The application of this sensor for total suction measurement of unsaturated soils has come to light only a couple of decades ago. WP4 dew-point potentiometer is commonly used in unsaturated soil testing, which is based on the principle of CMH.
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The soil specimen is placed in the sensing sealed chamber and brought to vapor pressure equilibrium prior to cooling. The instrument is provided with an internal fan in the sample chamber to reduce the equilibrium time. At equilibrium, the water potential of the ambient air in the sealed chamber is equal to the water potential of the sample. The mirror temperature is controlled by a thermoelectric cooler. Photoelectric cell is used to detect the condensation on the chilled-mirror. The thermocouple records the temperature at which condensation occurs. The water potential in kPa and temperature in degree Celsius of the soil sample are automatically displayed on the screen. Additional advantage of this instrument is that the sample temperature can also be controlled and, thus, thermal studies can be carried out.