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*! •* <br /> A Nilsson Sail. Resistivity Meter, Madel 400 is used for direct resistance <br /> moments. The meter incorporates a 97 Hertz generator to eliminate <br /> inb Terence from stray DC or 60 Hertz AC. <br /> IL Potential <br /> Shsu potentials are DC voltages used to evaluate the carrceion <br /> pattern along underground tanks. Properly interpreted and correlated with other <br /> measurements, stru potentials give an indication of the severity of <br /> both galvanic and electrolytic corrosion cells. <br /> Measurements must be recorded with a high input resistance voltmeter. The <br /> negative barminal of the metes is connected to the tank and the positive <br /> terminal to a saturated copper/copper sulfate reference electrode placed in <br /> contact with the smL Copper sulfate half-cells are used for underground <br /> cccrosion testing because they are stable and yield reproducible resLdts. <br /> Cell placement is important when collecting the data.. When best borings are <br /> made, the reference electrode can be placed near the taWsoil interface. <br /> Typically the gest hole is drilled one foot from the edge of the tank to a depth <br /> of two feet below the tank. Tank-to-sail potentials are then recorded at various <br /> depths to establish the potential profile from grade to below the tank. <br /> Structure-to-scil potentials are typically recorded with a Beckman, Model HD <br /> 110. This instrument is designed for corrosion testing and has an internal <br /> resistance of 22 megohms. <br /> III. Stray C ur ent <br /> Stray DC earth currents can emanate from the operation of DC transit systems, <br /> cathodic protection rectifiers, welding and DC motors. When discharged from <br /> the surface of a steel tank, these currents wM consume approximately 1 gram <br /> of metal per ampere-flour. Concentrated electrolytic corrosion can cause rapid <br /> deterioration of underground tanks. <br /> 2 <br />