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.

TN 5 Element Selection ZIN

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.

May Featured Product Model IR

Model IR
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.

TN 4 Element Selection AGD

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.

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.

TN 2 Element Selection – Copper/Copper Sulfate

Copper/copper sulfate elements are typically used in environments with less than 500 ppm chloride or other halides. They consist of high purity metallic copper immersed in a saturated copper sulfate (CuSO4) solution. Cu/CuSO4 elements can be used in portable, immersion or underground units.  Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Cu/CuSO4 electrodes which contain a liquid rather than a gelled electrolyte will require periodic cleaning and electrolyte renewal.  Use of Cu/CuSO4 elements in electrolytes with higher halide levels or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift. Because of their widespread use, Cu/CuSO4 electrodes are the ones upon which many cathodic protection criteria are based.

TN 1 How to measure tube sheet potentials

The potential across a tubesheet under cathodic protection can show large variations from one location to another. Different waterboxes of the same apparent design can produce different potential distributions. Areas with excessively electronegative potentials can cause hydrogen damage to titanium or ferritic stainless steel tubes. Other areas may have potentials insufficiently negative to adequately protect the copper alloy tubesheet. These potential gradients cannot be detected by a reference electrode mounted on the side wall of the waterbox.

EDI’s Model TE Tubesheet Mounted Reference Electrode is designed to mount any place on the face of a tubesheet. They are shipped with a double tube plug which is inserted into the end of a condenser tube and tightened. Tube plugs are available for common tube sizes between 5/8 inch and 1-1/4 inch. The electrode’s lead wire terminates in a waterproof connector which plugs into a mating connector on the Model TW wire which has been affixed to the tube sheet face.  This attachment system allows the electrode to be easily removed during scheduled outages.