Temperature Compensation in Sentek Soil Capacitance Sensors.

Capacitance sensors have long been found to be sensitive to variations in temperature due to changes in the dielectric properties of the surrounding soil media. Whilst this is a relatively minor effect and does not present a problem for the interpretation of soil water trend data by an experienced operator, it is a negative feature which the authors propose may be corrected for if something is known of the soil texture. Three approaches were taken in this investigation. Firstly, multiple Sentek EnviroSCAN™ and Drill & Drop™ sensors were installed into separate controlled atmosphere chambers in the laboratory so that the direct effect of temperature changes on sensor electronics could be evaluated. Additional probes were installed into laboratory soil columns of different textures where the temperature was carefully controlled (7^oC – 62^oC) using an external sealed water jacket. As well as this, probes were installed into real commercial field crops where natural soil temperature variations were high (10^oC – 35^oC). Volumetric Soil Water Content (VWC, measured in millimetres depth over one square metre, i.e. 1mm = 1L) data was collected on a near-continuous basis in all of these situations. Correction factors derived from the laboratory analyses were then applied to data collected from the field. Correction factors were found to be from 0.04 to 0.13mm per degree Celsius change for EnviroSCAN sensors and from 0.19 to 0.33mm per degree Celsius change for Drill & Drop sensors. A new standard measure for VWC at 18^oC is proposed to facilitate this correction. It was found that the correction protocols improved the variation in VWC previously attributed to diurnal fluctuation alone. It appears that diurnal fluctuations, although still present, have been overestimated previously. Remaining observed variation is proposed to be the result of real diurnal fluctuation and a time delay in the temperature readings between the internal sensors and the soil. This hysteresis has a different slope depending upon whether it is on a heating or a cooling cycle. In the EnviroSCAN sensor, this effect is largely mitigated by the insulation properties of the air jacket in the centre of the sensor. In the Drill & Drop probe, this hysteresis is more pronounced and arises from the thermal mass of the solid probe itself.

 

Keywords: capacitance, temperature effect, soil water management, dielectric.