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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TN 7 Meaning of Design Life of Reference Electrodes

Field grade reference electrodes contain a saturated salt solution in a gypsum-bentonite gel.  CuSO4 is the salt in copper/copper sulphate electrodes; KCl is the salt in silver/silver chloride electrodes.   The accuracy of a reference electrode depends upon this salt solution remaining saturated.  During use, salt will diffuse out from the reference electrode which can affect the concentration in the gel.  The design life of a reference electrode is an estimate of the time based on testing it would take for enough salt to diffuse out from the inner core to lower the salt concentration to below saturation.  At EDI, we use several techniques to extend this time as much as possible.  One of these techniques is to increase the amount of salt reserve contained in the gel.  This is one reason why longer life electrodes have physically bigger housings.  Download our paper Factors Affecting the Accuracy of Reference Electrodes from the Technical section of our website to learn more.

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TN 6 Importance of Backfill in Underground References

The backfill surrounding a bagged underground reference electrode is a mix of gypsum and bentonite. The primary purpose of backfill is to retain water which ensures that a low contact resistance between the electrode and the surrounding earth is maintained.  Additionally, backfill usually prevents the inner core of the electrode which contains a saturated salt gel from drying out.  However, during severely dry conditions, the electrode may still dry out.  The backfill will eventually rewet with local groundwater, and the electrode should re-activate.  However, local ground water will have many other chemicals dissolved in it that can affect the accuracy of the electrode.  If this situation is suspected, the electrode should be calibrated against a reference electrode of known accuracy to determine whether replacement is necessary.

When installing underground reference electrodes in areas known to have extreme seasonal dry periods, a good practice is to place additional gypsum-bentonite backfill around the reference bag.  This backfill is commonly known as driller’s mud.  This extra backfill will hold additional water around the reference electrode and extend the time before it dries out.

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TN 5 Element Selection Zinc

Zinc elements consist of high purity metallic zinc rod.  When these elements are used in our underground reference electrodes, the zinc element is encased in a gypsum-bentonite backfill. Reference potential of a zinc element encapsulated in backfill is about 1,100 mV negative to that of a saturated Cu/CuSO4 reference electrode.  The presence of halides in the environment will not affect the reference potential of an encapsulated zinc electrode.

In through-wall and immersion reference electrodes, the zinc element is directly wetted by the electrolyte.  The reference potential of zinc directly exposed to an electrolyte depends on the composition of the electrolyte.  The potential of zinc is also affected by temperature and can approach that of steel at around 60°C.  Bare zinc electrodes will perform best when their use is limited to clean full-strength seawater at ambient temperature.

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TN 4 Element Selection Dry Silver/Silver Choride

Dry silver/silver chloride elements consist of 99.99% pure silver coated with silver chloride.  Reference electrodes using this element are constructed so the external electrolyte comes into direct contact with the element.  Dry type elements are most commonly used in clean full strength seawater. Like the gelled Ag/AgCl elements, they are adversely affected by sulfides. The reference potential of dry Ag/AgCl elements immersed in full strength seawater is 70 mV negative to that of a saturated Cu/CuSO4 reference electrode.  As the ambient chloride level decreases, as would be the case when used in brackish water, the reference potential becomes less positive. Dry Ag/AgCl elements are only available in through-wall, immersion and tubesheet mounted reference electrodes.

Dry silver/silver chloride elements for concrete are a variation of our standard dry Ag/AgCl element which has been adapted for encasement in a cement-based grout. The reference potential depends on the pore water chloride level of the concrete structure in which it is embedded. In concrete immersed in seawater, the pore water chloride level equilibrates with that of the surrounding ocean so that the element provides long term stable service.     This element is only available in our Marine Concrete reference electrode.

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TN 3 Element Selection – Gelled Silver/Silver Chloride

Gelled silver/silver chloride elements are most often used in environments with more than 500 ppm chloride or other halides although they can also be used in chloride free environments.  They consist of 99.99% pure silver coated with silver chloride and immersed in a saturated potassium chloride solution. Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Ag/AgCl electrodes which contain a liquid rather than a gelled electrolyte are limited to laboratory use.

Silver/silver chloride elements can be used in portable, immersion or underground units.  Use of these elements in electrolytes with other halides (iodides or bromides) or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift.  The reference potential of Ag/AgCl/sat. KCl elements is 105 mV negative to that of a saturated Cu/CuSO4 reference electrode.  Use of sodium chloride rather than potassium chloride electrolytes can cause a junction potential error.

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