## 4 Discussion  
The information presented from the literature suggests the following aspects that deserve attention when planning and implementing AE or GUW for long term SHM or CM: (1) performance of the couplant material and of the mounting device; (2) assuring sufficiently constant sensor sensitivity; (3) hardware and software development; (4) signal transmission and data storage; and (5) maintenance and repair concepts. In addition to planning and respective design of the components and software, for all these aspects, periodic performance checks are essential. These comprise sensor performance and coupling, typically with automatic sensor test routines at defined time intervals, and periodic equipment performance characterization. These, e.g., apply standard procedures or equivalent tests implemented by the equipment manufacturers. Data backups are also essential. Procedures for specifically checking on proper operation of AE or GUW signal transmission and software performance are not available in standards. Operator experience is, even though a \"human factor\", probably the best current approach for this. Of course, double checking by another operator (\"four eyes instead of two\") may also help in detecting malfunction of any component of the measurement chain. The scant information available in literature seems to suggest bonding with thermoset epoxy-based adhesives as the first approach for coupling sensors for long-term monitoring. Epoxies, as other polymers, may age and degrade, particularly at elevated temperatures or under hygro-thermal changes of the environment. For special applications or service conditions, it might be useful to contact strain gauge manufacturers or suppliers for recommendations on specific surface preparation and choice of the adhesive. For the latter, adhesives manufacturers may also provide useful information. Note that complete removal of the adhesive after the test without any damage to the test object and its surface may be an important requirement. However, for typical long-term monitoring tests, e.g., on infrastructure, this likely is less important. If adhesive bonding is used, the question can be asked whether an additional sensor mount device is still necessary. Without mount device, potential interference with signal propagation is automatically eliminated. There are also cases where a couplant may not be necessary or suitable, as noted above, but then a mounting device is necessary. Magnetic sensor holders are used in many AE applications that involve test objects made of magnetic materials. Again, service temperature may limit the use of magnet holders. For example, Neodymium-Iron-Boron (NdFeB) magnets start to demagnetize at temperatures around +80 \({}^{\circ}\)C. Adding Dysprosium (Dy) and Terbium (Tb) improves temperature stability to at least +200 \({}^{\circ}\)C but is likely to also increase cost. NdFeB-magnets further are sensitive to corrosion. These effects may be reduced by adding cobalt or by coating the magnets with a suitable surface protection layer [118]. In the literature, there is no information on long-term performance of magnet holders to the best knowledge of the author. Sensor mounts may simultaneously have to hold the transducer and protect it against the environment, accidental damage caused by other equipment or service operations or against interference by non-authorized personnel or vandalism. As noted above, for PZT-based transducers, the operating temperature range is limited by the Curie-temperature of the piezo-material, typically to about +100 \({}^{\circ}\)C or less. Recent developments of special AE sensors extend the limits to higher temperatures up to +150 \({}^{\circ}\)C [119] or +180 \({}^{\circ}\)C [120]. However, the maximum and minimum operating temperatures are not the only criterion. PZT is a piezoelectric and pyroelectric material [121].