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 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 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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May Featured Product Model IR

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-AGD Immersion Reference may be a more economical alternative.