A Miniaturized Soil Moisture Sensor Based on Time Domain Transmissometry

Delay time measurements are a powerful method for soil moisture measurements. Besides the well-known time domain reflectometry (TDR), a new method, namely the time domain transmissometry (TDT) captures the market for soil moisture sensors. The key benefit of transmission measurements is their robustness against multiple reflections. However, the development of TDT sensors for measurements inside soils is a challenge due to specific geometric requirements. While transmission measurements are in general performed in two port setups, measurements inside soils require an one port setup. Hence, time domain sensors are in general based on reflection measurements. This contribution describes a soil moisture sensor, which combines the advantages of TDR and TDT measurements regarding the suitability for soil moisture measurements. The sensor consists of a concentric coaxial line assembly resulting in a compact setup. The measuring path is realized as an one-wire line to obtain a large cross sectional area and thus, a large soil sample volume.