TN 21 Maintaining Liquid Filled References

Portable, liquid filled, reference electrodes must be cleaned and refilled regularly.  The copper sulfate solution contains dissolved oxygen.  Oxygen gradually reacts with the copper to form copper oxide which shifts the potential of the reference.  The more copper oxide the greater the shift.  Potential shifts of up to 10 mV in a week’s time are possible.  This problem can be prevented by cleaning and refilling the reference electrode regularly (preferably weekly but at least monthly).  The newer gelled filled portable reference electrodes do not have this problem since the element and gel have minimal contact with the atmosphere.

Another problem occurs on reference electrodes that have a ceramic tip.  The insulating tip is porous so that the copper sulfate solution will leak through and allow conductance.  If the tip dries out, the holes can become partially plugged with copper sulfate salt which increases the electrical resistance through the reference.  The process is progressive, with the resistance increasing a bit more with each dry-down until, eventually, the tip becomes fully insulating (totally resistive).  Boiling the ceramic tip in distilled water for an hour or two will restore it.  A similar problem to this occurs when the ceramic tip becomes plugged with either dirt or oil.  When this happens, the ceramic tip should be replaced.

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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 18 How Fast is Instant Off

The instant off method of removing external IR drop in potential measurements involves interrupting the cathodic protection current.  This action produces an instantaneous voltage drop which is considered to be the external IR drop.  The potential measured immediately after this instantaneous drop is considered to be the “IR drop free” potential of the structure.  Clearly, this method only works with an impressed current cathodic protection system where all the rectifiers on that system can be interrupted simultaneously and there are no other sources of current flowing through the electrolyte.

An issue which should be considered when using current interruption for instant-off measurements is:   What is meant by instantaneous?  The answer is not simple since it depends upon the structure, the electrolyte and the method of interrupting the current.  Putting the answer in electrical terms, it depends upon the capacitance and the inductance of the circuit.  IR drop free measurements can be made microseconds after current interruption on small uncoated specimens in a low resistance electrolyte.  Large coated structures, such as pipelines, or high resistance electrolytes, such as concrete, usually require several hundred milliseconds or more for IR-drop free measurements.  Interrupting current on the AC side rather than on the DC side of the rectifier will increase the time delay because the circuit inductance is higher.

For situations where current interruption cannot be reliably used to minimize external IR drop error in potential measurements, cathodic protection coupons are frequently used.  These are small pieces of metal similar to the structure which are electrically bonded to the structure through a switch.  Measurements are made as above except that the coupon rather than the rectifier is momentarily disconnected.  These measurements are termed instant disconnect measurements in order to distinguish them from instant off current interruption measurements.

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TN 17 Using Reference Electrodes in Oil-Water Mixtures

Making potential measurements in an oil water mixture can be very difficult.  The surface energy of oils is much lower than that of water. When they are mixed, the two liquids will separate into distinct phases rather than dissolving into each other. While the addition of surfactants can overcome this somewhat, their use would defeat the purpose of oil-water separators where these mixes are encountered in industry. The lower surface energy of oil will make it preferentially wet any solid surfaces in contact with an oil-water mix. Since relative wettability is a property of the liquids rather than the solids, there are no materials which will preferably be wet by water rather than oil.

When installing a cathodic protection system in the water zone of oil water separators, oil can coat the membrane of the reference electrode during the initial filling of the vessel.  The oil film increases the resistance of the measurement circuit making measurements difficult. A work-around which can be used is to coat the membrane end of the reference electrode with clay prior to installing it. As the vessel is refilled and the oil phase rises up past the reference electrode, it will coat the clay on the end.  Once the vessel is filled so the reference electrode is in the water phase, turbulence will remove enough clay so that measurements are possible.

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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 11 Potentiometric Voltmeters

Most general purpose digital meters used today for corrosion measurements have 10 mega-ohm input resistance.  While this may seem high, it may not be adequate in some circumstances.  Consider a structure with a 900mV potential and a circuit resistance external to the meter of 10 kilo-ohms.  Ten kilo-ohms is 0.1% of 10 mega-ohms.  Thus 0.1% of the voltage will be dropped in the circuit external to the meter and 99.9% will be measured by the meter.  Now let’s consider the case where the structure is in concrete, rock or dry soil.  The external resistance in this situation could be 1 mega-ohm or higher.   The total circuit resistance would now be 11 mega-ohm with 90% being in the meter and 10% external to the meter.  The voltage drop in this case would be similarly divided with 810mV across the meter and 90mV external.  Measurement circuit IR drop errors always result in a lower apparent potential reading.  This can result in unnecessary and costly up-grades and/or replacements of CP systems.

