Technical Practice Guide

Instant-Off Potential: Field Method, Purpose, Interpretation, and Common Errors

An instant-off potential is one of the most important cathodic protection measurements because it helps separate the polarized condition of the structure from voltage-drop error caused by current flow through the electrolyte.

Overview

An instant-off potential is a structure-to-electrolyte potential measured immediately after cathodic protection current is interrupted, before substantial depolarization occurs.

The meter still displays a voltage difference between the structure connection and the reference electrode. The interruption changes the current condition during that measurement; it does not turn the reading into a direct view of the pipe surface by itself.

The purpose of the instant-off reading is to estimate the polarized potential of the protected structure while reducing the voltage error caused by current flow through the electrolyte.

In cathodic protection work, instant-off potentials are commonly used to evaluate whether a structure satisfies the −850 mVCSE polarized potential criterion.

The instant-off reading is important because an ON potential can include IR drop. If that voltage drop is not considered, the measured potential may appear more electro-negative than the actual polarized condition of the structure.

Core idea:

An ON potential is a live measurement with CP current flowing. An instant-off potential is intended to capture the polarized structure condition immediately after interruption, before significant depolarization occurs.

Technical Basis

When CP current flows through soil, water, concrete, or another electrolyte, voltage drop occurs between the structure and the reference electrode. This voltage drop is commonly called IR drop.

An ON potential includes several things at once:

  • the polarized potential of the structure
  • voltage drop caused by current flowing through the electrolyte
  • possible influence from nearby current sources
  • measurement effects from reference electrode location and contact

When CP current is interrupted, the current-related voltage drop is reduced. The potential measured immediately after interruption is intended to represent the polarized structure-to-electrolyte potential before significant depolarization occurs.

Practical field interpretation:

ON potential includes CP current effects and IR drop. Instant-off potential is intended to remove, or at least reduce, the IR drop component.

This is why instant-off readings are generally more useful than ON readings for evaluating polarized potential, when the interruption is valid and the reading is captured at the correct moment.

Timing, interruption quality, reference electrode contact, nearby current sources, and field conditions can all affect whether the value is representative. An instant-off value should therefore be interpreted with its measurement context, not as a standalone answer.

When Instant-Off Testing Is Used

Instant-off testing is commonly used during:

  • annual CP surveys
  • close interval surveys
  • interrupted structure-to-electrolyte surveys
  • tank-to-soil potential surveys
  • system commissioning
  • troubleshooting low or questionable potentials
  • criterion evaluation
  • interference investigations
  • verification after rectifier adjustment

It is especially important where impressed current CP systems are used because impressed current systems can create significant voltage gradients in the electrolyte.

Equipment Typically Used

Equipment Purpose
High-impedance voltmeter Measures structure-to-electrolyte potential.
Reference electrode, commonly CSE Provides a stable reference potential.
Test leads Connect the meter to the structure and reference electrode.
Current interrupter Cycles the CP current source ON and OFF.
Test station access Provides electrical connection to the structure.
Rectifier access Allows interruption of impressed current output.
Field log or survey software Records values, timing, equipment, and abnormal conditions.

For pipeline surveys, synchronized interrupters may be required when more than one current source affects the structure.

General Field Method

  1. Confirm the structure being tested.
  2. Confirm the test station, coupon, tank, pipeline, or other connection point.
  3. Place the reference electrode in contact with the electrolyte near the structure.
  4. Measure and record the native or static potential if required.
  5. Measure and record the ON potential with CP current applied.
  6. Interrupt the applicable CP current source or sources.
  7. Capture the instant-off potential immediately after current interruption.
  8. Restore CP current.
  9. Confirm that the current source resumed normal operation.
  10. Record conditions that may affect the reading.
The critical variable is timing.

The instant-off value must be captured immediately after interruption, before the structure begins to significantly depolarize.

Valid Data Conditions

An instant-off potential is more trustworthy when:

  • the correct structure was contacted
  • the reference electrode was in good condition
  • the reference electrode had good electrolyte contact
  • the correct current source was interrupted
  • all significant influencing current sources were interrupted
  • interrupters were synchronized where needed
  • the reading was captured immediately after interruption
  • the meter was functioning properly
  • test leads were properly connected
  • abnormal conditions were documented

A questionable instant-off reading may result if only one rectifier is interrupted while other CP current sources continue to influence the structure.

Field notes should make the reading context visible: ON value, instant-off value, interruption timing, current sources interrupted, current sources left operating, reference electrode location, soil or electrolyte condition, and any unstable or unusual behavior.

Common Errors and Misinterpretations

Error Why It Matters
Calling an ON potential an instant-off potential May overstate the level of protection.
Reading too late after interruption The structure may have already begun depolarizing.
Interrupting the wrong rectifier The measured value may not represent the intended CP system.
Failing to interrupt foreign current sources Remaining current may still create IR drop.
Poor reference electrode contact Can produce unstable or inaccurate readings.
Measuring the wrong structure Can invalidate the survey data.
Assuming one low reading proves failure Additional context may be required.
Assuming one good reading proves full protection Coverage may vary across the structure.
Treating instant-off as automatically conclusive Timing, interruption quality, IR drop, field conditions, and representativeness still matter.
Common misunderstanding:

An ON reading more electro-negative than −850 mVCSE does not automatically prove adequate CP because the ON potential may include IR drop.

