## 1 Introduction  
Safety monitoring of historical buildings is a topic of considerable importance for the scientific community. Recently, several studies available in the literature focus on the effectiveness of different technological systems proposed for Structural Health Monitoring (SHM) of existing masonry structures with particular reference to those belonging to the cultural heritage [1; 2; 3; 4].  
The new generation of SHM is based on the installation of inexpensive sensors that allow the acquisition, elaboration and transmission of the measurement signals at low cost and in many points of the monitored structure. Generally, sensor devices operate through two methods, the active one [5] and the passive one [6], recording information useful to detect the health of existing buildings.  
The SHM systems allow the non-contact survey and continuous monitoring of existing structure, identifying the performance level, the local damages and the possible degradation of material components. This information refers to specific points of the building where the devices are installed by facilitating the maintenance plan and the localization of damage after earthquakes [7]. Sophisticated algorithms provide an accurate time-correlation of the state of the building [8]. In some cases, the storage of this information is essential for post-processing through statistical methods in order to investigate the link between different phenomena that can happen at the same time within the monitored structure and to provide protection from future earthquakes [3].  
The architectural heritage often suffers material deterioration, deformations and critical crack patterns that require urgent strengthening interventions ensuring minimal invasiveness. In this background, Italian guidelines promote the use of SHM systems as aids in the maintenance of new buildings as well as in the preservation and conservation of historic constructions. However, the application of SHM systems in existing structures has several issues related to the minimal invasiveness in the sensor installation and the presence of heterogeneous materials and proper strategic installations [9]. With the aim of applying SHM to historical masonry, this study examines the post-installation of two innovative stress sensors, i.e., ceramic and capacitive, on pre-stressed calcarenite and clay brick masonry walls, simulating a realistic situation in which the sensor is installed on an existing wall under load. Good performances of pre-installed ceramic and capacitive sensors was already observed for monitoring new masonry elements [10; 11]. However, the use of sensors for monitoring new masonry structures is not yet a common practice. Conversely, the post-installation of sensors in existing masonry is increasing, and new commercial solutions are arising in order to overcame some of the drawbacks that may affect the accuracy of the sensor measurements. In this paper, critical issues related to the post-installation operating phases are discussed, and the results of the experimental campaign are presented. Traditional monitoring devices are employed to assess the comparison with data recorded by sensors and to detect drawbacks related to the test setup.