A preferred way of making potential measurements in high resistance circuits is with a potentiometric-voltmeter.  This type of meter was the standard field meter for corrosion personal up until the mid-1970s.  An internal battery in this meter applies a voltage with opposite polarity to that being measured. The applied voltage is adjusted to exactly balance the potential being measured.  The applied battery voltage is then read.  With no current flowing through the circuit, there is no measurement circuit IR drop.  Potentiometric-voltmeters may cost significantly more than a general purpose digital meter so it is easy to understand why they are not as widely used.  It is possible to convert an ordinary voltmeter to a potentiometric-voltmeter with the addition of a simple inexpensive converter circuit.  Two such approaches are shown above.  The circuit using a single meter requires that the potential read on the meter be reversed to give the actual potential.  When using the circuit with two meters, the potential can be read directly.  These are basic conceptual circuits; it may be necessary to adjust the values of some components to suit particular circumstances.

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TN 10 Data Logger and RMU Errors

Special precautions should be taken when using data loggers and remote monitoring units (RMUs) with reference electrodes.  The amount of current flowing through a reference electrode is inversely proportional to the circuit impedance.  This current causes the reference to polarize, i.e. shift potential, and the amount of shift is proportional to the amount of current flowing.  If the shift is small, or of short duration, the reference will usually recover.  When the shift is large and/or of long duration, the reference may be permanently damaged.

Most modern potential measuring devices have an input impedance of at least 10 MΩ in order to minimize IR drop error due to current flowing through the circuit.  These high input impedances are used during the measurement cycle to minimize errors from voltage drops in the measuring circuit while the measurement is actually being made.  When the unit is in stand-by or off mode, the impedance may be significantly lower (sometimes only a couple thousand ohms) depending upon the individual components used and the overall circuitry.  A drop in impedance during stand-by or off mode will not affected many transducers but a reference electrode will polarize if this occurs.  If a data logger or RMU is to be used with reference electrodes, the input impedance must be a minimum of 10 MΩ at all times including during active measurements, standing by between measurements, shut down with power connected or shut down with power disconnected.

Testing for changes in input impedance of a data logger or RMU as it goes through its cycles is relatively simple.  A 1½ volt battery is connected to the input terminals through a 10 MΩ resistor (see figure).  The voltage across this resistor is monitored with a portable voltmeter as the unit is put through its cycles.  This voltage should be about 0.15 volts for units claiming 100 MΩ input impedance during measurements.  If the measured voltage increases to greater than ¾ volt during standby or off cycles, then the unit is capable of damaging a reference electrode connected to it and should not be used.  Many RMUs have multiple independent input channels.  Each channel must be separately tested through all operating modes to be assured that all of the channels are suitable for reference electrodes.

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TN 9 Effect of Meter Impedance

Some meters designed for corrosion potential measurements have selectable input impedance with the highest value typically being 250 MW.  However, general-purpose digital meters with a fixed 10 MW input impedance are frequently used for corrosion measurements.  While this may seem high, it is not adequate in many situations.  Electrolyte resistance is highly variable ranging from quite low for potential measurements in seawater to very high for potential measurements in dry soils.  The proper strategy is to select a meter whose internal resistance (input impedance) is several orders of magnitude higher than any other resistance in the circuit so that voltage drop across the meter will, for practical purposes, represent the entire voltage drop in the circuit.

For example, consider a structure with a 900mV potential and a circuit resistance external to the meter of 1 MW which is typical in concrete, rock or dry soil.  The total circuit resistance when using a 10 MW meter would be about 11 MW with 90% being in the meter and 10% external to the meter.  The voltage drop in this case would be similarly divided with 810mV across the meter and 90mV external to it.  If a 100 MW meter were used, the circuit resistance would be 101 MW so that almost all of the voltage drop would occur in the meter.  Measurement circuit IR drop errors result in a more positive apparent potential reading.

When using a meter with selectable input impedance, successive readings can be made, each time increasing the input impedance.  When two successive readings are the same, the measurement can be presumed to be free of measurement circuit IR drop error.  If it is not possible to obtain two successive readings that are the same, there are two methods that may be used to eliminate this error.  A potentiometric-voltmeter will eliminate these potential measurements errors.   Alternatively, a correction factor can be calculated from measurements made with a meter that has variable input resistance.  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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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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