UFC 3-570-06
JANUARY 31 2003
commonly experienced variations in soil resistivity affect the operation of cathodic
protection systems, manifesting in variations in structure-to-electrolyte potentials or
rectifier outputs measured during routine system inspections. Electrolyte resistivity
measurements are taken to determine the cause of improper system operation.
7-11.1
Four-Pin Method. The most commonly used method of measuring soil
resistivity is the four-pin 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. A specialized instrument is used to supply the current and measure
the potential drop. To reduce the influence of any stray currents in the area, the
instrument supplies alternating current. The arrangement of electrodes is shown in Figure
7-26.
Figure 7-26. Soil Resistivity by the Four Pin Method (Wenner)
C2
P2
P1
C1
D
D
CURRENT
FLOW
D = PIN SPACING AND DEPTH OF READING
C = CURRENT PINS
P = POTENTIAL PINS
The soil resistivity is calculated from the indicated reading by using the
following formula:
Resistivity (ohm-cm) = 191.5 pin spacing (in feet) meter reading
In this 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, then there is a region of higher soil
resistivity at depth. If the average soil resistivity decreases with depth, then there is a
region of lower soil resistivity at depth. For multipliers for common distances and
distances for even multipliers, see Table 7-2.
7-42
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