# Lab  Measurements

Resistivity of soil samples can be measured in the lab. The values determined will be only an approximation of the true in-situ values. The lab samples will be disturbed, will not be compacted to the exact same extent as in the field, will not have the same moisture content and will not be at the same temperature.

A sample box is used that contains contacts for the current and potential cables. The geometry of the box and positions of the electrodes will determine the factor that must be used to convert the meter reading to the resistivity value.

For a sample of uniform cross section A and distance between potential electrodes L the resistivity of the sample is given by r = AR/L where R is the value read from the meter.

A box may be constructed with opposite ends of stainless steel and the long sides and bottom of non-conducting material such as Lucite or Nylon plastic. The stainless ends should have connectors to the current cables (C+ and C-). Small stainless pins can be used for the potential electrodes (P+ and P-). The sample should be compacted to approximately the same density as the soil in-situ. Moisture content should be as close to field moisture as possible. The easiest way is to preserve original moisture by transporting and storing the soil sample in a plastic bag.

Typical internal dimensions of the box may be 18 inches long, 4 inches wide and 4 inches high. L may be 12 inches. Resistivity in Ohm-feet would then be r = .333 x .333 R. This can be multiplied by 02.54 to convert the result to Ohm-centimeters, the unit most commonly used by engineers in the United States.

Typical internal dimensions of the box may be 18 inches long, 4 inches wide and 4 inches high. L may be 12 inches. Resistivity in Ohm-feet would then be r = .333 x .333 R. This can be multiplied by 30.48 to convert the result to Ohm-centimeters, the unit most commonly used by engineers in the United States

If the sample has very high resistivity and the MiniRes indicates an excess of voltage, the “L” dimension should be reduced to six inches or even 3 inches. The calculated resistivity would then be multiplied by 2 or 4, respectively.

If a box with stainless steel ends is not available, you might try several pins or nails at each of  the two ends of the box. Connect the pins together at each end of the box for current “electrodes”

Following is a photo of a “soil box”.