Measurement of electromagnetic properties of heat exchanger tubes with model-based eddy current technique

Electromagnetic properties of heat exchanger tubes with outside diameter in the range from 15.8 mm to 25.3 mm and wall thickness in the range from 0.63 mm to 1.6 mm made of SAF 2205, Type 439 and Sea-Cure were determined with model-based eddy current technique. Two encircling eddy current measurement sensors with uniform field were optimized and designed based on initial computer modeling results. A precision measurement circuit was assembled with the sensors and a lockin amplifier. Sensor computer models were built and validated through direct magnetic and voltage measurements. The sensor magnetic field uniformity was tested and agreed very well with model predictions. The sensor resonance frequencies were verified to ensure resonance effects would not interfere with the measurement process. The tubes' electrical conductivity and initial magnetic permeability were determined through optimization process minimizing the “cost” (objective) function at three frequencies 200 Hz, 1 kHz, and 2.5 kHz. The agreement between the eddy current and other conventional destructive magnetic and electrical measurements was very good where relevant comparison was possible.