On vs Instant-Off Potentials: What Field Testers Need to Capture

On and instant-off potentials can tell different stories about a pipeline’s cathodic protection. For field operators, the difference matters: an on reading includes voltage drops in the surrounding electrolyte, while a properly timed instant-off reading can reduce their influence. Cathodic Protection Potential Testers must capture more than a single value, however. Test location, reference electrode placement, interruption timing, current sources, and site conditions all affect how readings are interpreted. Understanding what to record helps teams produce useful evidence for assessing protection against the applicable criteria.

Why the on reading can be misleading

An on potential is measured with the cathodic protection system operating. The tester records the voltage between the pipeline and a reference electrode in contact with the surrounding electrolyte, usually soil for a buried line. That voltage is useful: it shows what the instrument sees under normal operating conditions and can help identify changes from earlier surveys.

It is not necessarily the polarized potential at the pipe surface. Current flowing through soil creates an electrical potential gradient, commonly described as an IR drop. The measured value includes contributions from that gradient as well as from the pipe-to-electrolyte interface. Soil resistivity, coating holidays, current distribution, and electrode position can all affect the result.

Consider a test station near an impressed-current groundbed. An on reading may be substantially more negative than a reading taken elsewhere on the line. That difference might reflect protection current in the soil rather than an equivalent difference in polarization at the steel. It is a reason to investigate, not proof on its own that the nearby section is adequately protected.

This does not make on readings disposable. They are often easier to collect, can be compared with operational history, and may be part of an accepted assessment method. Their limitation is that the operator must understand which components are included before comparing them with a criterion.

What makes an instant-off reading different

An instant-off potential is sampled just after relevant protection current sources are interrupted. The aim is to remove, or substantially reduce, the voltage drop associated with that current while measuring before significant depolarization occurs. In the field, the useful sample is a timing window, not simply any value taken while a rectifier happens to be off.

A reading taken too close to the switching transition may contain switching transients. A reading taken too late may have shifted as the pipe begins to depolarize. The appropriate delay and sampling method depend on the equipment, the waveform, and the governing procedure. Operators should record the actual settings rather than describe every off-cycle value as “instant-off.”

A waveform or time-stamped series is particularly valuable when the transition is unclear. It lets the reviewer see whether the instrument sampled a stable portion of the off cycle or captured a spike. A tester that displays only a final number may still be suitable for some work, but it gives the crew less evidence when a reading is disputed.

Interruption has to match the electrical system

Turning off the nearest rectifier is not always enough. Current from other rectifiers, interconnected structures, bonds, or neighboring systems can continue to influence the measurement. Multiple sources may need synchronized interruption so the intended off interval occurs at the test point at the same time.

Before the survey, confirm which sources are within scope, how their timing will be coordinated, and whether interruption is authorized. Do not disconnect bonds, anodes, or other equipment merely to obtain a cleaner trace without an approved procedure. Interruption can affect adjacent assets and protection levels, so coordination and restoration checks matter.

Even synchronized interruption does not guarantee that every voltage gradient has disappeared. Uninterrupted sources and stray currents may remain. Record those limitations; do not label the result completely IR-free simply because the tester captured an off value.

Reference electrode placement is part of the measurement

A field potential has meaning only with its reference electrode identified. For buried pipelines, a copper/copper sulfate electrode is commonly used; other electrodes are used in other environments, including seawater. Values referenced to different electrode types are not directly interchangeable without an appropriate conversion and attention to conditions.

Place the electrode according to the survey procedure, normally as close as practicable to the pipeline route for a surface measurement, and document its position. Moving it even a short distance can change the contribution from the soil voltage gradient, especially near a groundbed, a coating holiday, or another current source. If pavement, a crossing, or access restrictions force an offset, record that offset rather than treating the reading as directly comparable with one taken over the line.

Good contact with the electrolyte matters too. Dry, frozen, or disturbed ground can produce unstable measurements. Check the electrode’s condition and follow the operator’s maintenance and verification procedure. A dirty or damaged electrode, poor contact, or a faulty test lead can create a problem that looks like a change in pipeline condition.

The connection to the pipeline deserves the same attention. Confirm the test lead is connected to the intended structure and that terminals are sound. At crowded test stations, a plausible-looking voltage is not proof that the correct wire has been selected.

What field testers need to capture

A useful field record allows another person to understand how the value was obtained and whether it can be compared with previous results. At minimum, capture the following where applicable:

• Location and asset: pipeline or structure identification, test station, survey point, date, time, and a location reference that can be revisited.
• Measured values: on and instant-off potentials with units, polarity, reference electrode type, and any relevant range or variability.
• Interruption details: sources interrupted, cycle settings, synchronization method, sampling delay, and any source known or suspected to remain active.
• Measurement setup: instrument identification, applicable calibration or verification status, reference electrode position, and test connection used.
• Conditions and exceptions: soil or surface conditions, access constraints, unstable readings, nearby electrical activity, and deviations from the procedure.

Where the tester records a waveform, retain it with the point identification and timing settings. A screenshot without a clear time scale or a file detached from its survey location may be difficult to use later. If the crew repeats a measurement after correcting a lead or moving the electrode, keep the original result and explain what changed.

For close interval potential surveys, position accuracy and the relationship between successive readings also matter. A single test-station value can miss variations between stations. Conversely, a dense set of readings is not automatically reliable if the interrupters drift out of synchronization or the reference electrode loses contact along the route.

Do not apply a criterion without its conditions

Pipeline cathodic protection standards and regulations may permit different assessment approaches, including approaches based on potential or polarization. The applicable requirement depends on the asset, jurisdiction, environment, standard edition, and operator procedure. Identify the criterion and its measurement conditions before the crew starts collecting data.

A commonly cited example in buried pipeline practice is a negative potential of 0.85 V with respect to a copper/copper sulfate reference electrode. It should not be applied as a universal pass/fail number: relevant IR-drop considerations, the applicable standard or regulation, and the measurement method must be addressed. A value measured against another electrode, or an on value containing an unknown voltage drop, is not automatically equivalent.

Likewise, an instant-off reading and a polarization shift are not the same measurement. Assessing polarization generally requires a suitable comparison between the polarized condition and a depolarized or native reference condition, obtained and interpreted under the applicable procedure. Recording an on/off pair alone does not establish that comparison.

Document the exact standard and edition used on the project, along with any governing local requirements. If the field procedure and the criterion appear to call for different measurements, resolve that before assigning a pass or failure to the asset.

When the trace does not behave as expected

If the off trace is irregular, check the measurement chain before drawing a corrosion conclusion. Verify the reference electrode and lead connections, then confirm interruption timing at the relevant sources. Compare repeated cycles. A change that tracks the switching cycle may point to timing or current-distribution effects; erratic movement may require investigation of interference or contact quality.

At a crossing with another protected structure, for example, the surveyed pipeline may be affected by current that the crew cannot interrupt. The right response may be to coordinate with the other operator, use an approved alternative assessment method, or report that the instant-off result has a known limitation. Selecting the most favorable value from a fluctuating display is not a substitute for that work.

Potential data also need to be viewed alongside the rest of the integrity record. Coating condition influences where current enters the pipe; electrical isolation and continuity affect its distribution; inspection and direct examination can provide different evidence about metal loss. None of those records turns a questionable potential measurement into a sound one, but together they can help determine where follow-up is warranted.

Before leaving a test point, make sure the saved record answers a practical question: could another technician repeat this measurement, using the same reference location and interruption setup, and understand any difference? If the answer is no, the missing field notes may be more consequential than another decimal place on the display.