Determining the Repeatability of the Matric Potential Sensor (MPS-1) in a Columbia Silt Loam Soil.

The purpose of this trial was to test the repeatability of the new Matric Potential Sensor (MPS-1) from Decagon Devices. This sensor measures the dielectric permittivity of a porous ceramic disk that is then related to matric potential of a soil matrix. A dielectric permittivity of a particular material is associated with its ability to transmit an electrical field. Matric potential is the affinity of the soil matrix for water, which leads to how water moves in the soil due to capillarity and adsorptive or desorptive forces. The calibration of the MPS-1 sensors was conducted in a pressure chamber using a 5-bar pressure plate with pressure steps ranging from 0 to 5 bars. The pressure steps used were 0, 50, 80, 100, 150, 200, 500, 1000, 2000, 3000, 4000, and 5000 mbars, respectively. A slurry of Columbia Silt Loam was made using 3250g of soil mixed with 1750mL of water. Ten of the MPS-1 sensors were mounted on a plastic ring at a height of 1cm above the pressure plate with a layer of soil above and below the sensor to ensure good hydraulic contact. In addition, to these 10 sensors there was an ECH20-TE sensor added to the plastic ring to give soil temperature, volumetric water content (VWC), and electrical conductivity (EC). Additional external measurements include three other temperature measurements and four different pressure readings, including two pressure gauges, a Tensimeter, and a pressure transducer, to ensure that the proper pressure was being applied at each pressure step. From the onset of the trial there were questions arising. The readings show that for the first two pressure steps two out of the ten sensors were showing changes in matric potential. Then, as the pressures were increased more of the sensors began to respond at pressures above 100mbars, which is acceptable since the accuracy of the sensors is noted to be from 100 to 5000 mbars. It was not until the 200mbar pressure step that all the sensors began to respond. Seven out of the ten sensors showed pressure or matric potential values within a finite range. The other three sensors were either significantly higher or lower than the other seven sensors. The readings from the MPS-1 sensors also show that there is a dependence on temperature, which shows that as temperature rises and falls so does the matric potential.