Environmental Engineering Reference
In-Depth Information
stuck to the surface of the material can be - in the elementary case - proof with the
shape of a thin sheet whose temperature difference between both faces will be
measured. The application of Fourier's law to the proof for which the thickness and
conductivity are known, enables us to assess the incoming and outgoing flux density
from the rock. The metrological interest of fluxmetry in comparison to simple
thermometry of temperature fields has been proved by Thery [THE 91]. It could be
necessary to measure thermal fluxes generated by the Joule effect and imposed on
the rock. It will consist of measuring electric power whether the transmitter is a
thread, film, tube, ribbon, etc. These imposed fluxes will be useful (see later) to
create disruption of the thermal field of sample.
10.2.3.1.2. Humidity
Humidity could be useful to measure the amount of water absorbed by a block of
stone or hydric fluxes (in kg/s or in kg/m
2
.s): a description of the various techniques
is given in [RIL 80]. There are tests titled II.1 “Coefficient of saturation S”, II.3 and
II.4 “Evaporation curve” (mass increasing or decreasing due to vapor pressure
gradients are measured). It is also possible to make a mass balance (gain or loss of
water vapor) of air circulation where it contacts a by rock using temperatures and
relative humidities measured with a thermohygrometer before and after contact, by
means of a humid air diagram and measurements of the air flow and rock surface
concerned [BAS 97].
10.2.3.2
. Low perturbations
The techniques presented are assumed to weakly disrupt the variable considered.
It can therefore make [10.3] linear. Using the fact that Luikov's number [LUI 66],
the ratio between thermal and mass diffusivities, is usually low (i.e. around 1/500)
for the materials studied, it is possible to get thermal sufficiently short perturbations
there is no effect on the hydric equilibrium (this is the advantage of a dynamic
method over a static one, for example a hot guarded plate [AFN 70]) and isothermal
hydric perturbations, the isotherm being controlled by a secondary device. The
conception and set-up, however, do not always enable the addition of such a device.
If we are dealing with materials far from saturation, the effects of gravity are
negligible, thus:
∂
θ
=∆
D
θ
[10.4]
θ
∂
t
10.2.3.2.1. Thermal perturbations
As indicated above, equation [10.3] can be reduced to:
