Measurement of CO2 , CFox module C-AQ-0001R

The module is no longer for sale, it is replaced by the C-AQ-0006R module.

The C-AQ-0001R module is a spatial sensor of concentration of carbon dioxide (CO₂) in the air. The principle of CO₂ measurement is based on the dependence of infrared radiation attenuation on the concentration of CO₂ (non-dispersive infrared radiation absorption). The CO₂ concentration testifies to the quality of air in an area, so the module can be used e.g. for controlling ventilation in rooms and buildings. The device is intended for mounting on the wall or on the flush box.

The module contains two measurement inputs. The first one is connected to the CO₂ sensor. The temperature sensor is connected to the second input, which is only intended for servicing. The sensor measures the temperature inside the device and provides information about the operating conditions of the module. The sensor is capable of measuring the concentration in the air in the range of 0 ppm up to 5,000 ppm.

The module for the measurement of carbon dioxide has been calibrated by the manufacturer for the range of concentrations from 400 to 2,000 ppm of CO₂ in the air.

Auto-calibration feature of the C-AQ-0001R module

The C-AQ-0001R module is equipped with an auto-calibration function, which compensates for possible drifting of the CO₂ sensor due to the inevitable ageing of the infrared radiation source; this discrepancy is in the order of several ppm/month.

Thanks to this function it is not necessary to recalibrate the sensor during the operation; the sensor automatically maintains its accuracy over a period of many years (typical duration is 15 years).

In simple terms, the auto-calibration feature works as follows: the sensor internally monitors the concentration of CO₂ 24 hours a day for a two-week period. The minimum concentrations

are then statistically evaluated to find out, whether there has been a shift of the "zero" in the sensor, and if so, a slight correction is made of the internal sensor calibration values. For this feature to work properly, it is necessary to ventilate to a level of 400-500 ppm in the interval of 14 days. Of course the sensor can easily eliminate days when there was no decrease in the concentration of CO2 to the expected

minimum and does not take them into account. The room must be periodically ventilated in order for the sensor to work properly, ideally when the interior space is not used for at least four hours a day.

What the sensor peforms is a light correction of the factory calibration values, based on the long-term trends in measured concentrations of CO₂ in points close to the outdoor environment, where it can be assumed that these values are constant over the long term.

This auto-calibration function can be disabled, however, then it is recommended to periodically recalibrate the sensor roughly once every three years. The following diagram illustrates the principle of the auto-calibration feature.

Fig. 1. An example of wiring of the air quality sensor C-AQ-000xR

Fig. 2. Box dimmensions and mounting holes location