AC Voltage Gradient (ACVG)
AC Voltage Gradient (ACVG) surveys can be used as an indirect inspection tool as part of the external corrosion direct assessment (ECDA) process for a pipeline. ACVG uses the same transmitter and receiver as the AC current attenuation (PCM) survey method with an addition of a pair of probes (often assembled at a fixed distance in the A-Frame unit). ACVG is used to evaluate protective coatings on buried pipelines similar to DCVG, but using alternating current from a signal generator. The intent of the ACVG survey is to identify coating anomalies.
The survey consists of using an A-Frame (or other paired probes for earth contact) with the PCM receiver to pinpoint coating defects. The survey procedure consists of setting up the signal generator transmitter at a point of connection to the pipeline. The signal generator is connected to the pipeline and an external structure appropriately grounded to earth. A rectifier location with connections to the pipeline and an isolated groundbed is often ideal. A low frequency (4 and 8 Hz) AC signal will be produced and transmitted along the pipe. The signals will be measured using a receiver unit.
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