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THEORY OF OPERATION
The 14.04.05 Soilmoisture Probe is a tensiometer-type instrument that reads
soil suction directly. The “soil suction” reading is a direct measure of the availability
of moisture for plant growth, and the standard unit of measurement is the “bar”. The
bar* is a unit of pressure in the metric system and is used to dene positive pressure
(above atmospheric pressure), or negative pressure or vacuum (below atmospheric
pressure).
The gauge on the Probe is calibrated in hundredths of a bar (or centibars) of vacuum,
and is graduated from zero to 100.
In scientic work, it is becoming customary to express pressures and vacu-ums in a
unit of measure called a “Pascal”, and a “Kilopascal”, which is 1000 times as large as
a Pascal. A “centibar”, as used above, is exactly equal to a Kilopascal. Therefore, the
dial gauge on the Probe also reads in kilopascals and is graduated from zero to 100
kilopascals (KPa).
Soil suction is actually created by the attraction that each soil particle has for the
water in the soil. Because of this attraction, water forms a lm around each particle of
soil and collects in the capillary spaces between the soil particles. As the soil be-
comes drier, these lms become thinner and the attraction or soil suction increases.
The plant root has to over-come this soil suction, or attraction force, in order to
withdraw moisture from the soil. The measurement of soil suction then gives a direct
indication of the amount of work the plant root must do to get water from the soil. The
only moisture measuring instruments that accurately measure soil suction are those
using the tensiometer principle. These instruments read centibars of soil suction
directly without calibration for soil type, salinity, or temperature.
When the Probe is inserted into the cored hole, there are various effects associated
with the movement of the porous ceramic sensing tip through the soil. The soil sur-
rounding the tip is slightly compacted and the wiping action of the porous ceramic
through the soil causes small thermal effects. It takes a few moments for these dis-
turbances to disperse, and this is the reason that it is not desirable to move the Null
Knob for the rst minute after insertion of the Probe.
In order to obtain a soil suction reading, it is necessary for a small amount of water
to transfer between the sensing tip of the Probe and the soil. When the Null Knob is
turned clockwise, water is forced out of the Probe sensing tip and into the surround-
ing soil. When the Null Knob is turned counterclockwise, a vacuum is created within
the Probe, which causes moisture to move from the soil through the ceramic sens-
ing tip and into the Probe. In order to obtain an accurate reading within the minimum
amount of time, one must be careful not to disturb the moisture conditions surround-
ing the sensing tip.
Steps for initial filling
(before installation)
When examining the Probe, DO NOT leave the porous ceramic sensing tip exposed
to the air for prolonged periods. When the Probe is removed from the Carrying Case
and the sensing tip is not kept moist, evaporation of moisture from the tip will pull the
dial gauge up to a very high centibar reading. Under these conditions, air can dif-
fuse through the water in the pores of the sensing tip and enter the Probe, which can
result in a decrease in sensitivity and require a relling cycle.
