Rev 6 – Feb 2022
Page 36 of 44
outputs a 320 Hz and 640 Hz signal on the pipe. Many pipe locator transmitters are capable of
producing the required signal.
The difference between the 'on' and 'off' potentials is recorded
at the test point nearest the survey start point, and all other test
points encountered. ACVG surveys measure the potential
difference to help determine which direction current is flowing
and therefor indicate the direction of a defect. ACVG
measurements are only taken during ON portion of the cycle.
The surveyor walks on top of the pipeline with both half cells.
When both half cells are in firm contact with the soil a reading
can be taken. If one half cell is in the air, the reading will be
incorrect. The half cells can be oriented to the pipe with two
methods; lateral and longitudinal. For a lateral DCVG survey,
one half cell is kept over the pipe and the other is to the side.
A longitudinal survey has both half cells over top of the pipe.
The half cells are spaced as far apart as possible.
As a defect is approached, a noticeable fluctuation is observed on the Hexcorder Pro at a rate similar
to the interruption cycle. The amplitude of the fluctuation increases as the defect is approached. The
direction of current flow may be determined by the sign of the DCVG readings. The size of the
defect can be estimated by calculating the %IR from the DCVG and CIPS data. Size cannot be
estimated from ACVG data as the AC signal is attenuated by the length of the pipe.
Additional channels are available to perform multiple simultaneous DCVG/ACVG surveys with
extra survey personnel. In complex piping situations, it can be beneficial to survey the gradient to
left and right at the same time to locate the defect on the surveyed pipe or an adjacent pipe.
Combined Surveys
Combining CIPS and DCVG allows
the surveyor to gather data on both the
cathodic protection and the coating
integrity in one survey. This allows
for accurate data correlation after the
survey. Combined CIPS and DCVG
can be performed by one surveyor or
with the assistance of additional
surveyors. A combined survey
requires a pipe connection for the CIPS
reading and two half cells on the soil to
capture the gradient between them.
Utilizing Y-cables can allow two surveyors to perform a combined survey and walk at a normal pace.
It is critical to know both sets of data; CIPS and DCVG/ACVG are complementary surveys. Only by
knowing the level of cathodic protection at a coating defect can intelligent choices for remediation be
made. The data gathered should be analyzed by a qualified corrosion professional.
