The potential recorded immediately after all influencing current sources are interrupted, once the switching spike has passed but before the structure depolarizes, typically a few hundred milliseconds after the switch. Because IR drop vanishes the moment current stops while polarization decays slowly, the reading approximates the true polarized potential. It is the number compared with the -850 mV polarized criterion on most pipeline surveys.
On is -1150, instant-off potential is -880, so we're meeting criterion at this post.
Ignore the first number that flashes up after the switch. That is the spike; the instant-off is the steady value right behind it.
Also heard as
- instant off
- off potential
- polarized potential
- IO
Related terms
IR drop
Voltage error in a potential reading caused by current flowing through the soil between pipe and electrode.
Interrupter
Timed switch fitted to a rectifier or bond that cycles CP current on and off for instant-off readings.
Ons and offs
Survey slang for paired on and instant-off potential readings taken while rectifiers are being interrupted.
-850 mV criterion
Steel counts as protected when its potential is -850 mV or more negative against a copper sulfate electrode.