Campbell Scientific EnviroSCAN Sentek Water Content Profile Probe - Manual (Page 6)

EnviroSMART TM Soil Water Content Profile Probes
4.1 Water Content
Sensor output is a dimensionless frequency (raw count) that is converted via a
normalization equation and then a default or user-defined calibration equation
into volumetric soil water content. The measurement unit is volumetric water
content (Vol %) or millimetres of water per 100 mm of soil. Acceptable data
range for scaled frequencies is >0 to 1.0. Water content measurements using the
default calibration range from 0 to 53%.
4.2 Salinity
Sensor output is a dimensionless frequency (raw count) that, in conjunction with
the water content output signal, is proportional to changes in soil water content
and salinity. A proprietary data model processes the changes of both output
signals simultaneously to reflect the changes in soil salinity. The output of the
data model is a nominal Volumetric Ion Content (VIC). Measurement units of
VIC can be quantitatively related (benchmarked) to the soil EC through site-
specific soil sampling and analysis.
Acceptable VIC data range from 1000 to 17,000. The effective salinity
measurement range is between 0 and 17 dS m
-1 in sand, loamy sand and sandy
loam textures (Australian Soil and Land Survey Field Handbook). Salinity
levels and soil textures outside this range are currently not supported by
TriSCAN.
5. Resolution and Accuracy
5.1 Volumetric Water Content
The sensor has a resolution of 0.1 mm of soil moisture. Consecutive readings in
equilibrated soil have a coefficient of variation of 0.1%. Accuracy of the system
is dependent upon the similarity of the soil site to that of the original default soil
type used by Sentek. Calibration coefficients based on this default soil type are
used in normal operation. If site-specific (quantitative) values are required, then
a calibration procedure is required to be performed (refer to “Calibration of
Sentek Probes” Manual). A high level of accuracy can be attained with careful
calibration.
5.2 Salinity
To date, statements on the resolution and accuracy of the salinity output can only
be made with regard to the particular type of sand that has been used to develop
the TriSCAN data processing models. These specifications do not necessarily
reflect the resolution and accuracy of the sensor on any other soil types.
Accuracy and precision of the sensor in the tested sand are as follows:
In saturated soil conditions at low EC 55 μS cm -1 :
Repeatable change of VIC to a resolution of 1 μS cm -1 (Accuracy
±1.8%)
In saturated soil conditions at medium EC up to 5600 μS cm -1 :
Repeatable change of VIC to a resolution of 25 μS cm -1 (Accuracy
±0.4%)
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