Interpretation

Instant-off potentials are commonly compared to the −850 mVCSE polarized potential criterion.

Instant-Off Potential General Interpretation
More electro-negative than −850 mVCSE May satisfy the −850 mVCSE polarized potential criterion.
Less electro-negative than −850 mVCSE Does not satisfy the −850 mVCSE polarized potential criterion.
Questionable or unstable Additional testing or verification may be required.

However, the instant-off value is not the only possible criterion. A structure may fail the −850 mVCSE polarized potential criterion but still satisfy the 100 mV cathodic polarization criterion if sufficient polarization is demonstrated.

Interpretation Example

  • Instant-off potential: −820 mVCSE
  • Depolarized potential: −690 mVCSE
  • Calculated polarization: 130 mV

The structure does not satisfy the −850 mVCSE polarized potential criterion, but it does satisfy the 100 mV polarization criterion. That distinction matters.

Worked Example

A technician records the following values at a pipeline test station:

Reading Value
Native potential −530 mVCSE
ON potential −1,120 mVCSE
Instant-off potential −870 mVCSE
Depolarized potential −690 mVCSE

Step 1: Evaluate the ON potential

The ON potential is −1,120 mVCSE. This value is very electro-negative, but it includes CP current applied and may include IR drop. It should not be used by itself to prove the polarized potential criterion.

Step 2: Evaluate the instant-off potential

The instant-off potential is −870 mVCSE. Because −870 mVCSE is more electro-negative than −850 mVCSE, the test point satisfies the −850 mVCSE polarized potential criterion.

Step 3: Calculate polarization

Calculated polarization is:

870 mV − 690 mV = 180 mV

The calculated polarization is 180 mV.

Step 4: Evaluate the 100 mV polarization criterion

Because 180 mV is greater than 100 mV, the test point also satisfies the 100 mV polarization criterion.

Conclusion:

The test point satisfies both the −850 mVCSE polarized potential criterion and the 100 mV polarization criterion. The data indicates adequate cathodic protection at this test location, assuming the measurement was valid and representative.

Practice Questions

Question 1

Why is an instant-off potential generally preferred over an ON potential for evaluating the −850 mVCSE criterion?

  1. Because the instant-off value is always more electro-negative
  2. Because it reduces the effect of IR drop
  3. Because it does not require a reference electrode
  4. Because it measures coating thickness

Answer: B

Question 2

A pipeline has an ON potential of −1,050 mVCSE and an instant-off potential of −810 mVCSE. What is the best interpretation?

  1. The pipeline satisfies the −850 mVCSE polarized potential criterion
  2. The pipeline does not satisfy the −850 mVCSE polarized potential criterion at that location
  3. The ON potential proves adequate CP
  4. The reference electrode is definitely failed

Answer: B

Question 3

A test point has an instant-off potential of −825 mVCSE and a depolarized potential of −690 mVCSE. What is the calculated polarization?

  1. 85 mV
  2. 100 mV
  3. 135 mV
  4. 1,515 mV

Answer: C

Question 4

Which condition can make an instant-off potential questionable?

  1. Correct test lead connection
  2. Good reference electrode contact
  3. Unsynchronized current interrupters
  4. Proper current restoration after testing

Answer: C

Question 5

A structure fails the −850 mVCSE polarized potential criterion but demonstrates 125 mV of cathodic polarization. What should the learner understand?

  1. The structure automatically has no CP
  2. The structure may satisfy the 100 mV polarization criterion
  3. The ON potential should replace all other readings
  4. The depolarized potential should be ignored

Answer: B

Related Pages

Tank Bottom CP Systems Ring anodes, grid anodes, reference cells, liners, and current distribution. AST CP Testing Tank-bottom reference cells, rectifier interruption, criteria, and interpretation. UST CP Testing Direct-connect systems, coupons, criteria, and UST CP interpretation. CP Coupons Coupon measurements, instant-off readings, depolarization, and interpretation. Current Requirement Testing Temporary current application, current distribution, and CP design interpretation. Interference Testing Foreign current influence, bonds, stray current, and interpretation. Casing Testing Carrier pipe isolation, casing shorts, and electrolytic contact interpretation. Electrical Isolation Testing Isolation joints, continuity checks, and CP current distribution. Current Interruption Rectifier interruption, synchronization, and instant-off timing. Rectifier InspectionICCP rectifier output evaluation and interpretation. Reference Electrode Use Reference electrode placement, electrolyte contact, and common measurement errors. Depolarization Testing Learn how calculated polarization is used to evaluate the 100 mV polarization criterion. Pipe-to-Soil Potential Surveys Field method, measurement quality, interpretation, and common errors for pipeline potential surveys. −850 mVCSE Criterion Understand the polarized potential criterion and its measurement limitations. 100 mV Polarization Criterion Learn how calculated polarization can satisfy CP criteria even when −850 mVCSE is not met. IR Drop Learn why current flow can distort structure-to-electrolyte potential readings. Reference Electrodes Review why reference electrode type, placement, and contact matter. Close-Interval Survey See how instant-off measurements are used in interrupted field surveys. CP Coupons Compare coupon interruption with structure interruption and polarized potential evaluation. Rectifier Output Connect interruption, output adjustment, and field measurement response. Reference Electrode Conversion Review reference-electrode conversion concepts for CP field data.