Measurement of resistivity (specific resistance) by the Wenner method
At this method, four electrodes equally spaced (L cm) and in the same direction are immersed in the soil. Current is applied from the outermost electrodes (C1 and C2), and the voltage is measured from the electrodes (P1 and P2) inside. Soil resistance is read directly from the measuring instrument by applying Ohm's law (R = V / I).
Resistivity = 2 x R x L x (ohm x cm). In Wenner method, buried steel structures under the ground should be known as to cause faulty at measurement. Electrodes should be placed perpendicular to the pipeline. Placing parallel to the pipe or to the pipeline causes to measure the resistivity lower than the actual value.
Measurements are made as described below with the help of devices developed with the four-point Weston bridge system. Since the information on the use of these devices differs from the model of the device and will be found in its own brochure, only the main parts of the measurement process is given here.
- The tips of the device are immersed to the soil at equal intervals. (The depth of immersion should not exceed 1/20 of the distance L.) The distance L = 159 cm between the ends is the reason for preference in terms of both ease of measurement and measurement at the pipe depth.
After the tips are immersed in the soil, the resistivity of the soil is measured according to the type of device (magneto sleeve or pill). The resistance measured and shown on the device's screen is multiplied by 1000 to find the resistivity at the depth of 159 cm in the place where the measurement is made. In the meantime, we have to pay attention to the multiplier stage of the device.
= 2 x x a x R (ohm x cm)
a = 159 cm R = Value read from the device (ohms)
? = 2x3.14x159 x R = 1000 x R (ohmxcm).
(if the multiplier of the device is 0.1, the read R value must be multiplied by 100)
Soil resistivity can also be measured by the soil box method in the laboratory.
Floor Type Ohm x cm VERY CORROSIVE <1.000 CORROSIVE 1.000-3.000
MEDIUM CORROSIVE 3.000-10.000
AZ CORROSIVE >10.000
Determination of pH values:
To obtain information about the chemical structure of the soil, the pH value of the soil is measured with the help of a special indicator paper called litmus paper or directly with a digital pH meter. Acidic and basic environments should be considered during protection. In order to do this measurement, the soil must be saturated with distilled water.