Evaluation Techniques CHAPTER 5 Part III Survey Methods and Evaluation Techniques

Measuring Electrolyte Resistivity Wenner Four-Pin Method Most soil resistivity measurements are made by the four-pin Wenner method. This method is used to determine the resistivity of soil within an area. The Wenner procedure involves driving four metallic pins into the earth, in a straight line, equally spaced. The pin spacing is equal to the depth of investigation of the average soil resistivity, as shown below. The average soil resistivity is a function of the voltage drop between the center pair of pins with current flowing between the two outside pins. After resistance is measured at a given spacing, then the spacing can be changed and resistance measured at the new spacing.

Wenner Four-Pin Method

Wenner Four-Pin Method It is important that the pins are placed in a straight line and that they are equally spaced. Nearby metallic underground structures will create a false reading since they become part of the measured circuit. Consequently, the nearest pin should be at least 1.5 times the pin spacing from any underground metallic structures. The resistance, ‘’R," at each pin spacing "a'', is the resistance from ground level to a depth equal to the spacing of the pins.

Wenner Four-Pin Method Using the Wenner method, the soil resistivity, ρ (Greek letter rho), in ohm-centimeters is determined by: ρ = 2 π a R = 6.28 x a x R For "a" in centimeters and "R" in ohms To obtain " ρ in ohm-cm, if "a" is in feet and ''R" is in ohms, the formula becomes: ρ = 191.5 x a x R

Wenner Four-Pin Method You might be asked to collect data so that the resistivities of various layers of soil can be calculated. Figure below shows a typical layer situation. This measurement is done by taking a series of measurements at ever-widening pin spacings. It is important that the pin spacing be centered on a fixed point between the center two pins. This means moving all four pins out to wider spacings so that the center point of your measurement location is always centered between the two inside pins. To ensure accuracy, it is good practice to take two sets of data, perpendicular to each other. This will help expose any anomalies in the soil layers.

Wenner Four-Pin Method Typical layer situation

Soil Box The soil box method is used to measure the resistivity of an electrolyte that has been removed from its natural environment. The soil box method can also be used to measure resistivity of a liquid. If a soil box is used for measuring resistivity of soil, the soil sample should be tamped in the box to simulate natural compaction and be flush with the top of the box. Because natural compaction and natural moisture content are not always accurately simulated, the test results may vary from in situ soil resistivity measurements. A soil box consists of two plates at the end of the box for current flow and two pins in the center for voltage measurement as illustrated in Figure below.

Soil Box If the cross-sectional area and linear distance between the voltage pins are equal, the calibration is a factor of l (other calibration factors may apply with different dimensions). Therefore, the measured resistance equals resistivity of the sample in ohm-cm. The soil box is connected to the resistivity test instrument in the same manner as in the Wenner method.

Resistivity Probe This single-probe method is used to determine soil resistivity in the immediate vicinity of the tip of a probe driven into the ground to a depth of the desired measurement. This method is useful for: Rapid determination of local resistivity at intervals along a pipeline trench during construction (for later use during cathodic protection system design). Spot checks of soil or water resistivity.

Measuring pH Electrolyte pH can be measured in several ways. For liquids, pH (litmus) paper or a pH meter may be used. For soils, a pH meter may be used, or a filtrate may be made from distilled water and a soil sample and the pH measured with litmus paper, pH meter, or a pH test kits. Note that a pH meter uses a glass electrode with a rather fragile glass bulb on the bottom. Care must be taken when using these instruments not to break the electrode bulb.

Measuring pH Soil pH may also be measured using an antimony electrode and a copper-copper sulfate electrode. The antimony electrode consists of a slug of antimony metal in the bottom of a nonmetallic tube. The slug is connected to a terminal on the top of the tube. It is important to keep the antimony shiny and bright. Use fine nonmetal bearing sand paper or emery cloth for cleaning. Do not use steel wool or other metallic abrasive since particles of metal may become embedded in the antimony and affect the reading. The two cells are placed close together on the soil and connected to a voltmeter. It doesn't matter which cell is connected to which terminal of the meter since it is the potential between the two electrodes that is of interest. Take care not to get any copper sulfate on the antimony slug. There is a scale on the side of the antimony electrode that is calibrated in millivolts and pH. Once the potential is obtained, the pH can be determined from the scale.

Use of Pipe Locating Devices During corrosion testing, it is often necessary to accurately determine the location of such buried items as a pipeline, conduit, or storage tank. Also, it is important to determine points of electrical continuity and discontinuity. A pipe and cable locator can be a great time saver for making such determinations. Most locators include a transmitter and receiver. The transmitter is a source of radio frequency AC that is used to impress a signal on the structure. The receiver picks up the signal on the structure and provides it to the user as an amplifier sound.

Use of Pipe Locating Devices The sound of the signal, such as the frequency and rate of the pulse, can be controlled by the transmitter. The receiver controls the volume of the signal The sound of the signal can be received through earphones or a speaker. In a noisy area earphones may be necessary. If your job requires constant use of a pipe locator, you may prefer the speaker type. There are two types of pipe locators. Some locators contain both types in one unit: • Conductive • Inductive.

Use of Pipe Locating Devices Conductive The conductive locator uses either a radio or audio frequency AC signal that is connected to the structure by a direct wire. The transmitter converts DC from dry cell batteries to AC by means of a vibrator circuit, this AC can then be passed through a transformer to give an output of several hundred volts, peak. This AC voltage is connected between ground and the structure to be traced. The AC signal will then flow through the earth to the structure and finally to the structure connection to complete the circuit. When the receiver is near the structure, the AC field around the structure induces a voltage in the pickup coil. This voltage is amplified and produced as an audible signal heard by the user.

Use of Pipe Locating Devices Inductive An inductive locator uses a radio frequency AC signal, which is induced in the structure to be located by an induction coil that is part of the transmitter. Inductive type locators permit the location of underground metallic structures where it is not feasible to attach a wire directly to the structure, as in the case of the conductive locator. This is accomplished by a coil in the transmitter that establishes a strong magnetic field that induces an AC current in the structure. The AC field surrounding the structure can then be detected in a manner similar to that described for the conductive locator.

Use of Pipe Locating Devices Conductive locator Inductive locator

Use of Current Interrupters It is often desirable to determine the effect of a current source at various remote locations. To accomplish this, a current interrupter is inserted in the current source. An interrupter is a switch that is alternately turned on and off on a regular timed cycle by some mechanical or electronic means.

Coupon Measurements Coupons are often used to check the effectiveness of cathodic protection. These are made of the same metal as the structure and are electrically connected to it. They are weighed prior to installation and then weighed periodically to determine if any weight loss has occurred. Corrosion is evidenced by weight loss. Some test stations are equipped with a coupon so that structure-to-soil measurements can be made with essentially no IR drop. Some of these test stations have a stationary reference electrode built into them and some have a nonmetallic tube from grade to the coupon which a reference electrode can be lowered into. Electric resistance probes are also used to monitor the effectiveness of cathodic protection. These can be placed underground, inside water storage tanks or other vessels, or on marine structures, an instrument is used to measure corrosion rate electrically.