FMG HYDRO Humidity mesuarements using capacitive probe
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Measurability of materials

Basic considerations for good measurability of materials : The material should have a dielectric constant ε which is much smaller in comparison with that of water (ε =80), the ohmic resistivity (Ohm/cm) should not be too low, and the density of the material should remain fairly constant within the field of influence of the sensor during measurements.

Simple tests:
A material is essentially measurable when samples with different moisture content produce reproducible output signals under identical conditions of measurement with a probe. The mapping of the probe output signal to a definite value of moisture content (compilation of the calibration curve) must then be done in the laboratory. This calibration is rather flexible: e.g. an output signal of 1 Volt can be assigned to a moisture value of 1% and an output signal of 10V to a moisture content of 10%. Alternatively, 1Volt = 2% and
10V = 20% is also possible according to the chosen range of measurement.
This simplified assignment of moisture values to signals is only possible provided the calibration curve is linear. This is the case with minor deviations for sands obtained from the central European regions, namely such as those used in the production of concrete.

In other cases it is necessary to build the calibration curve using at least three different values of moisture.

The moisture measuring processor FMP can accept up to six such calibration points for building the calibration curve. Alternatively, such linearisation can be achieved with the help of a computer or a programmable logic controller.

Not measurable are those materials for which the dielectric constant approaches that of water, where the conductivity is high (producing a short circuit for the high frequency measuring fields) or those with large process dependent density variations (e.g. due to the presence of varying amounts of air within the material).

Also, a material becomes unmeasurable when the moisture content is so high that the sensor enters the so called region of saturation. With sands this situation is approached when the moisture content is so high that water starts running out of it. In such cases we stop describing it as moisture content of the material and instead talk about the solid content of water. We suggest using the type OFS optical equipment for measuring the solid / fluid content of such materials.

Another source of measuring error could be the changing salt content of the material. This happens  because of the increased ionic conduction within the material, and can under circumstances lead to complete ionic short circuits.