Appendix A—Theory of Operation
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Appendix A—Theory of Operation
Raw counts
Raw counts
are the raw frequency readings by the sensor inside an access
tube exposed to the air-water-soil medium to be measured. Raw counts
range between 120 000 counts (taken in 100% water) and 160 000 counts
(taken in 100% air).
The sensor operates at a very high frequency (in excess of 100 Mhz). It is
not possible to tune all Diviner 2000 sensors to exactly the same raw count
response when measuring a particular standard, e.g. in a water bucket.
Therefore we have chosen not to work with raw data directly, but with
frequency ratios or normalized counts. This allows you to use different
Diviner sensors during the monitoring season and ensures meaningful data
continuity.
Normalized counts or SF (Scaled Frequency)
Normalized counts
represent the ratio of individual probe frequency
reading responses (inside the access tube) in soil (F
S
) compared with probe
responses in air (F
A
) and in water (F
W
). SF readings usually range between
the values 0 and 1. The frequencies in air, water and soil are passed through
a normalization equation to determine a normalized count or scaled
frequency (SF), defined as:
SF = (F
A
- F
S
) (F
A
- F
W
)
–1
Where
:
•
F
A
is the frequency reading in the access tube while suspended in
air
•
F
S
is the frequency reading in the access tube in soil
•
F
W
is the frequency reading in the access tube in the water bath.
Measurement technique
Volumetric soil moisture content
is measured by responses to changes in
the dielectric constant of the soil. The capacitance of a soil increases
considerably with an increase in the number of soil water molecules, which
are free to relax as their electric dipoles respond to the capacitor sensors
field reversal. This measurement is proportional to capacitance and is also
called specific polarization or electric dipole moment per unit volume.
With the advent of microelectronics it is possible to measure the responses
Summary of Contents for Diviner 2000
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