## 5 Conclusions  
What are the conclusions from the information presented and discussed regarding the long-term behavior of the sensors for AE and GUW, the sensor coupling agents, the measurement electronics and related software? The first is that there is scant quantitative, published information available on these topics. For the piezoelectric transducers, the conclusions are as follows: Sensor performance requires proper mounting providing sufficient protection against external mechanical impact, shocks or misuse. Nevertheless, sensors for long-term SHM or CM may have to be replaced from time to time even for test durations of less than two years. Therefore, periodic checks of sensor performance, ideally yielding a frequency-dependent sensitivity curve, are essential. The frequency of the checks, e.g., daily, weekly or monthly, depends on the application, on the service loads and on the ambient conditions. Therefore, maintenance and repair concepts for easy and quick sensor or sensor mount replacements are essential. Redundancy in the sensor array may be useful, especially when access to the test object and the sensors is limited to specific time periods due to operating conditions (e.g., certain parts of power plants) or would require a major effort (e.g., accessibility of the test object location). The cost for such redundancy in sensor setup and in respective data processing and analysis, however, can be significant and has to be carefully assessed when planning the monitoring. For long-term sensor mounting and coupling for SHM or CM, the available information likely points to adhesive bonding, preferably with thermoset epoxy-based adhesives, as a promising solution, at least for sufficiently low operating temperatures. An alternative includes mechanical fasteners, e.g., screws applying the contact pressure with suitable mounting devices, independent of whether a coupling agent is used or not. On test objects made from magnetic materials, magnetic sensor mounts will work if service temperatures are sufficiently low. No information on the long-term behavior of magnetic holders is publicly available. With respect to long-term adhesive bonding, it is likely that strain gauge manufacturers or suppliers may provide recommendations on the choice of the adhesive and the appropriate surface preparation for both, sensor contact plate and test object. Long-term operations of adhesively bonded strain gauges, e.g., on ropeway cable cars, are often subject to rather severe ambient conditions (large temperature and humidity variation). The main conclusions on electronic measurement hardware and software are as follows: Considering durability as well as the necessary reliability for long-term performance is recommended for specifying the components and parts. This has to be complemented by planning maintenance, repair and later upgrades of hardware and software. Modular design and extensive documentation are, hence, essential. A required reliability or service lifetime for hardware can be achieved by \"derating\" electronic components and the implementation of redundancy or stabilizing operating conditions. Use of \"standard\" parts available from several manufacturers or suppliers is one approach for dealing with supply chain problems. For software, ref. [113] clearly states (cite) the following: \"_Defects are thus generally caused by human errors (software developer or user).\"_ Hence, human errors have to be avoided in development and in application. This requires careful planning,organization and documentation of software development and extensive quality assurance shall validate that the code performs correctly. Even though this may seem costly, having full control of software design may yield higher reliability and, hence, less troubleshooting and maintenance cost. With respect to long-term SHM and CM, research providing quantitative performance measures, reliability or service life estimates for all components of the measurement chain seems highly desirable.