V IR. Fig 1 I.R. Drop Error Component of Potential Measurement. V M = Measured potential V = Ohmic component ( IR error ) V = Polarised Potential

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2 V M = Measured potential V = Ohmic component ( IR error ) IR V = Polarised Potential P V M = V IR + V P VM D.C Supply + - V IR + - Anode Pipe Fig 1 I.R. Drop Error Component of Potential Measurement

3 I.R Error Polarised or Instantaneous OFF Potential Depolarisation C.P.ON C.P. OFF Switch OFF Time - Milli- seconds Fig 2 Idealised Polarised Potential Measurement

4 Distance and contact wire Recording Voltmeter ( Data logger ) Test Post Reference Electrode Pipe / Soil Potential wrt Cu/CuSO 4 volts DISTANCE Soil Variance Metallic Contact Fig 3 Close Interval Overline Potential Survey

5 Distance and contact wire Recording Voltmeter ( Data logger ) Fence Transformer Rectifier Test Post Reference Electrode Timer/ Contactor Groundbed P ip e / S oil P oten tial wrt Cu/CuSO 4 volts Test Point DISTANCE Fence Soil Variance Metallic Contact Pipe /Soil Potential ON I.R. Drop Free Polarised OFF Potential Fig 4 Close Interval Overline Polarised Potential Survey

6 Pipe / Soil Potential wrt Cu / CuSO 4 volts Pipeline Reactance effect plus switching spike seconds Depolarisation This period significantly shortened in the presence of sulphate reducing bacteria C.P. ON 0 10mS Switch Off C.P. OFF Time not to scale Can extend to 50mS in some circumstances Fig 5 Polarised Potential Measurement with Pipeline Reactance and Switching Spike Effects

7 Pipe / Soil Potential wrt Cu/CuSO 4 volts * ON * OFF * ON * OFF Survey Characteristics Time 100x ms Measurements Distance Metres Nominal Speed 1 metre / second ( 3.6km / hour ) ON : OFF Ratio 4:1 OFF Period 0.2 second ON Period 0.8 second Measurement spacing. 1 metre nominal Measure both ON and INSTANT OFF every 1 metre Fig 6 Typical Combination of ON and OFF Potential Measurements During Fast Survey

8 Pipe / Soil Potential wrt Cu / CuSO4 volts Indicated Polarised Potential v optimum timing ( accurate) v 0-100mS early 1 Wait period 2 Measure period v > 100mS early late Milli - Seconds Fig 7 Polarised Potential Measurement Errors due to Inaccurate Timing Survey Characteristics OFF period 1 Second Wait period 0.1 Second Measurement period 0.1 Second

9 Mobile Data logger Transformer Rectifier Timer/ Switching Device Static Data Collector Stray Current Soil Variance Metallic Contact Mobile Static Precise Time Linkage Mobile and Static Fig 8 Close Interval Overline Polarised Potential Survey Mobile + Static + Switching Device

10 Transformer Rectifier Timer Switching Device Static Data Collector Stray Current Soil Variance Metallic Contact DCVG Precise Time Linkage Mobile, DCGV and Static Fig 9 Close Interval Overline Polarised Potential Survey and DCVG Mobile Data logger + Static + Switching Device

11 Approaching a Over a Past a Equipotential Gradient Lines Current Flow Through the Soil Current Flow Through the Soil

12 Pipe / Soil Potential at Test points only Survey Progress Transformer Rectifier Slow Non Precision Synchronised Timer/ Switching Device Soil Variance Metallic Contact No Recording Analogue Indication Only No Local Pipe / Soil Potential No Distance Measurement Peg / Mark and Manually Record

13 Repeat Measurements mV 8mV 3mV 1mV For Example = 27mV Analogue DCVG Characterisation 1. DEFECT IR : Overline to Remote Earth = 27mV. 2. Pipe / Soil Potential shift ( ON - OFF ) : Applied Signal at is ESTIMATED as straight line attenuation between test points, say 300mV Pipe 3. Size ( % IR ) = IR mv Applied Signal mv say 27 = 9% 300 Fig 12 Non Recorded Analogue DCVG Characterisation

14 Transformer Rectifier Precision Timer / Switching Device Optional Static Data Collector 0 Stray Current Soil Variance Metallic Contact Static Data on another graph DCVG Precise Time Linkage Mobile, DCGV and Static Fig. 13 Combined Enhanced CIPS & Recorded DCVG to Indicate Size.

19 Sizing (calculated radius(cm)) Sumed Pipeline PL2C - Sizing and Instant Off summary Distance (m) Instant Off potential (mv)

DCVG Coating Survey Data Sheet

DCVG Coating Survey Data Sheet DCVG COATING DEFECT SURVEYS Today, DC voltage gradient surveys have evolved as an accurate and economic means of locating coating defects. When a DC current is applied to