Soil resistance is a factor of soil moisture and the amount of soluble salts found within the soil. This measurement can indicate the corrosivity of the soil. Typically, soils with high resistivity and water content will be lower in corrosivity. Soil resistivity is measured to collect data that will used to determine bedrock location and material, such as iron ore. It also helps in predicating depth in placing pipes to limit corrosion activity. The Wenner 4-Pin or 2-Pin Method and a soil resistance meter is used to determine the resistance of soil.
Find a location that will allow suitable pin spacing without obstructions such as tree roots, rocks and other impenetrable objects in the soil.
Insert 4 pins. In a straight line, place the pins along the path to be surveyed. Make sure they are of equal distance from each other and their depth is only 5 per cent of their spacing. For example, if the pins are 5 feet away from each other, then the depth of the pin will be about 8 centimetres.
Connect the cables. Some meters will require cables from the pins to be connected to the terminals. These will be colour-coded or, in some cases, the meter will state which cable goes where.
Turn dial for function. Many modern meters will have dial to turn for the 4-pin, 3-pin, and 2-pin method. Select the 4-pin method.
Find the range. Using a push button test or turning a range switch setting, find the balance between a high and low resistance. For LCD models, an actual number will flash. For other models, a knob will need to be turned until a dial is balanced.
Record resistance. The number on the range selector, either by LCD or the number on the knob, should be written down.
Multiply for voltage. Multiply the number on the range selector by 191.5 and by the number of feet. For example, if the meter shows 6.4, then 6.4 x 191.5 x 5 feet = 6128 Ohm-cm.
Select an area. Find a location that is free of rocks, tree roots or any other non-conductive materials.
Insert 2 pins. In a straight line of each other, insert the pins along the area to be surveyed. Measure the distance between the two pins. Be sure the pins are set to a depth of 5 per cent of their spacing. For example, if the pins are 10 feet away from each other, then insert pins about 15 centimetres deep.
Attach cables to terminals. Attach the cables leading from the pins to terminals on the soil resistivity meter. These will be colour-coded or, in some cases, the meter will state which cable goes where.
Select the function. If the meter has a dial to determine which method will be used, turn the dial to the 2-pin method.
Balance the range. Whether by using a push test LCD display, or turning an Ohms balance knob for a null indicator, do so to determine the result.
Record result. Write down the result shown or dialled to convert to Ohms-cm.
Find voltage. Multiply the result by 191.5 by the distance of the pins. For example, if the result is 6.4, then 6.4 x 191.5 x 5 feet = 6128 Ohms-cm.
It is best to be familiar with the soil resistance meter before going out into the field.
Do not measure across energised circuits.