Principle of operation (two-clamp stakeless resistance measurement)
In this example, the electrode under test is connected to a network of other electrodes. It is either impractical or unsafe to
disconnect an individual electrode for testing. Also, there might be insufficient space to perform a classic three-terminal
resistance measurement. The stakeless test method using both MVC CLAMP and MCC CLAMP can be used to obtain a
measurement for the electrode under test.
A defined test voltage is injected into the system using the MVC CLAMP, inducing a current, I, to flow and be measured
by the MCC CLAMP. The model shown in Figure 7 can be simplified to the resistance of the electrode under test, Rx and
the resistance of the other electrodes in parallel, i.e. R1 || R2 || … || Rn. Therefore, the current induced by the test voltage
is I=V/[Rx+(R1 || R2 || … || Rn)]. It follows that as the resistance of the other electrodes in parallel approaches zero, then the
resistance measured, approaches the value of the electrode under test.
Figure 7: schematic for two-clamp stakeless resistance measurement
Principle of operation (four-terminal resistivity measurement)
The soil resistivity measurement works on a similar principle to the other measurements which use stakes: a current is
injected around an outer loop and a voltage measured, shown in Figure 8. In this case, however, the measurement made by
the instrument requires further conversion using a formula to derive the volumetric soil resistivity from the resistance value
display.
Figure 8: schematic for 4-terminal resistivity measurement
For this test, the relative spacing and depth of the stakes is important. When configured as shown in Figure 8, the soil
resistivity can be calculated from the resistance value, R, displayed on the instrument as p = 2 x π x A x R.
MCC
MVC
