and slowly corrects for any long-term drift detected when compared to the expected fresh air value of 400 ppm (or 0.04%vol) CO₂. In normal indoor applications, the carbon dioxide level drops to nearly outside air at some point during a week. By sampling the values for an 8-day period and then comparing the lowest value with the sensor’s 400 point, the sensor can understand if it needs to adjust the zero point.

If the sensor is detecting hydrocarbons, the level of these gases is probably near 0 ppm most of the time. In all cases, the ABC set point is adjustable. The algorithm takes advantage of the fact that the CO₂ level stabilizes in buildings when unoccupied.

What causes sensor drift?

Rough handling, transportation, and ageing might result in a reduction of sensor accuracy. With time, the ABC-function will tune the readings back to the correct numbers.

What if a room is occupied 24 hours per day?

If a space is constantly occupied and there are no periods when outside levels drop to background levels, the ABC-algorithm will not work. This is the case for greenhouses or in closed confined spaces where CO₂ levels may always be elevated. For these applications, the ABC-function can be turned off. Without the ABC operating, the sensor should be calibrated every two to three years.

What if outside/background levels are higher or lower than 400 ppm?

Sometimes, the lowest level may be higher or lower than the 400 ppm assumed by the self-calibration algorithm. CO₂ based ventilation control is based on measuring the difference between inside and outside concentrations. It really does not matter if the lowest level of the concentration is 450 ppm; just knowing the difference between the baseline period and the occupied period is enough to provide adequate ventilation control. This algorithm has the same effect as placing a sensor outside to constantly measure the difference between inside and outside CO₂ concentrations.

How long will the sensor maintain calibration?

The sensor will maintain calibration as long as it periodically senses a background level of CO₂. The electronics and components in the sensor are rated for a minimum 15-year life. The sensor should maintain calibration over its working life.