Field shorthand for the two numbers recorded at each reading point while current sources cycle: the on-potential with current flowing and the instant-off with current momentarily cut. The gap between them is the IR drop. Crews talk about grabbing ons and offs at every test station on an annual survey, and an odd spread or a flat off reading points to interference, poor electrode contact or an interrupter out of sync.
Grab ons and offs at every post today, the interrupters are cycling on all three recs.
Also heard as
- on/offs
- on and off readings
Related terms
Instant-off potential
Structure potential read in the split second after CP current is switched off, free of IR drop.
Interrupter
Timed switch fitted to a rectifier or bond that cycles CP current on and off for instant-off readings.
IR drop
Voltage error in a potential reading caused by current flowing through the soil between pipe and electrode.
CIS
Close interval survey: potential readings taken every metre or so along a pipeline to find protection gaps.