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The use of a PVC housing may cause artifacts since this material can degrade in high concentrations of Evodiamine O3 and NO2, so that interaction between the gas molecules and the housing surface may be supposed. Indeed, this choice was motivated to minimize the cost of the single nodes, making future deployment of very dense monitoring systems economically sustainable. Use of highly inert plastics, such as Polytetrafluoroethylene (PTFE), which would considerably increase the cost of individual nodes, will be considered for comparison purposes in future experiments. In order to obtain an acceptable accuracy, each sensor node should be individually calibrated. The calibration phase is often overlooked in articles regarding sensor networks. Unfortunately, a common characteristic of the miniaturized and inexpensive sensors that are typically mounted on sensor nodes is the large parameter tolerance. This means that the response curve of different samples of the same kind of sensor (e.g., NO2 sensor) prior calibration are so poorly matched that comparison of data produced by distinct sensor nodes Epigenetics Compound Library nmr are generally meaningless. For this reason, we preliminarily performed two-point calibration of all the sensors used in this work. Note that all the considered sensors are of resistive type and their responses are nearly linear in a log-log plot, as reported in datasheets. For this reason, the following expression was considered for the calculation of the gas concentration: log10(C)=k1log10(R)+k2 (3) where C is the gas concentration, R the sensor resistance, whereas k1 and k2 are constants depending on the sensor type. The calibration procedure consisted in exposing the whole sensor nodes to different gas concentrations inside a small sealed chamber. The first step consisted in determining the optimal value of load resistance, which is a resistor placed in a series with the sensor resistance in order to form a voltage divider. The input voltage for the divider is a constant reference voltage, so that the output voltage is a function of the sensor resistance. An initial value for the load resistance was selected using the information provided by the sensor datasheets; this value is used to estimate the sensor resistance in the reference concentration. The final value of the load click here resistance is chosen in order to maximize the estimated output voltage swing across the operating concentration range. This is an important step, considering the relatively low-resolution AD converters (10 bit) that equip the sensor nodes. Tests were practically performed using two gas concentrations: (i) zero pollutant level, obtained exposing the sensor to synthetic air and (ii) reference level (i.e., 1.45 ppm for NO2 and 100 ppm for CO) obtained by using gas cylinders, containing a known concentration of the pollutant in synthetic air.