First, let’s make sure we all understand what is meant by a frequency channel. One frequency channel will generate one frequency which is sent out to the eddy current probe. The signal that comes back is detected by two phase sensitive detectors, resulting in two analog signals. These are ultimately displayed as a horizontal and vertical signal in the impedance plane as in Figure 1,
or as an impedance plane and a strip chart representation as in Figure 2.
Naturally, as one frequency channel generates two analog signals, there are two traces in the strip chart screen. In this case, the signal happens to be from a through wall hole which is close to a support plate.
If the defect was at a support plate, the shape of the signal and the angle of the signal which is used to measure the depth of the defect would be altered. This makes it difficult to detect and accurately measure defects, especially if they are somewhat small when they are at support plates.
To resolve the problem of detecting defects at support plates, a second frequency channel is used. The second channel is lower in frequency than the first channel. The support plate signal in the second channel is subtracted from the support plate signal in the first channel in a Mixer, usually implemented in software. Figure 3 shows the signals from Channels 1 and 2 in which you can see the support plate and the hole signal. Also shown is Mixer 1, in which the support plate signal has been largely mixed out; therefore, what you see in the mixer is the hole signal without any indication from the support plate.
Figure 4 shows signals from Channels 1 and 2 in impedance plane format on the left.
On the right are the strip charts from Channel 1 and Mixer 1. The signal closer to the top of the screen is from the hole. The signal halfway down the screen is from the support plate. Note that in Mixer 1, the support plate signal has been mixed out. This makes it easier to detect and measure defects close to the support plate.
Differential is the mode to use to find small defects in a tube. Differential uses two coils in the probe and compares one coil to the other; however, differential is not good for finding general erosion corrosion defects. These defects vary gradually along the tube, and as both coils sense it simultaneously, the defect signal is canceled out. To detect general erosion corrosion defects, it is necessary to inspect in Absolute Mode. In Figure 5, Channel 3 has been added in Absolute Mode.
The defect is general corrosion on the outside of the tube in an area about two inches long. In Channel 1, some background noise can be seen; whereas the general erosion corrosion is clearly seen in Channel 3, the Absolute Channel.
A third Differential Channel, Channel 4, is used as a Confirmation Channel. This channel is set to a frequency of 1/4 of that used in Channel 1. At this low frequency, the angle of defect signals in the tube wall vary over a fairly narrow range. For example, in Figure 6,
which shows the 100 per cent, 80 per cent, 60 per cent, 40 per cent, and 20 per cent outside defects from an ASME calibration tube, these vary over an angular range of 20 degrees for the through wall hole to 36 degrees for the 20 per cent outside defect. Figure 7 shows the signal from an internal magnetic inclusion.
By looking at Channel 1 only for the analysis, this appears to be a very deep outside defect, approximately 90 per cent through the wall. Obviously, the tube should be plugged and removed from service. However, by looking at the Confirmation Channel, Channel 4, in the lower right hand screen it is seen that the angle of this defect signal is 44 degrees, which falls outside of the range of tube defects shown in Figure 6; therefore, it is known that this is a magnetic particle imbedded on the inside of the tube wall. This type of defect is not detrimental to the tube, and the tube may remain in service.
Note that it is possible to inspect in Absolute Mode with a single frequency channel, so it would be possible to detect small defects in Differential Mode with a single frequency channel, and then switch to Absolute Mode and to inspect the tube a second time in Absolute Mode to find general corrosion defects. However, this can be done faster if more frequency channels are available by inspecting in Differential Mode and Absolute Mode simultaneously.
Obviously, four frequency channels gives you advantages over fewer frequency channels in that you can inspect at two frequencies in Differential Mode, mixed to eliminate support plate signals for high defect sensitivity at support plates. You can also inspect in Absolute Mode to detect general corrosion and erosion, and you have a Confirmation Channel for distinguishing between tube defects and magnetic inclusions. All of this can be achieved in one single pass of the probe through the tube.