A TDR wireless device for volumetric water content sensing

Abstract

Time Domain Reflectometry (TDR) is a widely used technique to estimate the soil volumetric water content (θ) and bulk electrical conductivity (σ). This paper presents TDR-WiFi, a wireless, portable and low cost interface to ease sensing of θ and σ by connecting TDR cable tester with a smartphone. TDR-WiFi consists of a microprocessor equipped with a WiFi microcontroller (M5Stack unit) and connected to the TDR device through an RS232-TTL adapter. The M5stack firmware is programmed in MicroPython language to act as a server between the user and the TDR device through a web page accessible from any smartphone web browser. TDR-WiFi is compatible with the Campbell TDR100 device and allows storing the TDR waveforms as well as estimating θ and σ. A complementary web page for subsequent analysis of the TDR waveforms was developed and is publicly available at http://swi.csic.es/tdrwifi/. The system was satisfactorily demonstrated in laboratory and tested in the field. The field trial allowed water content measurements in a steep road slope. TDR-WiFi demonstrated to be operative in the field and it can be satisfactorily used in difficult access areas.