TN 20 Potential Errors in Concrete

A common source of error encountered when making potential measurements in or through concrete is junction potential; this topic has been discussed in Technical Note 19.  There are additional sources of error in concrete potential measurements.  The mobility of all ions in concrete is retarded due to the material’s cellular microstructure.  Consequently, concrete has high electrolyte resistivity so the presence of any internal current flowing through it will be marked by relatively high IR drops.  Sources of these internal currents are often corrosion currents from corrosion of rebars.  The associated IR drops become incorporated into corrosion potential measurements and can result in a several hundred millivolt error.

Errors of the same magnitude have been documented for measurements made through concrete, such as to a structure buried beneath a concrete slab.  In a detailed study on this topic1, potential measurements were made on buried tanks at nine different service stations in the northeast.  When the measurement was made with the reference electrode contacting the concrete slab, the potential was from 20 to 260 mV more negative than when the reference electrode was directly contacting the soil through an access hole.  The rectifier was off during these measurements to eliminate CP currents as a possible error source.  In the same study, potential measurements were made on a pipe located beneath an airport runway.  Measurements made through the concrete near the edge of the runway were about 200 mV more negative than the same measurement made through the grass adjacent to the slab.  Wetting both the concrete and the grass did not significantly change the measured values.  Clearly, measurements of buried structures should never be made through concrete without using a soil contact access port.

  1. B. Husock, “Techniques for Cathodic Protection Testing Over Airfield Pavements,” US Air Force Report CEEDO-TR-78-31, Tyndall AFB, FL, July 1978.

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TN 16 Interrupting CP Coupon Current

The most common method to reduce voltage drop error in potential measurements is to use a CP coupon.  These are small pieces of metal electrically bonded to the structure so they come to the same potential as the structure.  They are placed within a few centimeters of a reference electrode.  When the coupon potential is measured, the short distance between the reference and the coupon reduces, but does not eliminate, voltage drop error in the measurement.

Voltage drop error can be further reduced by interrupting CP current flowing to the coupon and measuring its potential immediately before it depolarizes.  This measurement is often referred to as instant-disconnect potential.  The potential between the reference and coupon is measured as the connection between the coupon and structure is disconnected.  This task is much easier if the connection is made through an EDI Model SM Magnetic Switch.  This is a sealed reed switch for use in above and below ground test stations. The switch is activated by holding a magnet next to the color band.  Green bands denote normally closed switches which are momentarily opened with the magnet. These are most often used for instant-disconnect cathodic protection coupon measurements.  Red bands denote normally open switches which are momentarily closed with the magnet. These can be used to electrically isolate a reference electrode in test stations which may become submerged.  For best results, the use of model SM-MAG magnets is recommended.

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TN 8 Measurement Circuit IR Drop

The components used for making potential measurements and the equivalent electrical schematic are shown below.  A reference electrode located close to the structure is connected to the meter by a test lead.  A second lead wire connects the structure to the meter.  In this simple DC circuit, the driving voltage is the potential that exists between the reference electrode and the structure.  When a measurement is being made, current will flow through the circuit as a result of this potential. The magnitude of the current flow follows Ohm’s law, I = E/R.  The current is proportional to the driving voltage and inversely proportional to the sum of all resistances in the circuit.  For example, if the circuit potential is one volt and the sum of the resistances is ten mega-ohms (MW), a tenth of a micro-amp will flow through the measurement circuit.

Voltage drops occur across each of the resistive elements in the measurement circuit.  These voltage drops are separate and distinct from the more commonly discussed voltage drops, or IR drops, which are due to external current flowing through the electrolyte.  In the figures, the external current is shown as ie.  Both measurement circuit voltage drops and external voltage drops become incorporated into potential measurements causing errors.  Different methods must be employed to minimize errors caused by each type.  Download our paper  Effect of Measurement and Instrumentation Errors on Potential Readings from the Technical section of our website to learn more.

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