A close interval survey, or CIS, is a field survey that maps how well a buried pipeline's cathodic protection is actually working, foot by foot along its length. A technician walks the right-of-way with a reference electrode, taking pipe-to-soil potential readings every few feet, so that instead of a handful of measurements at fixed test stations the operator gets a near-continuous profile of the protection level over the whole segment. Those closely spaced readings reveal exactly where protection is adequate and where it falls short, flagging coating faults and cathodic protection shortfalls that a sparse survey would miss. This page explains how a CIS is walked, how the on/off technique removes measurement error, and how the results are read against the accepted criterion.
Close Interval Survey (CIS) in one line: A close interval survey (CIS) is a cathodic protection survey in which a technician takes pipe-to-soil potential readings every few feet along the pipeline right-of-way to map protection levels continuously. Using the on/off technique to eliminate IR drop error, the readings are compared against criteria such as the negative 0.85 volt polarized potential to find coating faults and cathodic protection gaps.
The purpose of cathodic protection is to hold a buried steel pipe at a potential negative enough that corrosion is effectively stopped, and the way to check that is to measure the pipe-to-soil potential, the voltage difference between the pipe and a reference electrode placed on the ground above it. Fixed test stations give this measurement at intervals of perhaps a mile or more, which is fine for confirming the system is broadly on but far too coarse to find a localized problem. A close interval survey closes that gap literally: a technician walks directly over the pipeline with a reference electrode, usually a copper-copper sulfate cell, and takes a reading every few feet, connecting back to the pipe through a trailing wire from a test point.
The result is a nearly continuous plot of potential against distance for the entire surveyed segment, rather than a few isolated numbers. That density of data is the whole point, because the failures that matter, a damaged spot in the coating, a place where the protection current cannot reach, a section shielded by rock or by a foreign structure, are local. They show up as a dip or a spike in the potential over a short stretch of pipe, and a survey that only samples every mile will step right over them. Reading every few feet catches the meter-scale features that determine where corrosion is actually being allowed to proceed.
A CIS is deliberate, physical work. The technician correlates each reading to a precise location along the pipe, so the finished survey can point maintenance crews to the exact station where protection is weak. Modern surveys log position along with potential so the profile can be laid over the pipeline's route and the alignment sheets, letting an integrity engineer see not just that a low reading exists but where it sits relative to road crossings, other utilities, and known coating history. The survey turns an invisible, buried condition into a map.
A naive potential reading has a built-in error called IR drop. When protection current flows through the soil to the pipe, that current across the soil's resistance produces a voltage that gets included in the measurement, making the pipe look better protected than it really is. To measure the true, polarized potential of the steel, the survey uses the on/off technique: the cathodic protection current sources are cycled on and off in synchronized fashion, usually with an interrupter, and the technician records both the on potential and the instant-off potential at each point. The instant-off reading, taken in the brief moment after the current stops but before the pipe depolarizes, is largely free of IR drop and represents the actual polarized state of the pipe.
Having both readings is what makes a CIS diagnostic rather than merely descriptive. The on potential shows the level with current flowing, and the difference between on and instant-off reveals how much IR drop is present at that spot, which itself is a clue to soil conditions and current distribution. The instant-off potential is the number that gets compared against the protection criterion, because it reflects whether the steel is genuinely polarized to a protective level rather than just sitting in a field of current. Interpreting the two together is a skill, and it is why survey design, interrupter cycling, and technician technique all matter to the quality of the result.
The most widely referenced criterion is the negative 0.85 volt polarized potential relative to a copper-copper sulfate reference: a pipe polarized to at least that negative value is generally considered adequately protected. It is one of several accepted criteria used in cathodic protection practice, and the exact criterion applied and how it is demonstrated depend on the operator's program and the applicable standards. A CIS is the tool that shows, foot by foot, whether the pipe meets the chosen criterion along its whole length, and where it does not, the profile marks the stretches that need attention rather than leaving the operator to guess between distant test stations.
A close interval survey earns its cost by turning a suspicion into a work list. Where the instant-off profile dips below the protection criterion, the survey has flagged a location that is not adequately protected, and the shape of the dip hints at why: a sharp, narrow drop often points to a coating holiday where bare steel is exposed, while a broad shallow shortfall over a longer stretch suggests the protection current simply is not reaching that section adequately. Those findings drive concrete action, digging to repair a coating fault, adjusting or adding rectifier output, installing additional anodes, or investigating interference from a foreign structure. Without the closely spaced data, the operator would know a segment was underperforming somewhere but not where to dig.
A CIS is a periodic snapshot, though, and cathodic protection is a system that runs continuously between surveys. The rectifiers that drive protection current can drift, lose output, or fail entirely, and a coating that surveyed well can be damaged by third-party excavation the week after the crew leaves. This is where continuous monitoring complements the walked survey: the CIS gives the detailed spatial picture at a point in time, and remote monitoring of the protection system watches whether that system stays healthy in between. The two together give both resolution in space and coverage in time.
This is where SCADA and cloud monitoring connect to corrosion control in the field. A cloud SCADA platform such as Merobix can bring rectifier output current and voltage, and potentials at instrumented test stations, back to the control room continuously, alarming when a rectifier trips or output drifts so a failing protection system is caught in days rather than at the next annual survey. Historizing those values also gives context for the next CIS, letting an engineer see whether a low reading in the field corresponds to a rectifier that had been running weak, and letting the operator schedule surveys and repairs against real system behavior. The walked survey and the continuous monitoring answer different questions, and reliable remote data ties them into one coherent view of whether the pipe is protected everywhere, all the time.
Fixed test stations are typically a mile or more apart, which confirms the cathodic protection system is broadly on but is far too coarse to find localized problems. The failures that matter, such as a coating holiday or a shielded section, occur over just a few feet and appear as a short dip in potential, so a survey that only samples every mile steps right over them. Reading every few feet produces a near-continuous profile that catches these meter-scale features and shows exactly where protection is inadequate.
The on/off technique cycles the cathodic protection current sources on and off in synchronized fashion using an interrupter, and the technician records both the on potential and the instant-off potential at each point. The instant-off reading, captured just after the current stops but before the pipe depolarizes, removes the IR drop error that inflates a normal reading, so it reflects the true polarized potential of the steel. That instant-off value is the number compared against the protection criterion.
It is a widely used cathodic protection criterion stating that a pipe polarized to at least negative 0.85 volt, measured against a copper-copper sulfate reference electrode, is generally considered adequately protected against corrosion. It is one of several accepted criteria, and the exact criterion an operator applies and how it is demonstrated depend on their program and the applicable standards. A close interval survey shows, foot by foot, whether the pipe meets the chosen criterion along its length.
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