Humboldt H-4385 Resistivity Meter - Manual (Page 4)

2) When the approximate resistance (say of the local soil) is not known, move the range
selector switch (labeled “Ohms Multiply By”) to the 100K setting and position the
“Balance Dial” knob at “10”.
3) Pull the “Null Sensitivity” switch down to the “Low” position and note that the null
indicating meter needle moves to the right, indicating too high a resistance setting.
4) While holding the “Null Sensitivity” switch in the “Low” position, step down through
the resistance ranges (10K, 1K, 100Ω etc.) until the needle moves to the left of the null
position (left of the center position) and then step back up one range.
5) Adjust the position of the “Balance Dial” until the needle is positioned at the null
(center) location on the meter.
6) Multiply the “Balance Dial” setting by the range setting (setting on the switch labeled
“Ohms Multiply By”) to obtain the resistance value. For example, for a “Balance Dial”
setting of 4.5 and a range switch setting of 100Ω, the resistance value is 450Ω
7) Apply the resistance value to the calculation of resistivity using the appropriate
formula for your application (see the Applications section below).
Note: You can increase the sensitivity of the resistance reading by holding
the “Null Sensitivity” switch in the “High” position and fine tuning the
balance, after finding the balance point in the “Low” sensitivity position.
Applications
4-electrode Applications
1. Earth Resistivity Measurement
This application uses 4 electrodes (pins). The electrodes are driven down into the
earth the same distance and are evenly spaced in a straight line.
A schematic of this arrangement is illustrated in Figure 1 below.
The H-4385 can be used in conjunction with the 4-lead (color-coded) test reel and
four heavy-duty (stainless steel) electrodes (soil pins) – (H-4388). The test leads are
connected to the H-4385 as shown in Figure 1. With this arrangement, the H-4385
effectively measures the earth’s average resistance to a depth equal to the electrode
spacing (S).
Dr. Frank Wenner of the U.S. Bureau of Standards developed the theory behind this
test in 1915 [1]. He showed that, if the electrode (pin) depth (d) is kept small relative
to the separation between the electrodes (S), the earth’s average resistivity to a depth
equal to the electrode spacing (S) can be obtained by applying the following formula:
ρ = 2 π S R
where R is the resistance value in ohms as determined using the H-4385, is the resistivity
in ohm.cm, π is the constant 3.1416, and S is the electrode separation in cm.
Typically, the electrode (pin) spacing is not measured in centimeters but, rather, in feet (in
the U.S.) or in meters (in most other countries).
U.S. Example (electrode spacing measured in feet):
Since there are 30.38 centimeters in 1 foot, the above formula can be written as:
ρ (Ω.cm) = 2π x 30.38 x (electrode spacing in feet) x R (ohms), or
ρ (Ω.cm) = 191.5 x (electrode spacing in feet) x R (ohms)