Direct Current Voltage Direct Current Voltage GradientGradient

DCVG

DCVG SurveysDCVG Surveys

With a Pipe Connection

Without a Pipe Connection

DCVG SurveysDCVG Surveys

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

Preparing to Undertake a SurveyPreparing 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

DangerDanger Do not survey

during a thunder storm or if thunder can be heard

DCVG SurveysDCVG 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

Installing A GPS Synchronized Installing A GPS Synchronized InterrupterInterrupter

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

Starting the HexcorderStarting 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.

Select Type of SurveySelect Type of Survey

Select the survey type

C = Close Interval D = Data Logger V = CIPS+DCVG G = DCVG S = Side Drain

DCVG Surveys

Readings are usually taken 1 metre apart.

Always have both half cells on the ground when recording the voltage gradient.

Reference Electrode CalibrationReference 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.

Put on Your Survey EquipmentPut on Your Survey Equipment

Hexcorder Half Cell extension

poles Connecting cables Radio transceiver

Connect the Half Cell’sConnect 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

Half Cell Extension PoleHalf Cell Extension Pole

Connect the cables to the half cell extension poles and to the Hexcorder.

DCVG Without A Pipe DCVG Without A Pipe ConnectionConnection

DCVG Used to locate

coating defects and damage in pipeline coatings.

Does not indicate level of Cathodic Protection

Your Now Ready to SurveyYour Now Ready to Survey

The pipe locater should be just ahead of the surveyor to locate the pipe for the surveyor

Two Surveyors are Two Surveyors are QuickerQuicker than than OneOne

• Purchase Cath-Tech Duel Surveyor Kit

Quick MenuQuick 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.

DCVG No Pipe ConnectionDCVG 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

DCVG No Pipe ConnectionDCVG No Pipe Connection

Surveyor Walks over pipeline

IR drop creates voltage gradient in soil

Voltage gradient leads to epicenter

Soil contact important One surveyor

Equipotential Lines at a DefectEquipotential Lines at a Defect

Equipotential lines are established in the soil around a defect.

As the probe spacing increases the voltage gradient increases.

Defect EpicenterDefect 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.

Equipotential VariationsEquipotential 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

DCVG No Pipe ConnectionDCVG No Pipe Connection

Advantages

No trailing wire Less sensitive to

AC and DC interference

Disadvantages

Laborious

Survey rate affected by terrain and coating condition

DCVG No Pipe ConnectionDCVG No Pipe Connection

Voltage gradients are measured in the soil around the pipe line

DCVG No Pipe ConnectionDCVG 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 0 - 150 mV.

DCVG No Pipe ConnectionDCVG 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 0 - 150 mV.

Bar Graph ScaleBar Graph Scale

The bar graph has 3 ranges: +/- 10.0 Millivolts +/- 50.0 Millivolts +/- 100.0 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)

Gradient Display & Storage ModesGradient 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

The Hexcorder DCVG or MillenniumThe 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.

DCVG No Pipe ConnectionDCVG 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

DCVGDCVG

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

SoftwareSoftware

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

Data SecurityData 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.

Data FormatData Format

The data can be exported in three formats:

ASCII comma delimited Hexadecimal ProActive

Downloading Data To A PCDownloading Data To A PC

Connect the Hexcorder to your PC with the supplied 9 Pin Serial cable.

The CATH-TECHThe CATH-TECH™ ™ Hexcorder Survey SystemHexcorder Survey System

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

Thank you from the staff at:Thank you from the staff at:

Cathodic Technology Limited

15-1 Marconi Court

Bolton, Ontario

Canada L7E 1E2

Ph. 905 857-1050

Fax 905 857-3499

Email ctl@corrosion-rust.com

Web Site www.dcvg.net