Instant-Off Potential and IR Drop
Separates the IR drop in an ON reading from real polarisation and checks both criteria on the instant-off potential.
Example
You enter
- ON potential (V, negative) -1.15
- Instant-off potential (V, negative) -0.88
- Native potential (V, negative) -0.62
- Absolute criterion (V, e.g. -0.850) -0.85
- Polarisation criterion (mV) 100
You get
- IR drop in the ON reading 270 mV of soil, not steel
- Real margin, on the instant-off PASSES by 30 mV
- Apparent margin, on the ON reading 300 mV -- 10.0 times too optimistic
- Polarisation above native 260 mV -- clears the 100 mV criterion
- Optimism ratio 10
Details, formula, and sources
The worked station passes by 30 mV while the ON reading suggests 300 -- ten times too optimistic. One rectifier left running voids the whole reading.
IR drop = instant-off minus ON potential; -0.850 V criterion applied to the instant-off potential against Cu/CuSO4; polarisation = native minus instant-off.
NACE SP0169 (now AMPP) and the CP designer govern.
The criteria are in NACE SP0169; public reference cells and interrupters make the measurement.
Estimate. AHJ and licensed professional govern.
Field names used by the API: on_potential_v, instant_off_potential_v, native_potential_v, criterion_v, polarization_criterion_mv, ir_drop_mv, off_margin_mv, on_margin_mv, polarization_mv, optimism_ratio
- Reference electrode copper-copper sulfate; other references shift every figureNACE SP0169
- Interruption every influencing source must be interrupted synchronouslysurvey procedure