The expected CO2 fresh air value is approx. 400 ppm (0.04 %vol) these days. This is assumed by the meter as the lowest limit value (baseline). A special algorithm now permanently monitors the lowest detected CO2 reading of the sensor over several days over a preconfigured time interval and slowly corrects any long-term drift detected compared to the expected fresh air value of 400 ppm (or 0.04%vol) CO₂. In normal indoor applications, this will cause the carbon dioxide level to drop to near outdoor air levels at some point over the course of a week. By recording the values over a period of 8 days and then comparing the lowest value with the 400 ppm point, the measuring device recognises whether or not it needs to adjust the zero point. This happens completely automatically. The algorithm thus makes use of the fact that the CO₂content in buildings/rooms regularly stabilises at a minimum for a certain period of time when they are not occupied. In rooms that are permanently occupied or in which there is a constantly increased CO2 concentration (e.g. greenhouses), this principle of automatic balancing does not work.