General NDT Knowledge and Basic Physics
General NDT Knowledge and Basic Physics
The general NDT domain covers the physics and cross-method concepts every Level III must command. Understand the families of NDT methods (RT, UT, MT, PT, ET, LT, VT, AE, etc.), what each detects, and their relative strengths and limitations. Know the basic physics: pressure, force, temperature, gas laws, the ideal gas law, viscosity, density, and the behavior of gases under vacuum. For leak testing, grasp partial pressure, Dalton's law, and how gas mixtures affect tracer-gas detection.
Understand probability of detection (POD), sensitivity, resolution, and the difference between a discontinuity, a defect, and a flaw. Recognize the role of calibration, traceability, and measurement uncertainty. Know the electromagnetic spectrum basics for RT and how radiation safety principles (time, distance, shielding) apply broadly. Be comfortable with the concepts of signal-to-noise ratio and threshold setting, which recur in LT and other methods.
Worked example / rule of thumb
Use the ideal gas law, PV = nRT, to reason about pressure changes. If a sealed volume at constant temperature leaks gas, the pressure drop is proportional to the moles lost; a 10% pressure drop corresponds to roughly a 10% loss of gas moles (at constant T and V). This supports pressure-decay reasoning.
Common question traps
- Mixing up POD with measurement accuracy.
- Forgetting that temperature changes mimic leaks in pressure-decay testing.
- Confusing partial pressure with total pressure in tracer-gas work.
- Assuming a method detects all discontinuity types equally.