Instant-Off Potential

An instant-off potential is measured immediately after cathodic protection current is interrupted to reduce the effect of voltage drop in the electrolyte.

Quick Definition

Instant-off potential is the structure-to-electrolyte potential measured immediately after CP current is switched off, before significant depolarization occurs.

Why Instant-Off Potential Matters

ON potentials are measured while CP current is flowing. Because current is flowing through the electrolyte, the measured value may include IR drop. That voltage drop can make the structure appear more negative than its true polarized condition.

Instant-off readings are used to reduce this error. By interrupting the CP current and measuring immediately after interruption, the voltage drop associated with current flow is reduced while the structure is still polarized.

Poor instant-off technique can invalidate the measurement. Timing, interrupter synchronization, current sources, and meter response all matter.

Core Concept

ON potential

An ON potential is measured while CP current is being applied. It includes the structure potential plus voltage effects caused by current flowing through the electrolyte and other circuit paths.

Current interruption

Current interruption temporarily stops CP current from one or more current sources. In impressed current systems, rectifiers are commonly interrupted. In galvanic systems, anode connections may be disconnected where the system is designed for that test method.

Instant-off reading

The instant-off reading is collected immediately after the current is interrupted. The goal is to capture the polarized potential before the structure begins to depolarize significantly.

IR drop reduction

IR drop is voltage drop caused by current flowing through resistance. Interrupting current reduces the current-flow component of the measurement. This makes instant-off readings more representative of the polarized structure potential than ON readings.

Timing and Synchronization

Instant-off testing requires correct timing. If the reading is taken too late, the structure may begin to depolarize and the value may not represent the intended instant-off condition.

If multiple current sources affect the structure, they must be interrupted in a coordinated way. Failure to interrupt all significant current sources can leave voltage drop in the measurement.

Interrupters must be synchronized when evaluating structures affected by multiple rectifiers or current sources. Unsynchronized interruption can produce unstable or misleading readings.

Field Application

Instant-off potentials are commonly used in pipeline close interval surveys, annual CP surveys, tank-bottom testing, current requirement testing, and troubleshooting.

Field personnel must identify which current sources were interrupted, the interruption cycle, the measurement timing, reference electrode type, and test location.

Instant-off readings are especially important when ON readings are strongly affected by voltage gradients from nearby anodes, rectifiers, bonds, or other current sources.

Common Mistakes

  1. Calling an ON reading an instant-off reading.
    Why it is wrong: The current condition is different, and ON readings can include voltage drop.
  2. Taking the reading too late after interruption.
    Why it is wrong: The structure may depolarize, so the reading may no longer represent the instant-off condition.
  3. Interrupting only one current source when multiple sources affect the structure.
    Why it is wrong: Remaining current sources can leave voltage drop in the measurement.
  4. Ignoring interrupter synchronization.
    Why it is wrong: Unsynchronized current sources can create unstable or invalid instant-off readings.
  5. Assuming instant-off removes every possible error.
    Why it is wrong: Instant-off reduces current-flow voltage drop, but measurement errors, interference, shielding, and reference electrode problems can still exist.

Standards Relevance

This page is educational and does not replace the applicable AMPP, NACE, ISO, DOT, API, regulatory, or project-specific requirements.

Instant-off measurements are commonly used when evaluating polarized potential criteria. The applicable standard or project requirement determines when instant-off testing is required and how readings should be interpreted.

Field Example

A pipeline test station shows an ON potential of −1,060 mVCSE. After synchronized current interruption, the instant-off potential is −830 mVCSE.

The difference indicates that the ON reading included a voltage-drop component. If the applicable criterion requires a polarized potential of at least −850 mVCSE, the instant-off value does not satisfy that criterion.

Practice Questions

  1. What is an instant-off potential?
  2. Why are instant-off readings used instead of relying only on ON potentials?
  3. What can happen if the instant-off reading is taken too late?
  4. Why must multiple current sources be synchronized during interruption testing?
  5. Does instant-off testing eliminate every possible measurement error?

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