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Appendix B: Soil Specific Calibration Process
Introduction
The GPLP probes are initially calibrated in fine sand, as this provides an easy to reproduce ref-
erence point. For this reason, there may be slight differences between the readings given by
the sensor and actual moisture content due to differences in the measured soil's dielectric.
These differences are generally small, on the order of a few percent, however for highest accu-
racy the user may wish to calibrate for the specific soil type used in their own application. The
process described here may be used to custom calibrate the
GroPoint™
Lite Profiling probes
for specific soil types.
As with any calibration process, it is vital to ensure the highest level of accuracy at all stages of
the process. Any errors introduced, for example in the manual determination of volumetric
moisture content, or in soil compaction around the probes, will affect the accuracy of the cali-
bration.
Important First Step:
Before proceeding with this process, you must disable the use of the third order
polynomial in the sensor measurement. This is done by issuing the following SDI-
12 extended commands (assuming the sensor has the default address = 0): 0XM0!
You can also use the GPLP Reader program to disable the third order polynomial via the check-
box on the “Coefficients” dialog.
Overview
Sensor readings are taken in a variety of soil moisture levels and are compared with manually
determined volumetric moisture contents. The first soil moisture level is the initial moisture of
the dry soil, after it has been allowed to air dry to ambient conditions for several weeks. It does
need to be completely dry, but it should be less than 5% volumetric moisture content (VMC).
The last moisture level should be at or very near saturation.
Calculate the volume of water to add based on the measured volume of the soil used. The de-
sired increase for each moisture level should be approximately 4-5%. For each test moisture
level, water must be carefully and evenly applied to the soil using a spray bottle. The soil is well
mixed, returned to the soil box and allowed to equilibrate for at least an hour (longer if the soil
is clay or clay loam).
When the soil is ready, remove the top half of the soil, place the probe in the soil container,
then replace the soil over the probe. Ensure that the sensor is in good contact on all sides with
the soil (no air gaps), and that there is at least 7-8 cm of soil in all directions around the probe.
Try to ensure uniform soil compaction when replacing the soil. Take several measurements
from the sensor and record the results. Take several samples (at least three) of the soil and de-
termine the volumetric moisture content (see Appendix A). Log the manually determined mois-
ture content along with the results measured by the sensor.
