Humboldt H-4385 Resistivity Meter - Manual (Page 6)

2. Soil Sample (or Liquid) Resistivity Measurement
This application also uses 4 electrodes, however, in this case, the electrodes are an
integral part of an electrolyte box, which is more commonly referred to as a soil box.
For this application, the H-4385 can be used in conjunction with one a Humboldt soil box
(H-4386 or H-4386SM) and 4 test leads (H-4388).
A schematic of the test arrangement is illustrated in Figure 2 below.
For this application, the test leads are connected to the H-4385 as shown in Figure 2.
With this arrangement, the H-4385 determines the resistance of the soil sample, or of the
liquid that fills the electrolyte box.
In general, for a particular volume of soil sample (or liquid), as defined by the geometric
constraints of the electrolyte box, the sample’s resistivity can be calculated from the
resistance value determined using the H-4385 by applying the following formula:
ρ = R A/L
where ρ is the resistivity in ohm.cm, R is the resistance in ohms, A is the cross-sectional
area of the current electrodes in cm squared, and L is the separation between the
potential electrodes in cm.
Consequently, the ratio A/L represents a multiplication factor that needs to be applied to
the resistance reading in order to obtain the sample’s resistivity value.
Conveniently, for both soil boxes (H-4386 and H4386SM), the A/L ratio is exactly 1cm.
Consequently, when H-4386 and H4386SM soil boxes are used, the resistance reading in
ohms determined using the H-4385 becomes the resistivity value in ohm.cm.
If the resistivity value is required to be expressed in ohm.m, rather than ohm.cm, the
ohm.cm value is divided by 100. For example a resistivity value of 2500 ohm.cm would
be equivalent to 25 ohm.m
Figure 2