Which characteristic is not part of the thermal stability requirement for NDT methods?

The characteristic that is not part of the thermal stability requirement for non-destructive testing (NDT) methods is compatibility with cooling techniques. This choice focuses on how a specific NDT method interacts with cooling systems rather than its inherent ability to withstand high temperatures or heat stress associated with the testing environment.

In the context of thermal stability, the other characteristics are crucial. Performance under heat stress refers to the method's capability to operate effectively when exposed to elevated temperatures, which is vital for maintaining measurement accuracy and reliability during inspections in heated environments. Resistance to thermal degradation indicates how well the materials and components used in the NDT method can endure high temperatures without losing their functional integrity, ensuring that the method remains effective over time.

Long-term efficiency and reliability also relate directly to thermal stability, as NDT methods must consistently produce accurate results even after being subjected to varying thermal conditions over extended periods. This ensures the continued effectiveness of the testing process, which is essential for safety and compliance standards in various industries.

In summary, compatibility with cooling techniques does not directly address the inherent stability of an NDT method under thermal conditions, making it the outlier among the choices presented regarding thermal stability requirements.