Volume 26, Issue 1 (March 2003)
Polymer Capacitance Sensors for Measuring Soil Gas Humidity in Drier Soils
Polymer capacitance sensors were assessed in this study for measuring soil gas relative humidity and total suction in soils that are too dry for thermocouple psychrometers. Capacitance of the sensors varies as the relative humidity of the gas surrounding the sensor is varied. Soil gas relative humidity is inferred through a calibration curve relating soil gas relative humidity and capacitance. Tests conducted with six finer textured soils and three sands showed that the capacitance sensors can be used to obtain unbiased estimates of relative humidity for a broad variety of soils with a precision of approximately ±3%. Tests also showed that the sensors are non-hysteretic and relatively insensitive to temperature, except when the temperature approaches the freezing point of water. An example is provided illustrating how capacitance sensors can be used to monitor soil gas relative humidities in alternative earthen final covers that employ passive flow of atmospheric air to remove water.