PASCO PS-2163 Soil Moisture Sensor - Manual (Page 2)

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Soil Moisture Sensor Introduction
2
Introduction
The PASCO Model PS-2163 Soil Moisture Sensor works with a PASCO interface to
collect soil moisture data. The sensor consists of the Soil Moisture Sensor box and the
the Soil Moisture Probe (cable assembly).
The sensor measures volumetric water content (VWC) percentage of a soil sample.
Volumetric water content is the volume of soil water per unit of total volume. In sim-
ple terms, dry soil consists of solid material and air pockets. A typical volumetric
ratio would be 55% solid material and 45% air pockets. As water enters the soil, the
air pockets begin to fill with water. An example of 10% volumetric water content
would be 55% solid material, 35% air pockets, and 10% water. The maximum volu-
metric water content would therefore be 45%. All the air spaces would be filled with
water and the soil would be saturated.
About the Sensor
The Soil Moisture Probe uses capacitance to measure the dielectric permittivity of the
surrounding soil. The volume of water in the total volume of soil most heavily influ-
ences the dielectric permittivity because the dielectric of water (80) is much greater
than the other constituents of the soil (mineral soil, 4; organic matter, 4; air, 1). Thus,
when the amount of water changes in the soil, the Soil Moisture Probe will measure a
change in the capacitance (from the change in the dielectric permittivity) that can be
directly correlated with a change in water content. Circuitry inside the Soil Moisture
Probe changes the capacitance measurement into a proportional millivolt output.
The Soil Moisture Probe averages the volumetric water content over the entire length
of the probe, with a zone of influence about 2 cm around the probe. The zone of influ-
ence is with respect to the flat surface; there is little or no sensitivity at the edges of
the probe. The electromagnetic field (EM) produced by the probe decreases with dis-
tance from the probe surface.
Installing the Soil Moisture Probe
When selecting a site for installation of the probe, it is important to remember that the
soil adjacent to the probe’s surface has the strongest influence on the sensor reading.
Therefore, any air gaps or excessive soil compaction around the probe can profoundly
affect the readings. Because the probe has a gap between its prongs, it is also impor-
tant to consider the size of the media you are inserting the probe into. It is possible to
get sticks, bark, roots, or other material stuck between the probe prongs, which will
adversely affect readings. Finally, be careful when inserting the probe into dense soil,
as the prongs will break if excessive sideways force is used when pushing them in.
When installing the probe, it is best to maximize contact between the probe and the
soil.
Vertical Orientation
Insert the probe into the soil, making sure that the probe is completely buried at least
3 cm below the soil surface. The tip of each prong is sharp - be careful!
The probe may be difficult to insert into extremely compact or dry soil. Carefully
loosen the soil before inserting the probe. Never pound the probe into the soil!
Make sure that there is good contact between the probe and the soil. Use your fingers
and fist to tamp the soil down around the probe.
The copper traces used to measure
water content are sealed between
two pieces of epoxy-impregnated
fiberglass. The electromagnetic
(EM) field generated by the traces
travels through the fiberglass and
into the soil surrounding the probe.
Do not install the probe adjacent to
large metal objects such as metal
poles or stakes. This can attenuate
the probe’s electromagnetic field and
adversely affect output readings.
3 cm