A laboratory procedure for the calibration of EC-10 and EC-20 capacitance sensors in coir
R. J. van der Westhuizen, Leon Daniel Van Rensburg · European Journal of Horticultural Science · 2011
Summary A new laboratory calibration procedure is presented to calibrate soil water sensors in coir. Water content was measured gravimetrically with load cells during a drying cycle determined by the evaporative demand of the environment as well as the water retention characteristics of the growth medium (coir). Sample mass and capacitance sensor output (mV) was measured and logged hourly. Excellent precision fits, indicated by R 2 values greater than 0.99, for sensor response versus volumetric water content (θ v ) were achieved by 4th degree polynomial curves. Accuracy of sensor-specific calibration was within 0.002 to 0.005 m 3 m -3 root mean squared error (RMSE) for the EC-10 sensors and 0.002 to 0.003 m 3 m -3 RMSE for the EC-20 sensors. This indicated that the instruments are very reliable and capable to measure water content of coir over the entire range from saturation to the lower limit of plant available water. Accuracy of a universal coir calibration was 0.05 and 0.025 m 3 m -3 RMSE for the EC-10 and EC-20 sensors, respectively. Although the universal equations were less reliable than the individual equations, they still outperformed the equations provided by the manufacturer. The manufacturer's defaultcalibraton for soil underestimated θ v of coir by a large margin of 0.25 to 0.29 m 3 m -3 RMSE for the EC-10 sensors and 0.27 to 0.33 m 3 m -3 RMSE for the EC-20 sensors. It was therefore strongly recommended that the manufacturer's equations should be replaced by the universal equations when using coir. Regular re-calibration was found to be important for organic growth mediums, since they may deteriorate over time. In conclusion, it stressed the need for a reliable and accurate determination of calibration equations.