1. Direct Current Voltage Gradient
DCVG

2. DCVG Surveys
With a Pipe Connection
Without a Pipe Connection

3. DCVG Surveys
A DCVG survey is an electrical method of locating holidays in a pipeline coating.

4. Preparing to Undertake a Survey
Hexcorder (DCVG)
Half Cell Poles
Cables
Calibrated Half Cells
Safety Equipment
Alignment Drawings
Pipe Locator
Radio
Test leads
GPS Current Interrupters

5. Danger
Do not survey during a thunder storm or if thunder can be heard

6. DCVG Surveys
To easily locate coating defects you need pulsed DC current. This can be provided by the cathodic protection rectifiers using GPS Synchronized current interrupters.
If the pipeline is protected by sacrificial anodes then a temporary rectifier and interrupter needs to be installed. b

7. Installing A GPS Synchronized Interrupter
Sufficient current interrupters need to be installed such that all cathodic protection current is halted at the point of measurement during the instant OFF potential measurement

8. Starting the Hexcorder
Turn the Hexcorder ON by pressing the upper right yellow key, before suiting up with your equipment, to allow the GPS engine time to acquire the satellites.

9. Select Type of Survey Select the survey type C = Close Interval
D = Data Logger
V = CIPS+DCVG
G = DCVG
S = Side Drain

10. DCVG Surveys Readings are usually taken 1 metre apart.
Always have both half cells on the ground when recording the voltage gradient.

11. Reference Electrode Calibration
If the potential difference between the half cells to be used during the survey and a new half cell is more than 5 mV then the half cells need service before use.

12. Put on Your Survey Equipment
Hexcorder
Half Cell extension poles
Connecting cables
Radio transceiver

13. Connect the Half Cell’s
Connect the half cells to the Hexcorder
Left to left connector, right to right connector
Tip Spray with DW-40 if the vegetation is wet

14. Half Cell Extension Pole
Connect the cables to the half cell extension poles and to the Hexcorder.

15. DCVG Without A Pipe Connection
Used to locate coating defects and damage in pipeline coatings.
Does not indicate level of Cathodic Protection

16. Your Now Ready to Survey
The pipe locater should be just ahead of the surveyor to locate the pipe for the surveyor

17. Two Surveyors are Quicker than One
Purchase Cath-Tech Duel Surveyor Kit

18. Quick Menu
Pressing the white key labeled “MENU” allows comments to be quickly appended to your data.
You can enter your own comments Press 5 (Test) from the Main menu to edit the quick menu comments.

19. DCVG No Pipe Connection
The surveyor monitors the voltage gradient measured between two half cells over and beside the pipe.
When a voltage gradient is found the surveyor prospects for the location and size of the holiday

20. DCVG No Pipe Connection
Surveyor Walks over pipeline
IR drop creates voltage gradient in soil
Voltage gradient leads to epicenter
Soil contact important
One surveyor

21. Equipotential Lines at a Defect
Equipotential lines are established in the soil around a defect.
As the probe spacing increases the voltage gradient increases.

22. Defect Epicenter
Where the probe placemat crosses the equipotential lines a voltage gradient is measured.
When the probes are parallel to the equipotential lines a null is observed.

23. Equipotential Variations
Uniform equipotential lines are observed when a small defect is located on the top or bottom of the pipe.
Defects on the side give elliptical equipotential gradients

24. DCVG No Pipe Connection
Advantages
No trailing wire
Less sensitive to AC and DC interference
Disadvantages
Laborious
Survey rate affected by terrain and coating condition

25. DCVG No Pipe Connection
Voltage gradients are measured in the soil around the pipe line

26. DCVG No Pipe Connection
The bar graph moving from the left to the right is a voltage gradient.
Current is flowing from left to right.
The scale of the bar graph is mV.

27. DCVG No Pipe Connection
The bar graph moving from the right to the left is a voltage gradient.
Current is flowing from right to left.
The scale of the bar graph is mV.

28. Bar Graph Scale The bar graph has 3 ranges: +/- 10.0 Millivolts
Press and hold 1 for 10 mV, press and hold 5 for 50 mV, press and hold 0 for 100 mV (confirmation left end of bar graph)

29. Gradient Display & Storage Modes
Press and hold “Q” display = O
Alternating ON then OFF bar Graph
Press and hold “P” display = N
Alternating (ON-OFF) then
zero bar graph

30. The Hexcorder DCVG or Millennium
Measures anodic and cathodic voltage gradients
Indicates current flow direction
Can record up to +/- 500 millivolts
Voltage gradient, direction, GPS coordinates and time can be recorded in memory simultaneously.

31. DCVG No Pipe Connection
DCVG has been used worldwide with success for over 20 years
When used correctly, it is a good coating defect diagnostic tool
DCVG helps prioritize remedial actions

32. DCVG
The Hexcorder DCVG and Millennium models can measure and record the voltage gradient and the GPS coordinates of each measurement

33. Software
Special software is not required with the CATH-TECH™ HEXCORDER survey system.
Windows Hyper Terminal is used to down load the data.
Microsoft Excel or other spread sheet program is used to graph the data

34. Data Security
At the end of the day or more frequently the data should be down loaded to a PC and backed up on the hard drive and on diskette.
For added security editing of the survey data can not be accomplished after it is stored in the HEXCORDER's memory.

35. Data Format The data can be exported in three formats:
ASCII comma delimited
Hexadecimal
ProActive

36. Downloading Data To A PC
Connect the Hexcorder to your PC with the supplied 9 Pin Serial cable.

37. The CATH-TECH™ Hexcorder Survey System
No special software is required to produce presentation quality graphs of your data gathered by the CATH-TECH™ Hexcorder Professional Survey Equipment

38. Thank you from the staff at:
Cathodic Technology Limited
15-1 Marconi Court
Bolton, Ontario
Canada L7E 1E2 Ph
Fax
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