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Principal of the DC Voltage Gradient Buried Pipeline Coating Survey Technique.

When a Direct Current (DC) is applied to a pipeline, as in the application of cathodic protection, a voltage gradient is created in the soil due to the passage of current from the anode bed through the resistive soil to the bare steel exposed at coating faults in the pipelines protective coating. The voltage gradient surrounding each coating fault becomes larger and more concentrated the greater the current flowing and the closer you are to a coating fault epicentre location. In general, the larger the coating fault, the greater the current flow and hence bigger the voltage gradient.

In the DC Voltage Gradient (DCVG) technique, the voltage gradient is traced to locate the epicentre in the soil surface, which is directly above the fault location on the buried pipeline. To ease interpretation and to separate the DC whose gradient is being monitored from other DC sources, such as long line cells, tellurics, other foreign CP systems, an Interrupter is inserted into the negative lead of the pipeline CP power source to pulse the DC in an asymmetric manner. The pipeline CP system is switched ON and OFF at the frequency of 1.25 hertz, (0.45 seconds ON and 0.8 seconds OFF). The pipeline CP system is usually used but in the case of pipelines that have no specific CP system a temporary CP system such as a rewired existing Transformer Rectifier or a portable Petrol DC Generator can be used.

The gradient is observed by measuring on a special milli-voltmeter the out of balance between two half cells placed in the soil in the voltage gradient at ground level. If one electrode is spaced about one metre from the other in the gradient, one half cell will adopt a more positive potential than the other, a fact that enables the size of the gradient and direction of current flow to be established. This is shown on the DCVG meter scale by the direction of the pulsing movement of the meter needle and the amplitude of the pulse.