Direct Current Voltage Gradient (DCVG) is a survey technique used to assess the effectiveness of corrosion protection on buried steel structures, such as pipelines. It measures voltage gradients set up in the soil by defects in coatings, which cause electrical currents to flow in the soils surrounding the structure. DCVG surveys require two data probes (one red-handled, the other green-handled), since these surveys involve measuring the potential difference between two locations on the surface of the soil above the pipe.
Close-Interval Survey (CIS) is a voltage potential survey technique performed at regular intervals, which are sufficiently small to permit a detailed assessment of corrosion protection flaws. CIS surveys can be performed using one or two data probes. A current-interrupted CIS survey is slower when only one data probe is used, as the operator must have the probe in contact with the soil for a complete interruption cycle after triggering a voltage recording. The CIS process is more efficient when two data probes are used, as at least one data-probe is in contact with the soil at all times as the operator walks down the length of the pipeline, and there is no delay in recording On/Off voltages. Also, by using two data-probes, voltage recordings can be triggered via the push-button switch on one of the probes while the location of survey flags (typically at 100-foot intervals) can be registered via the push-button switch on the second data-probe.
Combined DCVG/CIS Surveys can be performed using M.C. Miller equipment using two data probes and the M.C. Miller Switch Box Controller (Cat. #12555, illustrated). The Switch Box allows DCVG and CIS data to be collected on a single pass down the length of the pipeline. It includes a cable to connect the Switch Box to an M.C. Miller data-logger.