Integration of Heat Capacity and Electrical Conductivity Sensors for Root Module Water and Nutrient Assessment

Management of water content and nutrient status during space flight is a critical element for successful plant production systems. Our objectives were to determine if dual-probe heat-pulse (DPHP) sensors could improve water content determination accuracy over single-probe heat-pulse (SPHP) sensors, and to test a design using coupled heat-pulse and direct-current electrical conductivity sensors, paired as a 4-electrode array. The DPHP predicted water content correlated well with independently measured water contents based on a physically-derived one-point calibration model. SPHP water content prediction was comparable to the dual-probe sensors when using an empirical relationship. Pooled regression analysis showed that water content for both sensors was accurate with a root-mean square error of 0.02 cm³ cm⁻³. Electrical conductivity was measured in both saturated flow-through and static unsaturated measurements. Model predictions of solution electrical conductivity as a function of water contents were well correlated for water contents above 0.2 cm³ cm⁻³. Combining the dual-needle heat-pulse probe water content determination with electrical conductivity measurements provides improved root-zone environment assessment and management capabilities.