The time required for an electromagnetic signal to travel along the wave guide is measured. The rate at which the send signal returns is used to measure the water content in the soil. The return rate is dependent on the dielectric properties of the soil. The signal takes longer time in moisture soils and shorter time in dry soil. This pulse signal is then converted into soil moisture measurement. [4] TDR sensors give accurate readings faster and require very little maintenance. The major disadvantage of TDR sensors is that they require they require different calibrations depending on different soil types.