Instant-Off
Instant-off describes the structure-to-electrolyte potential measured immediately after CP current is interrupted.
Quick Definition
Instant-off is the potential measured immediately after CP current is switched off, before significant depolarization occurs.
Why This Term Matters
Instant-off readings are used to reduce IR drop effects in potential measurements. They are often more appropriate than ON readings when evaluating polarized potential criteria.
The word “instant” matters. If the reading is taken too late, the structure may begin to depolarize and the value may not represent the intended condition.
Core Concept
Current interruption
CP current sources are temporarily interrupted so the voltage drop caused by current flow is reduced.
Polarized condition
The reading is taken before the structure has significantly depolarized, so it can represent a polarized potential.
Synchronization
If multiple current sources affect the structure, they must be interrupted in a coordinated way.
Documentation
Reports should identify which current sources were interrupted, the cycle used, the reference electrode, and the timing.
Common Mistakes
-
Calling an ON reading instant-off.
Why it is wrong: ON and instant-off readings are different current conditions. -
Taking the reading too late.
Why it is wrong: Depolarization may begin, changing the reading. -
Interrupting only one source when several sources influence the structure.
Why it is wrong: Remaining current sources can leave voltage drop in the reading.
Field Example
A pipeline ON potential is −1,050 mVCSE. After synchronized interruption, the instant-off potential is −840 mVCSE. If the applicable criterion requires a polarized potential of −850 mVCSE or more negative, the instant-off reading does not satisfy that criterion.