# Development of an Automatic Low-Cost Air Quality Control System: A Radon Application
## 2 Materials and Methods
### The Devices  
The monitoring devices developed in this project can collect and display data related to air quality. They incorporate several environmental sensors:  
* The BME280 digital sensor [41] that measures relative humidity, barometric pressure, and ambient temperature. * The CCS811 digital gas sensor solution [42] that measures a wide range of Total Volatile Organic Compounds (TVOC), including equivalent carbon dioxide (eCO2) and metal oxide (MOX) levels. Volatile Organic Compounds, also categorized as pollutants, can come from various sources like construction materials, machines, and even people (breathing, smoking, and such). This sensor can give more precise measures if we provide it the temperature, so the monitoring devices use the temperature readings provided by the BME280 sensor to obtain preciser readings of the CCS811 sensor. Figure 2: The database schema of the air quality control system. * The RD200M radon sensor [43]. An instant radon sensor based on a pulsed ionization chamber. Its main characteristics are:
* Sensitivity: 0.81 cph at 1 Bq/m\({}^{3}\). * Measurement range: from 7.4 to 3.700 Bq/m\({}^{3}\). * Precision: \(<\pm\)10% at 370 Bq/m\({}^{3}\). * Default 10 min collision integration period. * Each sensor is individually calibrated. * It has a built-in vibration sensor for preventing errors in radon detection. In summary, we have sensors to collect the following data: radon concentration, temperature, humidity, barometric pressure, and TVOCs. The monitoring devices read the sensors in a continuous mode, but it needs to integrate some sensor readings over time. For instance, radon measures (collisions per second or Becquerels, Bq) need to be integrated over time, usually 10 min to 1 h, to obtain an accurate reading. The monitoring devices use a processing unit that integrates the sensors readings for 10 min and then sends all the readings to the server. The radon sensor manufacturer recommends this sampling period to integrate the measured instant collisions for a reasonable amount of time. The processing units were developed using the Arduino MKR microprocessor family. A family of microprocessors based on the ARM Cortex-M0+ CPU that can run up to 48 MHz. We chose this platform for the monitoring devices due to the high-performance of the ARM Cortex-M0+ processor and its energy-efficiency. This high performance is needed to integrate and process all the data from the different sensors and to manage all the communications with the server. The monitoring devices use two different communication technologies: WiFi and Sigfox. The WiFi devices can be used in locations where a WiFi network is available. WiFi technology is widely used and allows the exchange of higher amounts of data. The Sigfox devices use the Sigfox Communication Network, a low power wide area networks (LPWAN), which uses an ultra-narrow frequency band to achieve high coverage while consuming low energy. Sigfox communications use a public band (868 MHz). The European Telecommunications Standards Institute (ETSI) regulations stipulate that devices operating on a public band can only transmit 1% of the time over the course of 1 h. In Sigfox, this translates into six messages, of 12 bytes each, per hour, or 144 messages per day. This restriction is not imposed as a hardware limitation, but it is a gentleman's agreement. Despite the low data exchange rates, it is sufficient for sending the data that the devices developed in this work collect: our devices send data every 10 min, so we need to send 144 messages per day. This transmission speed and maximum data exchange are much lower than WiFi and even lower than other LPWANs such as LoRaWAN and NB-IoT [44], but the Sigfox network provides broader coverage. This characteristic is especially relevant for environmental monitoring because it allows using the monitoring devices in regions where other communication technologies are not available. For instance, there are some rural areas in the northwest of Spain without internet access.