## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Durability of Electronic Data Acquisition Equipment for AE and GUIW  
Signal transmission to data acquisition and power supply for the electronics are also dependent on the choice of transducer or sensor and, hence, on the test object and its environment. For long-term operation and durability of electronic measurement equipment, the reliability of the electronic components and modules is essential for providing sufficient technical availability. There are standard procedures [111; 112] for the verification of AE equipment operating characteristics; they recommend periodic verification. Certain procedures for AE testing of safety-relevant infrastructure explicitly require this verification at least once per year as well as in case of malfunction of the system. For electronic components and systems, ref. [113] discusses basic reliability issues. However, no electronic equipment operates without some maintenance and repair for several decades. NASA's deep space network is probably the electronic signal recording and analysis system with the longest operating duration known to date. It was set up in 1958 [114] and is still in operation today. However, as discussed in [115], there has been virtually continuous maintenance work and several upgrades have been implemented, the latter mainly for adapting the system to new communication technology for satellites launched more recently. To some extent, the deep space network system requirements differ from those for equipment transmitting and recording AE or GUW signals (except for space applications), since the sensor or, more generally, the measurement and wireless signal transmission systems on most satellites cannot be changed, repaired or upgraded anymore. Of course, depending on the object to be monitored by AE or GUW, the effort to change sensors and signal transmission devices can be quite substantial and may only be feasible at certain times, e.g., during shutdown from service. Nevertheless, if properly planed and prepared, repair and upgrade installations are feasible in these cases. Data files from AE and GUW for long-term SHM or CM tend to be \"large\" (of course, due to development of evermore computationally powerful data processing and storage technology, this term is not quantified here). This may not only pose respective problems for data storage and processing but also for signal transmission, the latter especially for wireless modules. Currently, for AE systems with twenty or more channels operating at sampling rates of 10 MHz or more (AE equipment currently provides up to 40 MHz sampling at 18 bit amplitude resolution) with signal rates of 1000 or higher per second and channel require signal transmission rates clearly above 1 GBit/second. This exceeds the transmission rates provided by the latest commercial wireless technologies, e.g., 5G as discussed in [116]. Potential solutions are interim storage of data at the sensor node with signal transmission in periods of lower activity, signal preprocessing and respective data reduction at the node before transmission or reduction in wave signals to selected signal parameters instead of transmission of full waveforms. Interim storage on the wireless node with delayed signal transmission as a solution of course inhibits real-time signal processing and analysis. Maintenance and repair of hardware benefit from standardized and modular design with standard components [113]. This also implies respective supply chain considerations. Standard components available from several manufacturers, maybe even from companieswith production facilities \"nearby,\" are one key issue. The supply and delivery problems of electronic parts and components experienced during and in the aftermath of the worldwide COVID pandemic in 2020 and 2021 nicely illustrate this point. For application-specific parts or components, a \"local\" manufacturer is preferable, possibly even at somewhat higher costs.