AEMC 6472 Manuale di servizio

2

“WATTS CURRENT” TECHNICAL BULLETIN

Issue 09 Summer 2016

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S

oil resistivity is a key factor for determining what the resistance of a grounding

electrode will be, to what depth it must be driven in order to obtain low earth

resistance, and even the type of grounding system that should be designed for

this location and facility.

The resistivity of the soil varies widely throughout the world and changes

seasonally. Resistivity is strongly affected by the content of electrolytes in the

soil, its moisture content, and even its temperature. Soil resistivity measurements

are therefore critical for designing and constructing a grounding system

appropriate for your site and requirements.

This article describes a simple way to measure soil resistivity known as the

Wenner method. Our test instrument will be the AEMC®Ground Tester Model

6472. This instrument automatically calculates and displays soil resistivity, with

no manual calculations necessary.

The Model 6472 also features advanced capabilities such as test currents up

to 250 milliamps for testing soils with high resistivity, automatic test frequency

selection for testing in high EMI environments, and the ability to connect to a

computer via AEMC’s DataView®data analysis software.

Soil Resistivity vs Resistance

In this article, we refer to both soil resistivity and soil

resistance. To avoid possible confusion, it’s important to

have a basic understanding of what these terms mean

and how they differ. Simply put, resistance (represented

by the letter R) is a property of a speciﬁc physical

item of a deﬁnite size and shape; while resistivity

(represented by the Greek letter ρ), is a general

property of a material irrespective of its shape or size.

For example, a 500-foot length of 10-gauge copper

wire has a speciﬁc resistance, measureable in

ohms (Ω); while copper as a material possesses

a general resistivity, expressed in units such as

ohm-centimeters (Ωcm) or ohm-meters (Ωm).

Changing the length or gauge of the wire

changes its resistance, while the resistivity of

the copper from which it is made remains the

same.

Performing a Wenner Soil Resistivity

Test with the AEMC®Model 6472

“WATTS CURRENT” TECHNICAL BULLETIN

Issue 09 Summer 2016

3

Wenner Test Setup

The Wenner test involves placing four equally spaced and in-line electrodes into the

ground. The two outer electrodes (called the current electrodes) inject current into the

soil. The two inner electrodes (called the potential electrodes) measure voltage, which is

then used to calculate soil resistance.

The Model 6472 automatically calculates soil resistivity using a formula that assumes the

auxiliary electrodes are inserted into the ground to a depth that does not exceed 1/20th

of the spacing distance between electrodes. So for our demonstration, we will place the

electrodes 10 feet apart and no more than 6 inches deep. Other distances can be used,

as long as depth does not exceed 1/20th of the spacing. Note that when calculating

the spacing-to-depth ratio, the spacing distance is the primary value. Determine the

electrode spacing ﬁrst, and then adjust the depth accordingly.

The two outer electrodes are connected to the instrument’s outer terminals, labeled H

and E. The inner electrodes are connected the inner terminals, labeled S and ES.

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“WATTS CURRENT” TECHNICAL BULLETIN

Issue 09 Summer 2016

To enable the instrument to accurately calculate soil resistivity, we must enter the electrode

spacing distance. We start by ensuring the instrument is conﬁgured for the correct units of

measure. Turn the instrument’s dial to the set-up setting. Then press the DISPLAY button. The unit

of measure, either meters (m) or feet (ft), appears blinking on the screen. Pressing the Up button

toggles between these two options. In our example we’re using feet, so if this is set to meters

press the Up button to change this setting.

Next, turn the dial to the 4-pole ρ setting. To enter the spacing distance, press the yellow button

labeled 2nd. The symbol 2nd appears on the screen, indicating buttons will now perform the

actions labeled in yellow.

Press the DISTANCE button. The word DISTANCE appears on the screen, and the ﬁrst digit in the

distance ﬁeld blinks, indicating it is in edit mode. Use the Up and Right buttons to navigate through

this ﬁeld and make selections. For example, to set the distance to 10 feet, press the Right button

to navigate to the second digit and press the Up button to change this to 1. The ﬁeld should now

be set to 10 feet.

“WATTS CURRENT” TECHNICAL BULLETIN

Issue 09 Summer 2016

5

You can save the setting by pressing the yellow 2nd button to enter 2nd mode and then pressing

the DISTANCE button.

You are now ready to take a soil resistivity measurement. To do this, push the START/STOP button

and hold it down for at least two seconds. After a few moments, the resistivity reading appears,

displayed in ohm-meters. The resistance of the soil between the two potential electrodes is also

displayed, in ohms.

To save the measurement in the instrument’s memory, press the MEM button twice. This saves

the reading, along with the date and time the reading was taken. This stored record can later be

displayed on the instrument’s screen, and downloaded to a computer using DataView®for further

analysis.

This concludes our quick explanation of how to perform a Wenner soil resistivity measurement with

the Model 6472. For more information about this instrument, please visit our Product Page.

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