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PCMx Operation Manual

There was a coated pipeline, which had three defects of

equal electrical resistance, one near the beginning, the

next in the middle and one at the end. If the current loss

graph is prepared using mA on the vertical scale the

second and third faults would be shown as progressively

smaller steps in the graph. This is due to Ohms law, and

the loss of current at the previous fault. See Fig. 8.4.

Fig. 8.4: mA fault graph

Using dBmA as the vertical scale would result in equal

steps in the graph for equal size faults, regardless of how

much current was lost at the first fault. See Fig. 8.5.

Fig. 8.5: dBmA fault graph

The graphs show three equal faults and the effects on a

graph showing mA and dBmA.

Note that the graph showing mA (Fig 8.4) at first glance

suggests that the faults are of diminishing magnitude. The

graph showing dBmA (Fig 8.5) clearly shows the

magnitude of the faults are equal. Therefore, dBmA shows

the ratio of the faults, whereas mA alone may give rise to

misinterpretation of data due to the high current loss near

the transmitter and lower losses further away.

In figure 8.6, the line marked ‘A’ to ‘E’ is the data

collected, whilst the line ‘1’ to ‘4’ is the interpreted data.

Note that at position ‘A’ the current falls and then returns to

almost its original level. This is probably due to field

distortion of the locate signal possibly caused by another

utility line above the pipeline and should be ignored or

further investigation undertaken.

Fig. 8.6: Collected data and interpreted results

Steps ‘B’ to ‘E’ show definite steps (with some field

distortion at the point of fault) and loss of signal. Note

the recovered reading is less than the signal before the

trouble.

Further investigations using the PCMx A-Frame

should now be undertaken so as to pinpoint the exact

position of the faults.

Radiodetection PCMx User Manual

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