The expected CO₂ fresh air value is about 400 ppm (0.04 %vol) nowadays. The device assumes this value to be the lowest limit value (baseline). Over several days, a special algorithm now permanently checks the lowest CO₂ value measured of the sensor over a preconfigured time interval and slowly corrects every long-term drift determined by comparison with the expected fresh air value of 400 ppm (or 0.04%vol) CO₂. In normal indoor applications, the carbon dioxide level usually decreases almost to outdoor air level at some point during one week. By recording the values over a period of 8 days and then comparing the lowest value to the 400 pm point, the device determines whether or not it needs to adjust the zero point. This takes place fully automatically. The algorithm therefore makes use of the fact that the CO₂ content in buildings and rooms regularly stabilises at a minimum for a certain period of time when they are not occupied. In rooms that are continuously occupied or where there is a constantly increased CO₂ concentration (e.g. in greenhouses), this principle of automatic balancing therefore does not work.