We utilize automatic, dynamic calibration logic in our CO2 sensor in order to achieve the maximum level of accuracy over the lifetime of the sensor. Basically, the sensor is continuously correcting itself over time. This logic is called Dynamic Automatic Background Calibration Logic (ABC Logic).
ABC Logic works on the sensor's dedicated on-board microprocessor to remember the lowest CO2 concentration that took place over the past 24 hours. The sensor assumes this low point is at outside levels. The sensor algorithm is also smart enough to disregard extended periods of elevated readings that might occur if, for example, a space was used 24 hours per day over a few days. Once the sensor has collected 14 days worth of low concentration points, it performs a statistical analysis to see if there have been any small changes in the sensor reading over background levels that could be attributable to sensor drift. If the analysis concludes there is drift, a small correction factor is made automatically to the sensor output to adjust for this change.
The figure below shows CO2 concentrations as they might occur over 14 days in a typical office space, with peak concentrations occurring in the morning and afternoon of each day. The dotted line is drawn through all the low points for each day as compared to an assumed background of 400 ppm (the global atmospheric background level). If a statistically relevant change in the data shows a shift above or below background, a slight adjustment is made to sensor readings, as shown by the solid level line. Every day the sensor looks at the past 14 days worth of data and determines if a calibration adjustment is necessary. A similar type of approach of nightly calibrations is often used for other HVAC equipment such as VAV boxes.
When you connect your product for the first time, the CO2 sensor will use the first 14 days (or less) of operation to calibrate itself to local background levels. You should not be concerned if readings on the sensors appear unusually high or low, or if a lack of agreement between sensors is found. Each sensor has gone through a complete quality check and factory calibration. Reasons that a sensor reads differently initially may include:
- Concentrations in each space may be different.
- A person or animal may be blowing into the sensor while setting it up or observing it in close proximity.
- If the unit went through some physical stress during the shipment, a slight shift in the original factory calibration may occur. ABCLogic will correct this shift within 14 days (or less).
The graph below shows the behavior of three sensors with ABCLogic installed in the same space over their first 14 days of operation. As can be seen in the initial few days, the sensor readings were at times quite different and in some cases over 100 ppm apart. However, by the 5th day they began tracking closely, and by the 10th, all sensors agreed very closely in their readings.