Measuring Electrolyte Resistivity

5.9.1 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 in Figure . 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.

Figure 5.14 Four-Pin (Wenner) Method of Measuring Soil Resistivity

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. Where this is not possible, the pins should be set at right angles to the underground structure.

The resistance, “R,” at each pin spacing “a”, is the resistance from ground level to a depth equal to the spacing of the pins.

Using the Wenner method, the soil resistivity,  (Greek letter rho), in ohm-centimeters is determined by:

 = 2  a R = 6.28 x a x R

For “a” in centimeters and “R” in ohms

To obtain “” in ohm-cm, if “a” is in feet and “R” is in ohms, the formula becomes:

 = 191.5 x a x R

You might be asked to collect data so that the resistivities of various layers of soil can be calculated. Figure 5.15 shows a typical layer situation.

Figure 5.15 Average and Layer Resistivity

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

 avg

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.

5.9.2 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 5.16. If the cross-sectional area and linear distance between the voltage pins are equal, the calibration is a factor of 1 (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.

Figure 5.16 Resistivity Soil Box

P1 P2

C1 C2

Soil Box

Resistivity Test Instrument

Current Plate Voltage Pins

If a soil box and resistivity meter are available, take time now to work with them; connect the instruments as shown in Figure 5.16.

Fill the soil box with water and any available soil samples.

5.9.3 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 pipe-line trench during construction (for later use during cathodic pro-tection system design).

• Spot checks of soil or water resistivity.

The single-probe method is illustrated in Figure 5.17.

Figure 5.17 Single Probe Soil Resistivity Measurement

5.10 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.

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 non-metal 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.