Determination of Electrolyte Resistivity
In Soils. Soil resistivity is best determined in situ using the
Wenner 4-pin method as shown in Figure 9. In this method, a current is passed
through two electrodes and a drop in potential through the soil due to the
passage of the current is measured with a second pair of electrodes. The
electrodes should be in a straight line. A specialized instrument is used to
supply the current and measure the potential drop. In order to reduce the
electrodes, alternating current is used in the instrument. The soil
resistivity is calculated from the indicated reading by using the following
a) Resistivity (ohms-cm) = 191.5 x pin spacing (feet) x meter
reading. Resistivity is measured either in ohms-cm or in ohms per cubic
centimeter (ohms/cm3), which are equivalent.
b) In the Wenner 4-pin method, the average resistivity of the soil
between the two center electrodes to a depth equal to the pin spacing is
measured. If the pin spacing is increased then the average soil resistivity
to a greater depth is measured. If the average resistivity increases as the
pin spacing increases, there is a region of higher soil resistivity at depth.
If the average soil resistivity decreases with depth, there is a region of
lower soil resistivity at depth. Other instruments using the 2-pin method of
soil resistivity measurement give less accurate results than the Wenner 4-pin
method in many circumstances and are limited to measurement of resistivity
only in the direct vicinity of the soil probe. Use of these instruments is
primarily limited to preliminary surveys.
c) When it is impractical to make field measurements of soil
resistivity, soil samples can be taken from an appropriate depth and the
resistivity of the sample can be determined by the use of a soil box. As
shown in Figure 10, the measurement made on the soil sample is essentially the
4-pin method. Metal contacts in each end of the box are used to pass current
through the sample. Potential drop is measured across probes inserted into
the soil. The resistivity is calculated using constants furnished with the
particular size of soil box being used. Due to the disturbance of the soil
during sampling and possible drying out of the soil during shipment, this
method of soil resistivity measurement is less accurate than actual field
tests by the preferred Wenner 4-pin method. To minimize drying out of samples
they should be placed in plastic bags and sealed prior to shipment. Soil
resistivities contained in routine geological surveys are normally taken at
shallow depths and should not be used as a basis for cathodic protection
Liquids. The resistivity of liquids can be measured using the
Wenner 4-pin method using a soil box or by using a conductivity meter which is
specifically designed for the measurement of the conductivity of solutions.
Conductivity is the inverse of resistivity.