Humboldt H-4386SM Soil Box User Manual
Page 3
Suggested Procedure for using Soil Box
Purpose:
Measurement of electrical resistivity (in ohm-centimeters) of liquid or earth, sand or other
particulate material which can be tamped into the soil box.
Equipment Required:
1.) Battery or DC power supply capable of putting out at least 20V DC at about 100ma
2.) Soil Box
3.) DC milliammeter with ranges of about 1ma to 100ma.
4.) DC Electronic Voltmeter with input resistance of at least 10 Megohms and ranges of
up to 20V .
5.) Four insulated test leads.
Procedure:
a.)
Pour or tamp material to be tested into soil box until flush with top of box.
Where soil or similar material is to be tested, the two brass potential pins should
be temporarily removed to facilitate filling the box. Material should be tamped
down until compacted to same degree as soil at test site.
b.)
Connect power supply and DC milliammeter so as to pass current between the
two end terminals of the soil box. (See diagram on page 3.)
c.)
Connect DC Electronic Voltmeter between the two brass potential pins, which are
located near the center of the soil box.
d.)
Using appropriate Milliammeter and Voltmeter ranges, measure the potential
between the two brass pins with no current applied and again with measured
current passed between the end terminals of the soil box.
Resistivity (in Ohm-cm) = *Change in Potential (mV)
Curent (ma)
* The change in potential is an algebraic difference, since initial potential difference may
be of opposite polarity to potential obtained when test current is supplied. It is usually
more convenient to express the current in ma and the change in potential in mV.
Resistivity in ohms/cm is average resistivity of soil to a depth equal to the, pin spacing
(equal spacing between pins),
Remarks:
The resistivity of both liquids and soils is a function of temperature. In order to obtain
results which best agree with field conditions, the sample should be at the same
temperature as would be experienced at the test site. The resistivity of soils is also a
function of moisture content; therefore, all soil samples should be kept tightly capped
and measured as soon as possible after they are removed from their containers. Soil
boxes should be thoroughly cleaned with distilled water after each use. The soil box may
also be used with four-terminal, AC meters, such as soil resistance meters as shown in
the instructions included with those meters.