Automatic fault detection in a low cost frequency domain (capacitance based) soil moisture sensor

Frequency Domain Analysis can be used to determine the moisture content of soils. At least two techniques can be used, the first using the soil capacitance as part of a low pass filter, measuring the attenuation of a fixed frequency signal, the second using the soil capacitance as the controlling component in a variable frequency oscillator. Whilst the two techniques demonstrate differing sensitivities to different conditions, they demonstrate an acceptably stable reciprocal relationship to each other over a wide range of soil moisture conditions. With insulated probes, it is possible under field conditions for these probes to be damaged or for moisture to creep into the electronics housing. Either of these conditions make the soil capacitor appear to ‘leak’ by providing a lower electrically resistive path in parallel with the soil capacitance. This resistance affects the measurements of the two techniques described above in different ways and thus readings from the sensors diverge from their normal relationships. These variations are measureable and thus the fault condition can be automatically detected. This can be used to flag potential problems in the soil moisture measurements raising an alarm condition, or stopping unnecessary irrigation based on erroneous results from a damaged sensor. This paper presents results demonstrating these phenomena using a Frequency Domain capacitance based sensor costing less than 10 Euros.