Mississippi Administrative Code › Title 11 - Mississippi Department of Environmental Quality › Part 5 - Underground Storage Tank Regulations › Chapter 2 - Mississippi Commission on Environmental Quality Underground Storage Tanks Regulations Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (UST) (Adopted March 22, 1989; Amended August 25, 2011, Last Amended August 23, 2018) › Appendix 11-5-2-280.1 - GUIDELINES FOR THE EVALUATION OF UNDERGROUND STORAGE TANK CATHODIC PROTECTION SYSTEMS › Section 11-5-2-280.1-6 - CATHODIC PROTECTION TESTING › Section 11-5-2-280.1-6.6 - Other Test Considerations

Current through June 25, 2024
Various other factors can affect the accuracy of
						structure-to-soil potentials. Listed below are some of the more common factors
						that you should keep in mind:
Contact Resistance
-
In
order
						to obtain an accurate structure-to-soil potential, a good (low resistivity)
						contact between the reference electrode and the soil must be made. Sometimes,
						the soil at the surface is too dry and water needs to be added in order to
						lower the resistance between the reference electrode and the soil. In addition,
						if the porous ceramic tip of the reference electrode becomes clogged or
						contaminated it should be replaced since this in itself can cause a high
						contact resistance.
Contaminated Soil
- You should ensure that
						the soil the reference electrode is placed in is free of contamination.
						Hydrocarbon contamination can cause a high resistance between the reference
						electrode and the soil.
Current Requirement Testing
- When a current
						requirement test is conducted on galvanically protected tanks (refer to STI
						R972-01 for a description of this test), the affected structure can exhibit an
						elevated (more negative) structure-to-soil potential during the test and for a
						period of time after the test is completed. This is due to a temporary
						polarization of the tested structure which will dissipate over a period of time
						ranging from a few minutes to perhaps a few days depending on several different
						factors. Therefore, time sufficient for the temporary polarization of the
						affected structure to "drain-off after a current requirement test is conducted
						must be allowed before an accurate structure-to-soil potential can be obtained.
						In addition, any potential measured with the battery connected should be
						disregarded as this measurement contains a large voltage drop. Only instant off
						voltages are meaningful when the battery is connected.
Drought Conditions
- On occasion, it has
						been observed that structure-to-soil potentials can be improved by running
						water into the backfill material of the tank bed when extended periods of no
						rain have occurred. This is commonly done by placing a water hose in one of the
						tank bed monitoring wells (or other access points) and allowing the water to
						run for a period of a few hours. This practice serves to lower the resistance
						of the backfill material. However, you should keep in mind that the resistivity
						of the soil is not appreciably lowered if the moisture content is 20[CENT] or
						higher.
Electrical Shorts
- When a substandard
						reading is observed on a galvanically protected system, it is common to find
						that some other metallic object is electrically connected to the protected
						structure. For instance, on sti-P3 tanks, the nylon bushings installed in the
						tank bungs were sometimes removed when the various risers and other tank system
						components were installed or an electrical conduit was buried in contact with
						the tank shell.
Electromagnetic Interference
- Overhead high
						voltage power lines, railroad crossing signals, airport radar systems and radio
						frequency transmitters (CB radios, cellular phones, etc.) can all cause an
						interference that will result in an inaccurate voltage reading.
Galvanized Metals
- Buried metals that have
						a high electrochemical potential can also influence the voltage observed if the
						reference electrode is placed in close proximity to such metals. For instance,
						the steel of some of the man ways that are installed to provide access to the
						tank appurtenances may be galvanized. If the reference electrode is placed in
						the soil of such a manway, an artificially high (more negative) potential may
						be observed. This is actually a raised earth effect although the galvanized
						metal is not acting to cathodically protect the buried structure of
						concern.
Parallel Circuits
- Care should be taken to
						ensure that the person conducting the structure-to-soil testing does not allow
						their person to come into contact with the electrical components of the testing
						equipment. If the person touches the electrical connections, an error may be
						introduced due to the creation of a parallel circuit.
Pea Gravel
- Because pea gravel or crushed
						stone typically has a very high electrical resistivity, it is necessary to
						ensure that it is saturated with water when attempting to measure
						structure-to-soil potentials with the reference electrode placed in the pea
						gravel. Evaluate any effect high contact resistance may have by changing the
						input resistance of the voltmeter as described in Section 6.1.1. As an
						alternative way to evaluate the effect contact resistance may have, place the
						reference electrode remotely. If the remote reading is substantially more
						negative than the local, high resistance is indicated. Placement of a saturated
						sponge on the surface of the pea gravel may help overcome high contact
						resistance.
Photovoltaic Effect
-
It
is
						known that sunlight striking the viewing window of a reference electrode can
						have an effect (as much as 50 mV) on the voltages observed when conducting
						testing. You should ensure that the viewing window of the reference electrode
						is kept out of direct sunlight. As an alternative, the viewing window can be
						covered with black electrical tape in order to prevent any sunlight from
						reaching the copper-copper sulfate solution.
Poor Connection
-
If
the
						observed structure-to-soil potentials are unsteady and the voltmeter will not
						stabilize, you should suspect a bad connection somewhere. Ensure that all
						electrical connections are clean and tight and good contact is made between the
						test lead and the structure.
Shielding
- Sometimes, a buried metallic
						structure that is between the reference electrode and the structure you are
						attempting to test will cause the reference electrode to be unable to "see" the
						structure you are testing. Shielding is commonly cited when low potentials are
						observed with the reference electrode placed locally over sti-Pj tanks due to
						the various tank risers, pump heads, piping, electrical conduits and metallic
						manways that are typically located over the tank.
Temperature
- The temperature of the
						reference electrode affects the voltages that are observed when conducting
						cathodic protection testing. You may need to make a correction to the observed
						potential in some extreme and/or marginal cases. The "standard" temperature is
						considered to be 77° F. For every degree less than 77 add 0.5 mV from the
						observed voltage. For every degree above 77 subtract 0.5 mV from the observed
						voltage. To illustrate this, consider the following (in order to simplify the
						calculation, the negative sign is dropped from the structure-to-soil
						potential): A voltage of 845 mV is observed when the temperature is 57° F.
						In this case the corrected voltage would then be 855 mV (20° X 0.5 mV = 10
						mV. Therefore: 845 mV + 10 mV = 855 mV).