The voltage equal to current (I) times resistance (R) that appears in any structure-to-electrolyte reading taken while protective current flows. It makes an on-potential look more negative than the true polarized potential at the pipe surface, sometimes by hundreds of millivolts in high-resistivity soil or near groundbeds. Techs remove it by interrupting current and reading instant-off, by using coupons, or by placing the reference very close to the surface.
That -1200 on-reading is padded with IR drop, the instant-off is only -790.
Also heard as
- ohmic drop
- IR error
- voltage drop
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.
CP coupon
Small bare steel sample buried beside a pipe and wired to it, used to read IR-free potentials.
-850 mV criterion
Steel counts as protected when its potential is -850 mV or more negative against a copper sulfate electrode.