Campbell Hydrosense, Instruction Manual

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HydroSense
6
4.5 Measuring Volumetric Water Content
To measure volumetric water content fully insert probe rods and press READ.
The correct rod length must be selected so the HydroSense display unit applies
the correct calibration to convert the probe output to water content.
4.6 Measurements in Special Materials
The HydroSense was principally designed for use in agricultural soils, but the
technology underlying the instrument supports many potential applications.
Soils with characteristics as described in Section 6.2 and many other porous
media can also be monitored with HydroSense using the period value presented
in the display. The period is directly related to the dielectric permittivity of the
material surrounding the probe rods and can be used as a relative value to
monitor changes in the material of interest. As water content increases so does
period. If actual water content values are needed, a calibration can be
performed using an independent measurement of the water content. A
calibration equation can then be derived using curve fitting methods to relate
the independently measured water contents to the probe output periods.
5. Water Deficit Mode
5.1 Description of Water Deficit Mode
To use the Water Deficit Mode, a lower reference level and an upper reference
level must be chosen and a HydroSense reading taken at each level and stored
in memory (see Section 5.4 for calibration). Measurements taken after
reference levels are stored will display the relative water content and the
amount of water in millimeters which must be added to bring the water content
to the same value as the upper reference level. The relative water content is a
value between 0 and 100 with 0 corresponding to the lower reference level and
100 the upper level. Upper and lower reference levels can be stored in
HydroSense for up to 5 sites.
A typical application of the Water Deficit Mode is irrigation scheduling. The
lower reference level might correspond to the lowest water content that would
be allowed for a particular crop and the upper level the point where irrigation
would be ceased.
5.2 Display Information
The two-line display provides the following information
1) The relative water content (RWC)
2) Which site reference level is selected
3) The water deficit for 12 and 20 cm profiles