−850 mVCSE Criterion
The −850 mVCSE criterion is one of the most recognized cathodic protection evaluation concepts for buried or submerged steel, but it is also one of the easiest to misuse.
Use Warning
This page is an educational study guide. It does not replace official AMPP, NACE, ISO, DOT, API, regulatory, owner, or project-specific requirements. Always consult the governing standard and project documents for exact wording, applicability, exceptions, current-interruption requirements, and documentation requirements.
Quick Definition
The −850 mVCSE criterion generally refers to evaluating whether a steel structure has achieved a sufficiently electro-negative potential relative to a copper-copper sulfate reference electrode under the test condition required by the applicable standard.
Why This Criterion Matters
Field personnel often hear the criterion reduced to a simple phrase: “meet −850.” That shortcut is dangerous. The number by itself does not tell the evaluator whether the reading was measured with the correct reference electrode, whether the value includes voltage drop, whether the structure was under CP current, or whether the measurement represents the structure being evaluated.
A complete CP conclusion requires more than a potential value. It requires the structure type, reference electrode, measurement condition, current source status, survey method, and governing acceptance criterion. A reading can appear acceptable as an ON potential while failing when evaluated as an instant-off or polarized potential.
For certification study, the most important lesson is this: −850 mVCSE is not merely a voltage number; it is a criterion tied to a reference scale and a valid measurement condition.
Core Interpretation Requirements
Reference electrode scale
The subscript matters. In −850 mVCSE, the CSE means copper-copper sulfate electrode. A value measured versus silver-silver chloride, zinc, or another reference electrode cannot be treated as the same value without proper technical conversion and applicability review.
Measurement condition
A potential value should be labeled as ON, instant-off, native, depolarized, coupon-based, or another defined condition. The same location can have very different readings depending on whether CP current is applied, interrupted, disconnected, or allowed to depolarize.
IR drop and voltage drop
ON potentials can include voltage drop through the electrolyte. That voltage drop can make the structure appear more electro-negative than the polarized surface condition actually is. When a polarized potential criterion is being evaluated, instant-off measurement techniques are commonly used to reduce the effect of voltage drop.
Structure and standard applicability
The criterion must be matched to the structure and governing requirement. Pipelines, facility piping, aboveground storage tank bottoms, underground storage tanks, marine structures, and reinforced concrete structures are not automatically evaluated by the same method.
Survey coverage
Passing at one test point does not prove that an entire structure is protected. A valid evaluation also considers test point spacing, current distribution, coating condition, shielding, continuity, interference, and whether the measurement location represents the area of concern.
ON Potential vs Instant-Off Potential
An ON potential is measured while cathodic protection current remains applied. ON readings are useful in some contexts, but they can include voltage drop. An instant-off potential is measured immediately after current interruption, before significant depolarization occurs. Instant-off readings are commonly used when the criterion requires a polarized potential.
A common field error is to record an ON reading that is more negative than −850 mVCSE and then call the location protected without confirming whether ON readings are acceptable for the governing criterion. In many technical evaluations, that conclusion is not defensible.
How to Document the Criterion Correctly
A defensible field record should not say only “−850” or “passes.” It should identify the measured value, reference electrode, current condition, and criterion being evaluated.
Strong documentation looks more like this: “The instant-off structure-to-electrolyte potential was −872 mVCSE; therefore, the test location satisfied the applicable −850 mVCSE polarized-potential criterion, subject to the limitations of the survey method.”
Weak documentation looks like this: “Pipe was −850, good.” That statement omits the reference electrode, current condition, measurement type, standard, and basis for the conclusion.
Field Example
A pipeline test station has an ON potential of −940 mVCSE and an instant-off potential of −820 mVCSE. The ON value is more negative than −850 mVCSE, but the instant-off value is less negative than −850 mVCSE.
If the applicable criterion requires a polarized instant-off condition, the ON value alone should not be used to claim compliance. The location may require additional evaluation, a valid 100 mV polarization assessment if allowed, or troubleshooting to determine why the polarized potential does not satisfy the fixed-potential criterion.
When the 100 mV Criterion Changes the Interpretation
A location that does not satisfy a fixed −850 mVCSE polarized-potential criterion may still satisfy a 100 mV polarization criterion if the applicable standard permits that criterion and the test method is valid. This is a major reason CP reports must distinguish between fixed-potential criteria and polarization-shift criteria.
The two criteria are not interchangeable. The −850 mVCSE criterion evaluates an absolute potential threshold relative to a specific reference electrode and measurement condition. The 100 mV criterion evaluates a change in potential caused by cathodic protection.
Common Mistakes
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Reporting “−850” without the reference electrode.
Why it is wrong: The value is incomplete without the reference scale. -
Using ON potentials where polarized potentials are required.
Why it is wrong: ON readings may include IR drop and can overstate protection. -
Calling every reading more negative than −850 mVCSE acceptable.
Why it is wrong: Applicability depends on structure type, material, environment, standard, and test method. -
Assuming one passing test point proves full protection.
Why it is wrong: Current distribution and shielding can vary significantly across the structure. -
Ignoring overprotection risk.
Why it is wrong: More negative potentials are not always better for every structure, coating, or material condition.
Study Checklist
- Can you explain why “−850” by itself is incomplete?
- Can you distinguish ON, instant-off, native, and depolarized measurements?
- Can you explain why IR drop affects ON potentials?
- Can you identify when the 100 mV criterion may be relevant?
- Can you state why reference electrode type must be documented?
- Can you explain why a single passing reading does not prove full-system protection?
Practice Questions
- Why is “−850 mV” incomplete without a reference electrode?
- Why can an ON potential appear more favorable than the actual polarized condition?
- What information should be recorded with a structure-to-electrolyte potential?
- Why might a location fail −850 mVCSE but still satisfy the 100 mV polarization criterion?
- Why should the governing standard be checked before applying the criterion?