Humboldt H-4385 Resistivity Meter - Manual (Page 8)

This type of plot assumes that the “Current Electrode” (see Figure 3) is positioned far enough
away from the electrode under test so that a “plateau” region is obtained in the plot. A
100 foot separation is typically recommended. However, it is suggested that a resistance
versus distance curve be generated for any given situation in order to verify the existence of
a plateau-type region which will allow an accurate determination of the electrode-to-earth
resistance.
Assuming that a plateau-type region is obtained, the resistance-to-earth value for the
electrode under test will be the resistance value on the plot corresponding to the plateau
region as illustrated in Figure 4.
A general rule-of-thumb, which assumes that the “Current Electrode” is positioned sufficiently
far away from the electrode under test, is that if the “Potential Electrode” is positioned at a
distance from the test electrode of 0.62 x D, where D is the distance between the electrode
under test and the “Current Electrode”, the resistance reading observed will correspond to
the resistance-to-earth value for the electrode under test.
This particular configuration is illustrated in Figure 3. For example, if the distance between the
electrode under test and the “Current Electrode” is 100 feet, the “Potential Electrode” should
be positioned 62 feet from the electrode under test. However, again, it is suggested that
resistance versus distance plots be generated in each instance.
2-Electrode Application
The 2-Electrode Application can be used to measure the resistance between two buried
electrodes, such as two ground rods, or two anodes, for example.
In this case, the two electrodes are the two buried components. Figure 5 below illustrates the
measurement configuration.
Figure 4