Calibrating ECH2O Soil Moisture Sensors Application Note

Onset adds smart sensor adapters to the ECH2O probes so that they are plug‐and‐play compatible with HOBO® U30 stations and HOBO Micro Stations. The S‐SMC‐M005 incorporates the EC‐5 and the S‐SMD‐M005 incorporates the 10HS. Onset also offers the W‐SMC HOBOnode wireless soil moisture sensor, which uses the EC‐5 and provides data in the same format as the S‐SMC‐M005. ECH2O probes measure the volumetric water content of the soil by measuring the dielectric constant of the soil, which is a strong function of water content.  However, not all soils have identical electrical properties.  Due to variations in soil bulk density, mineralogy, texture and salinity, the generic mineral calibration for current ECH2O sensors (EC‐5, 10HS) results in approximately ± 3‐4% accuracy for most mineral soils and approximately ± 5% for soilless growth substrates (potting soil, rock wool, cocus, etc.).  However, accuracy increases to ± 1‐2% for all soils and soilless substrates with soil‐specific calibration.  It is recommended that you conduct a soil‐specific calibration for best possible accuracy in volumetric water content measurements. Studies performed by independent researchers (Czarnomski et al., 2005) indicate that soil‐specific calibration of ECH2O sensors achieves performance results similar to that of TDR instruments ‐ at a fraction of the price.  Note that the resolution, precision, repeatability, and sensor‐to‐sensor agreement of the ECH2O sensors are excellent, so the soil specific calibration of one sensor can be applied to all other sensors of that type in that particular soil.  The purpose of this application note is to provide a step‐by‐step guide for performing a soil specific calibration on ECH2O sensors.

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