PASCO PS-2163 Soil Moisture Sensor - Manual (Page 6)

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Soil Moisture Sensor Setup
6
2. Air dry the soil (spread the soil in a thin layer and use a fan to move air over the
soil.)
3. Remove large objects from the soil and break up large clods so the soil can fit
through a 5 mm mesh.
4. Pack the soil into the calibration container at approximately the field bulk den-
sity.
5. Insert the Soil Moisture Probe fully into the soil. Connect the probe to the sensor
and the sensor to the interface and use the sensor to make a measurement in
millivolts (mV). Record the measurement.
6. Use the volumetric soil sampler to collect a sample near the probe. Place the soil
sample into a drying container. Measure and record the mass of the soil sample
plus the drying container and then replace the lid on the drying container*.
7. Wet the calibration soil by adding 200 to 300 milliliters of water to the soil as
evenly as possible. Thoroughly mix the soil.
8. Repeat steps 3 through 7 until the soil approaches saturation. This generally
yields five to seven calibration points.
9. Remove the lids from the drying containters. Dry the volumetric soil samples in a
105 C oven for 24 hours.
10. Remove the soil drying containers from the oven and replace the lids. Allow the
containers to cool.
11. Measure and record the mass of the dry soil plus the container (without lid).
Calculations:
1. Determine and record the volume of water by subtracting the dry soil mass from
the “wet” (pre-dried) soil mass. Convert the mass difference into a volume based
on the density of water (1 g/cm
3 ).
2. Calculate and record the volumetric water content by dividing the volume of
water (cm
3
) by the volume of the soil sample (cm
3
).
Analysis:
1. Plot volumetric water content versus the sensor voltage output (in mV). Open a
data table and enter the values for sensor voltage output (x-axis) and volumetric
water content (y-axis).
2. Open a graph display and use the “Fit” function to find the “best fit” for the plot-
ted data. (Try “Linear” first and then “Quadratic”). Record the slope and the
y-intercept. (If the best fit is quadratic, record the coefficient, a, b, and c.)
3. Create a calculation for VWC based on the slope and the y-intercept (VWC = mx
+ b). Let “x” be the sensor voltage output.
*Any water loss due to evaporation
after sampling introduces error to the
volumetric water content calculation.
If the best fit is quadratic, create a
calculation for VWC based on the
coefficients, a, b, and c.
VWC = ax 2 + bx + c
where x is the sensor voltage output.