## 4 Discussion
The latter property may be useful for, e.g., energy harvesting from temperature fluctuations [122] but produces noise signals in AE measurements where temperature changes or fluctuates sufficiently. Depending on the circumstances, these noise signals may occur much more frequently than the \"real\" AE signals from the test object. An analogous situation was the observation of large amounts of electromagnetic noise signals discussed in [91]. Noise signals constitute a potential problem for data processing (large amounts of signals) as well as for analysis (requiring criteria for defining noise signals). Possibly, AI algorithms can soon eliminate noise signals with an acceptable probability of detection. The last part of the discussion now focusses on practical experience that is not available in literature yet. A working group of the German Committee on AE (a member of the German Society for Nondestructive Testing, DGZfP) dealing with long-term performance of AE for SHM and CM has drafted a questionnaire that was sent to all members of the committee. So far, there were five questionnaires returned, both from AE service providers and research institutes. The responses received so far (end of October 2021) are briefly summarized and compared with the information and recommendations detailed above. The first response deals with process monitoring applications in various industrial sectors. The sensors for structure-borne sound are usually coupled with a M5 screw (tightened to 6 Nm with a torque wrench), and, for very rough surfaces, a tin washer may be used for coupling. Sensor tests are implemented in the system and service technicians check the measurement equipment once per year. If the signal amplitude in a specific frequency window falls below a defined limit by, e.g., 10% of the reference value, the sensor has to be checked. Often, the problem was not the sensor itself but a connection. Equipment development is continuous using commercial parts and components. Modules of the equipment are changed or upgraded about every two years. Hardware components that are exchanged are mostly motherboards, RAM and hard-disks. Due to failures of magnetic hard-disks, the use of SSD is recommended for data storage. Sensors are exchanged after five to seven years or sometimes earlier depending on use. Equipment malfunction is often found to be due to erroneous operation by the client. The second response deals with a three-year monitoring operation of a steam pipeline segment in a power plant. Due to the high operating temperature (above 500 \({}^{\circ}\)C), six high temperature sensors (type D9215) were coupled with a ceramic based adhesive (type Thermoguss 2000) and protected with a casing made from austenitic steel. Sensor tests during service were not possible. Data acquisition was operated with a front-end filter in order to limit the amount of noise signals recorded. Observation after demounting the pipe segment indicated that sensors were still coupled, but cracks had appeared in a few of the couplant adhesive. After removing the austenitic bands that held the sensors, four of the six sensors easily detached from the test object. The total amount of data recorded was about 220 GB. The data acquisition unit worked well, but data storage hardware failed a few times, resulting in loss of data. These data storage failures were attributed to ambient vibrations from power plant operation. The PC and monitor screen also had to be changed once. Data analysis is still in progress, however. The third response deals with concrete elements that were monitored for alkali-silicic acid reactions at elevated temperature (40-60 \({}^{\circ}\)C) and humidity (98-100% relative humidity). Monitoring duration was between six and twelve months (AE sensors type VS-150K). Results are presented and discussed in [123]. Automatic sensor tests were initially performed every five minutes and every twelve hours later. Due to chemical reactions, the wave propagation properties of the concrete changed, and this was reflected in the measurements.