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The widely practised technique to monitor pipe to soil potentials of a buried pipeline is to take measurements using a saturated copper/copper sulphate electrode and a high resistance voltmeter, making electrical contact to the pipeline at test posts that are usually spaced at regularly one to two Km intervals along the pipeline route. The problem with such measurements is that they reflect only what is happening one or two metres either side of the test post. Therefore what happens to the pipe to soil potentials between test posts are largely unknown. Further, local influencing factors such as a significant coating fault within a short distance of a test post can dominate the test post reading giving rise to limited information as to the true nature of the level of protection on a pipeline.
To overcome the limitations of test post readings the Close Interval Potential Survey (CIPS) technique was developed more than 45 years ago. In CIPS a long trailing cable is used to make contact to the pipeline via the test post and pipe to soil potentials are measured at regular short intervals along the whole pipeline route thus providing a more intimate view of the level of protection along the whole pipeline. The conventional voltmeter is usually replaced by a data logging voltmeter to gather the large quantity of measurements made. The technique is claimed to provide a detailed potential profile and the results are widely used to assess the overall effectiveness of the pipeline Cathodic Protection. The usual practice in CIPS is to determine the pipe to soil potential with all recognised DC sources (transformer rectifiers) operational (ON POTENTIAL) and then with all the DC sources interrupted (OFF POTENTIAL). The latter measurement, the OFF potential is now regarded world wide as the most important potential, and is thought to largely eliminate the IR voltage drop errors present in ON readings that are caused by current flow through the soil and films of corrosion product on the pipe surface. The concept of ON/OFF potentials as a technique is based on the principle that when the CP is switched OFF, the IR component in the potential measurements decays almost instantaneously but the pipe to soil interface polarisation decays very slowly over hours/days depending upon the soil composition. This allows what is considered to be a more correct pipe to soil potential (the OFF Potential) and hence a better assessment of the level of protection to be measured (free from the IR error). There is no industry standard identifying the switching frequency and the time after switching that the ON and OFF potentials should be measured. Each vendor of equipment have chosen their own time sequences which depends upon synchronisation method (and drift of Synchronisation) and accuracy not only of the interrupters used but also the synchronisation of the Data Logger with the Interrupters.
DCVG Ltd have designed and produced commercially available Quantum CIPS equipment. This equipment is Satellite controlled and capable of operating at variable ON/OFF sequences, in particular at the DCVG pulse frequency of 0.45 ON/0.8 OFF so two types of survey, true DCVG and CIPS can be run independently but as a one pass survey as required by the ECDA concept.
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