## 1 Introduction  
Preventive conservation methods are based on the concept that controlling some of the major deterioration causes in the environment it is possible to ensure the sustainability and durability of the art work [1]. Depending on the nature of cultural heritage, the deterioration causes are subjected to the influence of different physical parameters. For example, artworks are influenced by terms of stress caused by physical agents such as temperature, humidity, radiation, and chemical agents (e.g., CO\({}_{2}\), SO\({}_{2}\), O\({}_{3}\), mineral salts, _etc._) [2], meanwhile, temperature and humidity can reach especial importance on archaeological structures because of being majorly stone-built [3, 4]. But generally, abrupt changes of temperature and relative humidity (RH) may cause serious damage in all the different kind of objects, such as non-isotropic material deformation or detachment in materials of several layers [5]. In hygroscopic materials, such as wood panels, which are the mainstay of many artworks, mechanical changes and deformations could occur [6]. In the case of frescoes, soluble salts and moisture are the very common causes of deterioration; therefore an early detection of dangerous values of these physical parameters (together with other chemical parameters in some cases) is essential to avoid this kind of damage [7]. Thus, both long-term monitoring of the environmental parameters as well as further analysis of the recorded data are necessary [8]. The concept of \"Smart City\" is notoriously liquid, scarcely formalized and, in some degree, subject to different ideological interpretations [9]. Normally Smart City is associated with real-time monitoring and cloud computing [10]. However, Smart Cities should not necessarily be restricted to real-time monitoring. In fact, in the case of monitoring cultural heritage, historical data (without underestimating real-time alarms, _etc._) is more interesting as it is important to characterise the site for performing comparative studies in the future, either regularly, in order to verify whether the conditions are constant, or occasionally, when the boundary conditions are altered [11]. Thus, the aim of monitoring cultural heritage in a Smart City would be sending all these recorded data to internet so they can be checked and serve as an example for the conservation of other similar sites. The environmental monitoring is performed by a regular data recording, whose frequencies typically range between one datum every hour (1 datum/h) or every day (1 datum/day) [12, 13]. Monitoring with frequencies near 60 data per hour (1 datum/min) is interesting as power and confidence of the statistical analysis performed increases in direct proportion to the sample size used [8], and therefore develop a monitoring system with large grid of sensors and big memory to store large amounts of data from high recording frequencies is interesting. Different works monitoring cultural heritage are found in the literature [14, 15]. Most of them use individual data-loggers. Some authors [16] use Hobo data-loggers [17], and other studies [6, 11, 18] work with data-loggers DS1922L [19] and DS1923 [20]. Data from these data-loggers cannot be sent automatically to the cloud and due to they are commercial devices, its hardware and software is unknown and cannot be modified. There are also papers that have studied the microclimate in churches making use of a wired sensor network composed of different lines and sensors and a single microcontroller [7, 21]. The experience with the DS1923 [18] has demonstrated that individual data-loggers battery and memory get depleted around the third month of work in cases where data has to be taken with high frequency. Moreover, results of each data-logger must be combined into one file. Software to combine the data automatically once they are in the computer could be developed; however the hardest part of the work consists in loading data into the computer one by one.