page_title: DCVG
section_title: Practical Methods
hierarchical_section_title: DCVG / Practical Methods
caption_attribution_description: English: Author
context_page_description: DCVG stands for Direct Current Voltage Gradient and is a survey technique used for assessing the effectiveness of corrosion protection on buried steel structures. In particular, oil and natural gas pipelines are routinely monitored using this technique to help locate coating faults and highlight deficiencies in their cathodic protection strategies.
context_section_description: In theory, a standard analogue electronic multimeter could be used to perform a DCVG survey, but in practice it would be very difficult to take accurate readings and assess the direction of the voltage gradients correctly. A digital multimeter is completely unsuitable because of the difficulty in quickly assessing the direction of the voltage gradient. Specially designed DCVG meters are available, which have bespoke voltage ranges, specially designed transient response, rugged cases and (usually) a centre-zero meter movement for ease of use. The NACE method requires the measurements to be made using a pair of copper-copper(II) sulfate electrodes rather than simple metallic probes. In addition, the cathodic protection is switched on and off repeatedly using an electronic switch commonly referred to as an interrupter. Thus, two voltage readings (the "on" and "off" potentials) are taken at each fault position. Counter-intuitively, it is actually the "off" potential which is regarded as more indicative of the effectiveness of the CP applied to the pipeline.
  Pipelines which do not have any form of CP may be surveyed by using a temporary DC supply and anode bed. Long pipelines frequently have more than one DC supply for their CP, requiring a number of synchronised interrupters to perform a survey. DCVG surveys are often combined with other techniques, such as close interval potential survey and soil resistivity as part of a comprehensive corrosion protection program.
  Results of a DCVG survey often result in selecting locations to excavate pipelines, which can be costly in urban areas.  Collection of data and interpretation may be performed by pipeline companies themselves or, more usually, by independent specialists.
caption: A Purpose-Built DCVG Meter
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