Frequency Selection For Inspecting Ferromagnetic Tubes With Near Field™ Probes

The best frequency to use depends upon the thickness of the tube wall and the magnetic properties of the alloy. The magnetic properties of the alloy are not known; therefore, we can only recommend frequencies. The Inspector should select the final frequency based on the results achieved with the Calibration Tube.

.049 450 – 570
.065 210 – 270
.083 150 – 210
.109 90 – 150

Electrical power harmonics may create noise; therefore, it is best to avoid those frequencies which happen to be 60, 120, 180, 240, 360, 480, 540, and 600 Hz. It is best to select a frequency halfway between these harmonic frequencies. (Note that if you are in an area where the power frequency is 50 Hz, then your harmonics will be different than those listed here.)

By choosing a frequency halfway between power harmonic frequencies, then interference from the power system will be a noise in the eddy current signal at 30 Hz. You can minimize this noise by setting the LP Filter to a value lower than 30 Hz, for example, 25 Hz. Try changing the LP Filter up or down by a few Hertz, one Hertz at a time, in order to select the LP setting that has the lowest noise. It is best to use a low gain setting and increase the screen sensitivity (set the Volts Per Division to a lower number) for optimum performance. Adjust the Phase Control so the signal from a 70 per cent O.D. groove will be between 60 and 70 degrees, much like a similar groove in a non-ferrous tube would appear. You may fine tune the frequency so you get a good phase angle spread between a 75 per cent and 25 per cent O.D. grooves, but not too much. A good phase spread might be approximately 60 degrees.