Ssd Coefficient And Ssd Moisture Content; Storing Calibration Data; Figure 16: Calibration Inside The Sensor - Hydronix Hydro-Probe Configuration And Calibration Manual

Sensor Integration and Material Calibration

3 SSD Coefficient and SSD Moisture Content

In practice it is only possible to obtain oven dried moisture (total moisture) values for calibration.
If the surface moisture content (free moisture) is required the Saturated Surface Dry (SSD)
coefficient must be used. In some industries SSD is also known as the Water Absorption Value
(WAV).
Absorbed
moisture
The SSD coefficient used in Hydronix procedures and equipment is the Saturated Surface Dry
offset, which is the water adsorption value of the material. The SSD value can be determined
using industry standard procedures or obtained from the material supplier.
The surface moisture content refers only to the moisture on the surface of the aggregate, i.e. the
'free water'. In certain applications such as concrete production only this surface water is used in
the process, which is why it is this value that is generally referred to in concrete mix designs
Oven dried
moisture %
(Total)

4 Storing Calibration Data

There are two ways of storing the calibration data, either in the control system or in the sensor.
Both methods are shown below.
Calibration inside the sensor will involve updating the coefficient values using the digital RS485
interface. A value that is directly proportional to moisture will then be output by the sensor. To
communicate using the RS485 interface, Hydronix have a number of PC utilities, most notably
Hydro-Com which contains a dedicated material calibration page.
To calibrate outside of the sensor, the control system will require its own calibration function and
the moisture conversion can then be calculated using the linear Unscaled output from the sensor.
For guidance on setting the output see Figure 2.
Calibration inside the Sensor
4.1
+ Free moisture
- water adsorption value %
(SSD offset in the sensor)

Figure 16: Calibration inside the Sensor

= Total moisture
= surface moisture %
(free moisture)
Configuration and Calibration Guide HD0679 Rev 1.2.0 31
Chapter 3