From the UPCT, we have hardly worked in the calibration proccess of each sensor to achieve accurate information related to the quality air. This information follows the OMS regulations which analyze and measure gases such as CO2, CO, SO2, NOx and O3 in urban environments. However, the values captured by the sensor are not understanding theirselves; it is required to carry out a previous process. To this end, from decimal codification provided by Arduino resolution (between 0 and 1024) and for each collected measure, we must adjust this value to a specific voltage level. This voltage is that which the sensor really undergoes and, in turn, the resistance (Rs) associated to this level voltage is also calculated. As a second step, the sensor internal resistance (R0) must be derived; this parameter depends intrinsically on the manufacturer since is unique and characteristic for each sensor. To achieve the R0 value, concentration values of the gas to measure must be previously captured, thanks to the use and handle of commercial meters, also acquired by our work group.
Once the Rs and R0 values have been calculated, each sensor is calibrated from its datasheet to generate graphs (functions) which draw the sensor response as a function of Rs/R0 (x-axis) and ppm (y-axis). We must act so because the sensor datasheets are difficult of interpreting, and, in some cases incomprehensible, which have involved to plot the sensor response again. In detail, the following figures extract the gas levels (concentration in ppm) from the resistance values (Rs and R0).