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MP BLOG Continued from page 11
ZWW\ KI][M 1\ IT[W IXXMIZ[ \W JM QVÆ]MVK- ing one of the other big guys in the range of a 200-mV swing in P/S. The Hz is so low, I didn't think it could be associated with any kind of radio transmission, but I was not sure if some other large hardware/electronics systems might be involved.
A
I have taken probably thousands of P/S readings using an oscillo- scope. I was looking at a ripple from unregulated, unfiltered > )> [QVOTM XPI[M ZMK\QÅMZ[ 5W[\ WN
these readings were on natural gas distri- bution lines and not transmission lines. I have seen other than 60 or 120 Hz
ripples but they were the exception, cer- tainly not normal. I had brief exposure to main lines that were protected by [QVOTM XPI[M ZMK\QÅMZ[ \PI\ _MZM XZWJIJTa on different phases of the three-phase alternating current (AC) transmission TQVM[ IVL \PMa OMVMZITTa IK\ML \W ÅT\MZ the ripple. In one case on a gas main I found to be electrically continuous, I had
0b I\ \PM ZMK\QÅMZ[ IVL 0b KTW[M
\W I UQLXWQV\ JM\_MMV ZMK\QÅMZ[ \PI\ 1 suspected were on different phases of the AC side. 5a O]M[[ Q[ QN aW] IZM [MMQVO W\PMZ
than 60 or 120 Hz at multiple locations, something foreign to the pipeline is affect- ing the readings. We generally accepted that when we saw other than 60 or 120 Hz, other electrical activity foreign to the cathodic protection (CP) was near our structure. The next issue is what is happening
to your structure while the waveform is positive. I started my studies in 1993 and most CP experts I consulted believed that during the time the waveform was positive, no activity was occurring at the cathode. In other words, a structure with a negative protective P/S reading had enough polarization to either stop or mitigate anodic activity occurring at the cathode. We tried to investigate this in the labo- ratory. I helped with extensive lab stud-
ies. We published two papers presented at CORROSION 1997 and 1998. Our results were inconclusive, but strongly suggested that a high positive ripple did in fact affect polarization levels.
pipeline? connected in conductive mode?
frequency. Did you check this feature in the equipment datasheet? If not, there is a problem with the equipment or your wave is not 2 KHz.
A A A
I'll answer you as an electronic engineer. If you are seeing a 2-KHz wave, at least the equip- ment is taking samples at 4-KHz
Use a digital scope to read the wave print. Two KHz may have been the frequency used by the pipe locator. Was the transmitter
I think it may be the analog-to- digital converter inside the log- ger. Is there any significant induced AC voltage on the
12 MATERIALS PERFORMANCE March 2012
NACE International, Vol. 51, No. 3