Environmental Engineering Reference
In-Depth Information
11.1.2.
Follow-up of water contents after restoration
The capacitive sensors were installed inside the walls with the idea of following
the movement of water in the wall, rising damp in particular. It appeared that
unexpected water movements could be found.
11.1.2.1.
Measurement principle
The quantification of water content by capacitive sensors is based on the
measurement of the apparent dielectrical permittivity (ε') of a moistened material.
The technology is close to that of a capacitor: two metallic electrodes placed inside
the stone form a capacity, the dialectical part of which is the material in between the
two electrodes.
C = ε'ε° /
k
where ε' is the apparent dielectrical permittivity, ε° the vacuum permittivity
(ε° = 8.84. 10m
-12
F/m), and
k
the coefficient linked to the geometry of the
electrodes. These electrodes are connected to an oscillator, the nominal frequency of
which is equal to 35 MHz. At this frequency, the losses due to conductivity and
polarization are weak, and the relative permittivities of the constituants in the
material studied have the following order of magnitude: ε' air = 1, ε' ground or stone
= 2, ε' water = 80, and ε' ice = 4. A variation in water content (and thus of the
dielectrical constant) induces a variation in the frequency of the oscillator,
determined by the value of the capacity. The relationship between the frequency (in
Hz) and the dielectrical permittivity of the material has the following form:
F
= 1 / sqrt (αε' + β)
where α and β are coefficients linked to the geometry of the electrodes.
The oscillator is associated with a circuit, the function of which is to divide the
frequencies, thus preventing high frequencies from being transported. The variations
then observed are situated around 5,000 Hz. This method requires a calibration to
be performed on the material: a stone sample is saturated with water and slowly
dried. The frequency is measured with the capacitive sensor at the same time as the
water content, which is evaluated through weighing. A linear function is obtained:
F
= -
Aw
+
B
