Evaluation of the Dual-probe Heat-pulse Method for Measuring Water Content in Spaceflight Plant Growth Systems

The dual-probe heat-pulse (DPHP) method uses temperature increase with time after heating to measure soil heat capacity, which then is related to soil volumetric water content. The coarse-textured plant growth media being considered for spaceflight applications may lead to problems in contact resistance between the medium and the probes of the DPHP sensor, which may limit the effectiveness of the method. The DPHP method was evaluated in 0.25-1 mm and 1-2 mm fritted clay media. Specific heat, determined using differential scanning calorimetry, was 830 J kg⁻¹ °C⁻¹ at 20 °C for the 0.25-1 mm medium and 810 J kg⁻¹ °C⁻¹ at 20 °C for the 1-2 mm medium. Good agreement between sensor measurements and independent water content measurements obtained through oven-drying was indicated by a linear regression of y = 1.00x + 0.0087 and a standard error of 0.04 cm³ cm⁻³. The DPHP method can be used to accurately monitor water content in the coarse media being considered for spaceflight applications.