Temperature compensation in a low cost frequency domain (capacitance based) soil moisture sensor

Frequency Domain Analysis is a well established technique in soil moisture determination, using the change in electrical capacitance of probes inserted into the soil caused by the presence of water. However it is known that temperature affects the determination of this capacitance. Here two different techniques are used, the first passing a fixed frequency through the soil via insulated probes, then measuring the amplitude of the resultant signal. The second uses the soil capacitance as the controlling component in a variable frequency oscillator, measuring the resultant times to charge and discharge. The measured capacitance is seen to be affected both by the temperature of the soil and, due to the sensitive nature of the monitoring electronics, also the temperature of critical components in the measurement circuits. Results from these experiments show that these two effects are complementary, soil temperature adding to the measured capacitance, whilst electronics temperature effectively decreases the measured capacitance. The daily profiles of the soil and electronics temperatures, whilst both rising during the day, and falling at night, show significant phase difference and therefore do not simply cancel out. Further, the strength of temperature compensation required is shown to vary with technique and moisture level. This paper explores these phenomena using results from a recently developed, four probe Frequency Domain capacitance based sensor costing around 12 Euros. These measurements are compared to those achieved by a commercial soil moisture system costing over 250 times this price. Preliminary results are presented from temperature compensation algorithms intended to minimize these effects.