®
PS-2163
012-10538C
Setup
5
voltage output from the Soil Moisture Probe to VWC values for several soil
samples with different degrees of dryness, and then plot a graph of VWC
versus probe output in millivolts (mV).
The basic process is to collect a measured volume of bulk soil, measure its
mass, measure the probe reading in mV for the bulk soil, dry the soil to
determine the amount of water that was in the soil, and calculate the ratio of the
volume of water to the volume of bulk soil. Then, create a graph of VWC versus
probe voltage and determine the “best fit” for the graph. If the graph is linear,
determine the slope and y-intercept. If the graph is quadratic, determine the
coefficients of each term. A
linear
calibration curve would have a formula of
y
=
mx
+
b
where
y
is the VWC,
m
is the slope,
x
is the probe output in mV, and
b
is the y-intercept. Finally, create a calculation based on the formula for the soil
specific calibration curve.
Equipment: shovel and bulk soil container, calibration container, Soil Moisture Probe and PASCO
Interface, volumetric soil sampler
1
, soil drying containers with lids (e.g., baby food jars), scale or
mass balance, drying oven.
Procedure:
1. Collect approximately 4 liters of bulk soil from the depth/location where you
want to measure with the probe.
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
density.
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 containers. 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).
A
quadratic
calibration curve would
have a formula of
y
=
ax
2
+
bx
+
c
where
y
is the VWC,
x
is the probe
output in mV, and
a
,
b
, and
c
are
coefficients.
*Any water loss due to evaporation
after sampling introduces error to the
volumetric water content calculation.
