10 Jul The Secret to Maximizing Remote-Field Testing Inspection Speed

All of you who’ve been doing tube testing for a while know this: there’s nothing more risky than getting inaccurate inspection results because a probe is pulled too quickly. And yet, optimizing the inspection speed is important. Placing the finger on that magical maximum speed isn’t an easy task, but it’s not black magic either.

Speed Sensitivity—Low-Pass Filter

Remote-field testing (RFT) results are especially sensitive to speed. So it’s particularly important when using this technique to have as much as possible an accurate maximum pulling speed.

The probe’s excitation frequency dictates what low-pass filter is applied to inspection data to lower noise, and the frequency of the low-pass filter is directly proportional to the maximum inspection speed. Still with me so far?

Simply put, the lower the excitation frequency, the higher the noise from the utility frequency (50 Hz or 60 Hz power outlet). The higher the noise, the tighter the low-pass filter (lower frequency) around the excitation frequency of the probe. Consequently, the tighter low-pass filter can eliminate high-frequency signals. This calls for a lower inspection speed to reduce the defect’s signal frequency.

Ascertaining the Maximum Pull Speed

To rigorously ascertain the maximum inspection speed, you must take into account three variables:

  • The target defect’s size
  • The receiver coil’s physical size (which varies depending on the probe design)
  • The instrument’s bandwidth/low-pass frequency

Assuming 4 mm (0.158 in) defects and a typical RFT probe for 25.4 mm (1 in) tubing, here’s what you could expect to be your maximum pulling speeds:

  • An excitation frequency of 20–100 Hz calls for a low-pass filter of approximately 8 Hz, which is quite low, therefore a maximum pull speed of 0.10 m/s (4 in/s)
  • An excitation frequency of 100–200 Hz calls for a low-pass filter of approximately 12 Hz, therefore a maximum pull speed of 0.15 m/s (6 in/s)
  • An excitation frequency of 200–350 Hz calls for a low-pass filter of approximately 16 Hz, therefore a maximum pull speed of 0.20 m/s (8 in/s)
  • An excitation frequency of 350–500 Hz calls for a low-pass filter of approximately 20 Hz, therefore a maximum pull speed of 0.25 m/s (10 in/s)
  • An excitation frequency of 500–1000 Hz calls for a low-pass filter of approximately 25 Hz, therefore a maximum pull speed of 0.30 m/s (12 in/s)
  • An excitation frequency of 1 kHz or more calls for a low-pass filter of approximately 50 Hz, therefore a maximum pull speed of 0.50 m/s (20 in/s)

I hope these guidelines can save you some headaches! Take a look at our RFT single-driver probes, RFT double-driver probes, and RFT single-driver boiler probes, or you can use our tubing probe creator to find out which probe is best for your specific application.

 

Introduction image © Rémi Kaupp CC-BY-SA, Wikimedia Commons

Categories

Electromagnetic testing How-to NDT Tubing inspection

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