2010 Volume 59 Issue 5 Pages 187-194
In cathodic protection (CP) by impressed current method, polarization decay, a shift in potential following the switch-off of CP current, is used as parameter of adequate operation especially in the media of high resistivity, where iR-drop compensation is indispensable. Instant-off potential, sampled immediately after switch-off of CP current, is an essential parameter that represents effective ON-potential (with iR-drop compensation) and a baseline for the polarization decay. A detailed analysis of potential transient demonstrated that a transition from iR-drop to electrochemical relaxation starts within a millisecond in concrete. A novel portable instant-off potentiometer that allows direct reading of the instant-off potential was developed. The potentiometer consists of (a) generation of timing pulses for driving MOS switches and triggering an AD converter, (b) switching of CP current with variable ON/OFF interval and duty ratio, and (c) fast AD conversion triggered synchronously with ON/OFF timing followed by storing and computing the average. Noise level was minimized by properly selecting the sampling interval and by synchronizing the logging timing with the power-line wave but dispersing phase shifts of sampling points. A typical example of data logging interval and timing, duty cycle, and sampling number for averaging is shown for minimizing noise level of power-line origin. The potentiometer allowed direct display of the instant-off potential with the interval of 1.6 s. The measurements by model concrete block and concrete bridge on-site have proven the effectiveness and validity of the potentiometer.
