A thermodielectric analyzer to measure the freezing and moisture characteristic of porous media

Abstract

The freezing and moisture characteristics of porous media are difficult to measure. We have designed an instrument to measure the freezing characteristic of a porous medium in the range −20 to 0°C. Because of the similarity between the freezing characteristic and the moisture characteristic, the data obtained can be used to infer the moisture characteristic from about −30 to −23,600 J kg−1. Temperatures and unfrozen water contents are measured with a thermistor and a spiral‐shaped transmission line connected to an oscillator, respectively. Temperatures are converted to water potential using the Clapeyron equation. Experimental results obtained with the freezing technique were compared with vapor pressure methods. The four test media used, a silt loam soil, a silty‐clay loam, a clay, and a sandstone, showed good agreement between freezing and standard methods. The freezing technique described here has a water potential resolution of about −11 J kg−1. An entire characteristic can be determined within 24 hours.