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.
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.
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.
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.
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.
The Immersion Reference Electrode, Model IR, is designed for long term installation in an aqueous environment. Typical applications include elevated water tanks, standpipes, ground storage tanks, clarifiers, traveling screens, trash racks, submerged pipelines, locks, dams, and dock structures. The electrode can be directly suspended by its lead wire, cemented directly to 3/4 inch PVC conduit using the optional socket end termination, or securely attached to a steel structure with the optional magnetic mount. Antifreeze protection is also available for those situations where the electrode may be exposed to temperatures down to -30°F (-34°C). However, antifreeze may shift the reference potential by up to 12 mV.
The Model IR has a twenty year design life and uses #12 AWG RHW/USE lead wire. This electrode can also be fitted with a concentric cathodic protection coupon which minimizes voltage drop error in potential measurements (Model IRC). Another option is a copper sleeve which will reduce biofouling when the electrode is exposed for extended times in natural seawater (Model IRF). Potable water applications requiring NSF61 certification should use Model IRW.
Model IR can be used in all aqueous environments. For clean full strength seawater applications, our Model IP-AGDImmersion Reference may be a more economical alternative.