Connect the amplifier to Channels 1 and 2 of an ect MAD 8D Eddy Current System. Set Channels 1 and 2 to the same frequency.
Both frequency channels 1 and 2 must be set to the same frequency. It is recommended that the frequency controls only be adjusted in the Setup Menu. If you adjust the frequency controls in the Channel Menu, you will observe large amounts of noise when the two channels are not set to the same frequency.
Start with a frequency of approximately 90 Hz for a thick 0.125 inch wall tube or 1 kHz for a 0.049 inch thick wall. Do not exceed 1.2 kHz or the signal may become very noisy. Use frequencies in between for other wall thicknesses.
The optimum frequency for inspection cannot be calculated because in order to do so, it would require knowledge of the BH (magnetic) characteristics. Generally, the magnetic properties of these materials are not known.
Inspect the calibration tube. The watch point is looking for an approximate 45 phase angle difference between a 75 per cent and 25 per cent groove. If the phase angle is higher, it is recommended to decrease the frequency. If the phase angle is lower, it is recommended to increase the frequency.
This process is trading off decreased sensitivity to outside defects with the higher frequencies and a lower phase angle spread; hence, less resolution between defects of different depths with the lower frequencies. It is up to the Inspector to choose the frequency that he believes gives him the best result.
Because the frequencies are close to the frequencies used for electrical power distribution and the signal received back from the probe is very weak, the system may be subject to interference from power distribution systems. To avoid this problem, keep the equipment away from electrical devices that may emit an electromagnetic field, such as an electric motor.
Also, choose frequencies that are not close to your power distribution frequency or its harmonics. Specifically, if you have a 60 cycle power system, avoid a frequency close to 60 Hz. Its harmonics are 120, 180, 240, 300, and so on. Generally, there will be more noise at the odd harmonics, such as 180 Hz (3 x 60) and 300 Hz (5 x 60).
If you have a 50 Hz power system, then avoid frequencies such as 50, 100, and 150 Hz.
Choose an operating frequency between power harmonic frequencies if possible, and perhaps moving your inspection frequency slightly closer to the even harmonic than the odd harmonic.
Keep the LP Filter set below 30 Hz for a 60 Hz power system and below 25 Hz for a 50 Hz power system to filter out the noise.
Normally the Remote Field/Near Field™ switch will be set in the Remote Field position for Remote Field probes and Near FieldTM position for Near Field™ probes. It may be worthwhile to check the Alternate switch position before making a final decision on which switch position works best in a given application. Specifically, when working with a relatively thick wall tube, better performance may be achieved with a Near Field™ probe when the switch is set in the Remote Field position.
Using a low gain setting and a high sensitivity volts per division setting (low number) generally results in best performance. The use of high gain settings may cause distortion of the signals.
Setting up the system contrary to what is recommended above will not harm the system, but may result in more noise.