Comparison of calibration and direct measurement of cable and probe properties in time domain reflectometry

For the measurement of soil bulk electrical conductivity using time domain reflectometry (TDR), it is necessary to determine cable and probe properties. This can be done by direct measurement or by calibration of the TDR system in salt solutions. For a three-wire probe system, the results showed that with least-squares fitting, the electrical conductivity of salt solutions could be measured most accurately. Comparison of several calibration techniques suggested that directly measured cable properties deviate from their calibrated (optimal) values. The difference in accuracy between calibration and direct measurements of cable properties could not be explained with overfitting, as has been suggested. It was concluded that a reduction of calibration time by direct measurement of probe and cable properties is not advisable in case of three-wire probes. When necessary, a reduction of calibration time can be achieved by limiting the calibration procedure to two well chosen combinations of cable length and solution concentration.