Insight Vol 46 No 10 October 2004 1 Introduction Eddy currents are alternating electrical currents which can be induced to low in any metallic section, their low pattern being disturbed by the presence of cracks or other discontinuities (1-3) . Eddy current testing (ET or ECT) method has been used for many applications such as crack and corrosion detection in different industrial ields. Different eddy current methods have also been successfully implemented for the measurement of electrical conductivity, coating thickness assessment, surface roughness and some other material properties evaluations, material sorting, age-hardening condition, determining of heat treatment status of metals, etc (4-9) . Surface eddy current testing is very sensitive for the detection of tight surface cracks and corrosion. Both ferromagnetic and non-ferromagnetic materials can be inspected. It can be implemented through paint coating, but is only applicable to electrically conductive materials. The eddy current density decreases with depth as an exponential function. The STANDARD DEPTH OF PENETRATION is defined as ‘where the eddy current is 1/e (37%) of its surface value’ (2-3,5) and penetration and phase shift are given by the formulae: d= 50 ¥ 172.41 f ¥ IACS ¥m r ...........................(1) q= x p¥ IACS ¥ f ¥m r ...........................(2) where d is the depth of penetration (mm), f is the frequency (Hz), mr is the relative permeability of the material (dimensionless, for non-ferrous = 1), q is phase lag (degree) x is the distance below the surface (mm) and IACS is the conductivity in %IACS (IACS: International Annealed Copper Standard, conductivity of pure copper is 100 in this industry standard of units). d is in order of 1-10 mm for different metals in a midrange frequency. As it can be seen, different testing frequencies can give more information. Due to improvement in electronic technology in recent years, multi-channel, multi-frequency microprocessor based ET systems are available now at a reasonable price. Pulsed eddy current can acquire data at many frequencies simultaneously (as much as pulse rise-time is decreasing, the higher frequency components will be stronger (10-11) ). Remaining wall thickness measurement and corrosion/erosion evaluation is a vital component of in-service inspection of industrial plants. Heat exchangers, steam generators, condensers, air ventilation systems and chillers are widely implemented in the petrochemical and power generation industries and these devices have many thousands of tubes. Copper alloys tubes offer better resistance to corrosion and heat transfer performance, therefore they are widely used as heat exchanger tubes and the in-service inspection of heat exchanger tubes is one of the most important applications of ET. Eddy current testing is used almost exclusively for this purpose due to the speed of the test (up to 1-2 m/s), ability to introduce automation leading to clear economic and time-saving advantages (5-6,13) . The test is normally carried out using an internal probe (‘bobbin’ probe) which is inserted into the tube either manually or automatically by a driver system (13-16) . The probe diameter must be as high as practically possible (high fill factor) to increase the signal-to-noise ratio (SNR). The interpretation of eddy current signals is a major problem. Reliable analysis has to be performed for discrimination between different types of flaws and also measurement of remaining tube wall thickness. Although some techniques such as C-scan can help on this regard, but phase plane and amplitude analysis in a multi-frequency mode of operation are more common (5,13-14) . The major difficulty in using this ET test system for tubes is the need for precise calibration. For each metal, a suitable calibration block must be created with the same dimensions and material. Artificial discontinuities such as different diameters and depths flat-bottom holes through the tube wall and groove (inside and RADIOGRAPHY A radiographic calibration method for eddy current testing of heat exchanger tubes A Movafeghi, M H Krgarnovin, H Soltanianzadeh, K Edalati, B Rokrok, A Kermani and M Seidi The most technically available NDT method for in-service inspection of heat exchanger tubes is eddy current testing. Using a differential ID bobbin probe, tubes can be tested with high speed, but for accurate estimation of tube condition, precisely machined calibration blocks from the same material are necessary. The purpose of this work was to calibrate the eddy current system by radiography without the use of expensive calibration blocks. Wall thickness measurement and deterioration determination due to corrosion/erosion/pitting was evaluated in this study by using radiograph ie ilm density measurements. It was found that this method can determine the remaining wall thickness as well as pitting corrosion in the tubes using differential and absolute ilm density measurements. Results show that good precision can be obtained with this method of eddy current system calibration. Amir Movafeghi, PhD candidate, School of Mechanical Engineering, Sharif University of Technology, Tehran. NDT level-3 Certiicate Holder. Mohammad H Kargarnovin, PhD in Mechanical Engineering, Center of Excellence in Design, Robotic and Automation, School of Mechanical Engineering, Sharif University of Technology, Azadi Ave., Tehran 11365- 9567, Iran. Hamid Soltanianzadeh, PhD in electrical engineering, Radiology Research Lab, Henry Ford Health System, Michigan, USA also Faculty of Electrical and Computer Engineering, University of Tehran, Tehran, Iran. Kaveh. Edalati, MSc in Metallurgical Engineering, NDT level-2 Certiicate Holder. Behrouz Rokrok, MSc in Nuclear Engineering, signal analysis and simulation expert. Aboutaleb Kermani, MSc in Nuclear Engineering, NDT level-2 Certiicate Holder. Masoud Seiedi, Production Technician, expert in fabrication of test pieces. Contact Address: Department of NDT, Nuclear Safety and Radiation Protection Technological Center, AEOI, PO Box: 14155-1399 Tehran, Iran. Tel: (+9821) 8005053; Fax: (+9821) 8008376; E-mail: ndt99@aeoi.org.ir