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How to recalibrate a CO2 sensor?

Virtually all gas sensors are subject to some sort of drift, including CO2 sensors. The degree of drift is partially dependent on the use of quality components and a good design. But even with good components and excellent design, a small amount of drift can still occur in the CO2 sensor. This may ultimately result in the need for a sensor to be recalibrated. This manual recalibration process is simple but it is time-consuming and therefore it can turn into a significant expense if recalibration is required frequently. Thanks to the intelligent recalibration algorithm inside Sentera CO2 sensors, the sensor service life is extended and significant (maintenance) cost savings can be realized.

Sentera CO2 sensors are maintenance-free in normal environments thanks to the built-in self-correcting ABC algorithm. However, if a Sentera CO2 sensor is applied in an application that is unlikely to see regular outdoor CO2 concentrations, then the self-calibration algorithm should be deactivated. This can be done via Modbus Holding Register 40. In that case, manual recalibration of the sensor is necessary. 

Maintenance-free CO2 sensors
Sentera CO2 sensors have a built-in self-correcting ABC algorithm. ABC stands for automatic baseline correction. Thanks to this algorithm, Sentera CO2 sensors do not require manual recalibration. They automatically recalibrate themselves once they are installed. This feature eliminates one of the biggest concerns surrounding CO2 sensing: sensor drift and routine maintenance for recalibration.

Automatic recalibration algorithm
The microprocessor of the sensor remembers the lowest CO2 concentration that occurred during the past 24 hours. This low point is assumed to be the outside CO2 level (baseline). The sensor is smart enough to discount periodic elevated readings that might occur if, for example, a space was exceptionally used 24 hours per day. Once the sensor has collected 14 minimum CO2 levels, it performs a statistical self-analysis. If the analysis concludes that there is sensor drift, a small correction factor is made to the sensor calibration to compensate the drift or to correct the baseline.

Graph of CO2 readings
 














The CO2 sensor self-calibrating algorithm is designed for applications where spaces are periodically unoccupied for 4 hours per day or more so that indoor CO2 concentrations can drop to typical outside levels.

 
Manual recalibration process

As mentioned above, the ABC logic algorithm usually takes care of recalibration. There are, however, rare applications where the CO2 level does not fall below 400 ppm in a 24-hour period. These may include, for instance, offices where night guards reside, hospital rooms, and police headquarters. 

The process is relatively simple, yet time-consuming. If automatic recalibration has been turned off, recalibrate the CO2 sensor once every while. The more often the sensor is recalibrated, the more accurate the readings will be. 

The first method is simply to open a window until you are sure the CO2 concentration has fallen back to the outdoor levels. A few hours should be enough. Enable automatic recalibration. Disable the automatic recalibration after a few hours. 

If the first method is not possible or not reliable, dismount the sensor and put it in another room or in an environment where the sensor will be able to read CO2 concentrations that fall to the outdoor level. Enable automatic recalibration in that room and hang the sensor back in its original spot after calibration.

    
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