Sensor Integration and Calibration
44 Hydro-Probe XT User Guide HD0538 Rev: 1.6.0
The equation of the calibration line is defined by the Gradient (B) and the Offset (C), see
Figure 34. These values are referred to as the calibration coefficients.
The Gradient (B) is the relationship between the change in Moisture % to changes in the
Unscaled value.
Gradient =
Change in Moisture %
Change in Unscaled Value
The Offset (C) is the Moisture % value that corresponds to 0 Unscaled units.
The sensor will output 0 Unscaled units when in air. When loaded with dry material the
Unscaled value will increase and the Moisture % value will rise to indicate 0% Moisture.
The SSD value is a third coefficient used to define absorbed water within a material, this is
covered in section 2.2.
Using the coefficients the conversion from Unscaled to Moisture % is as follows:
Moisture % =
B
x (Unscaled Reading) +
C
–
SSD
In rare cases when the measurement of the material exhibits non-linear characteristics, a
quadratic term can be used in the calibration equation as shown below.
Moisture % =
A
x (Unscaled value)² +
B
(Unscaled value) +
C
–
SSD
Use of the quadratic coefficient (A) would only be necessary in complex applications and for
most materials the calibration line will be linear in which case
‘A’
is set to zero
2.2
SSD coefficient and SSD moisture content
Saturated Surface Dry (SSD) is an offset adjustment typically used in aggregates and hard
materials to define the % of moisture that is tightly bound to the material and not freely
available. The use of the SSD offset allows the % of free water to be measured.
For applications where Total Moisture is required the SSD Value is left to zero.
Bound Moisture
+ Free Moisture
= Total Moisture
In order to determine the moisture content of a material it is weighed, dried and re-weighed. In
practice it is not possible to easily determine the point at which the material reaches its SSD
condition. It is therefore often only practical to obtain an oven dried sample (Total Moisture).
SSD values are derived through more lengthy industry standard tests or supplied by the
material supplier.
Oven dried
moisture % (Total) -
Water Absorption Value %
(SSD offset in the sensor)
= surface moisture %
(free moisture)
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Страница 42: ...Chapter 4 Configuration 42 Hydro Probe XT User Guide HD0538 Rev 1 6 0 ...
Страница 52: ...Chapter 6 Optimising Sensor and Process Performance 52 Hydro Probe XT User Guide HD0538 Rev 1 6 0 ...
Страница 58: ...Chapter 7 Sensor diagnostics 58 Hydro Probe XT User Guide HD0538 Rev 1 6 0 ...
Страница 60: ...Chapter 8 Technical specification 60 Hydro Probe XT User Guide HD0538 Rev 1 6 0 ...
Страница 66: ...Appendix B Document Cross Reference 66 Hydro Probe XT User Guide HD0538 Rev 1 6 0 ...
