Equipment 4 of the GWT training series covers direction control, followed by the first two steps of the field procedure.
Direction control is something worth understanding, even if you never operate an MsS system.
Imagine placing a single probe ring on a pipe and sending a wave. The wave will travel in both directions along the pipe and reflect whenever it encounters a change in the pipe's cross-sectional area, such as a weld or a defect. That's simply how physics works. The problem is that, with only one probe ring, you cannot determine which direction a reflection came from.
So, how do we solve this problem?
Instead of using just one probe ring, we use two. Problem solved! Here's how it works.
The two probe rings are physically spaced one-quarter wavelength apart, but they are not fired simultaneously. The first probe is fired, followed by the second probe one-quarter of a period later.
Let's first look at the wave traveling in one direction, where the second probe ring is positioned ahead of the first. With the physical separation and firing delay combined, the additional travel distance is effectively compensated for. As a result, the two wave contributions arrive in phase and reinforce each other.
Now, let's look at the wave traveling in the opposite direction, where the second probe ring is positioned behind the first. In this case, the additional travel distance and firing delay combine to create a half-period phase difference. The two wave contributions are therefore completely out of phase and cancel each other out.
By using two channels—one for each probe ring—we can steer the wave in the direction we want.
Here is the most important part that many people overlook.
A quarter wavelength is not a fixed distance. Wavelength is determined by wave velocity divided by frequency. This means that the spacing that works at 32 kHz will be incorrect at 128 kHz—a difference of a factor of four.
In other words, the physical separation between the two probe rings must change when the operating frequency changes.
This is why directional control is most effective near the frequency for which the probe spacing was designed. As the operating frequency moves away from that target, directional control gradually becomes less effective.
That's why different probe spacings are needed for different operating frequencies, such as 32 kHz, 64 kHz, 90 kHz, 128 kHz, and so on.
This is not a defect or a system limitation. It's simply geometry and physics. Before relying on a directional indication in your inspection report, it's important to understand which frequencies your probe setup is actually optimized to steer.
The second half of this module covers the field procedures that follow: choosing the probe location, marking the reference line, preparing the pipe surface, and preparing the strips.
None of these steps is particularly complicated, but each plays an important role in determining the quality of your inspection data.