Calibration of thermal conductivity sensors with consideration of hysteresis

Abstract

A thermal conductivity sensor monitors soil suction by measuring the changes in thermal conductivity of the porous tip. The thermal conductivity of the porous tip is a direct function of its water content. It has long been recognized that the suction versus water content relationship of a porous material exhibits hysteresis of various magnitudes between wetting and drying processes. The sensor output may correspond to various suction values of the sensor porous tip, depending on the wetting or drying state of the porous tip. The current calibration procedure, however, represents only one of the drying processes of the sensor porous tip. A laboratory testing program was carried out to better understand the hysteretic properties of the sensor output voltage versus the suction and to further improve the calibration procedure. The output of the sensor was monitored as the sensor porous tip was subjected to various drying and wetting processes. The test results indicate an error of 30%–70% for suctions higher than 100 kPa if the hysteretic effects of the porous tip are not considered in data interpretation. Based on the laboratory testing results, a revised calibration procedure was proposed that takes into consideration the capillary hysteretic effects.Key words: thermal conductivity sensor, calibration, matric suction, capillary hysteresis.