Environmental Engineering Reference
In-Depth Information
The reader is recommended to look at some reference topics dealing with
metrology, particularly [ASC 84, BAS 66, HLA 92, LAB 85, LHE 66], and the
RILEM document [RIL 80].
10.2.2.
Independent variables
10.2.2.1
. Temperature: thermometry and calorimetry
For our purposes, the most appropriate sensors are those that are based on the
thermoelectric properties of conductors or semiconductors: metallic resistance,
thermistor and thermo-electric couple. These sensors will be set up by drilling stone
or fixing them to the stone surface [ASC 84]. Keep in mind that the information
delivered by a sensor is its own temperature: reduction of the difference between it
and the stone will depend on the experimenter's skill. Miniaturization of sensors is
interesting for rapidity of response and local damage of the material may become a
drawback if this material is not homogenous. In the case of unidirectional thermal
fields or radial symmetry, straight sensors may be used: stressed metallic thread or a
thin metallic tube. This tube can be also used as a “specimen” if instead of being
used directly as sensor it is used to support a sensor (e.g. thermocouple) [LAU 89].
Whether bought or specially made, sensors will be linked to a measurement system
- microvoltmeter or ohmmeter - that does not always have a directly standardized
temperature. Usually when temperature variations are low, except in the case of a
thermistor, linearization is possible. It is worth noting that the approach is different
for laboratory measurement and
in situ
work.
Calorimetry (measure of the mass of thermal capacity) [HLA 90] enables us in
principle to assess the c value (stone of given water content and temperature) or c
s
value (stone dried in a drying chamber), as the method does not change the water
content of the sample. Another approach will be indicated in section 10.2.3.
10.2.2.2
. Water content: gravimetry, tensiometry and hygrometry
A bibliographic study of this metrology was made by Aouaïssia-Abdallah [AOU
97]. Direct (gravimetry) or indirect measures have been listed in section 9.2.1.2 and
will not be described here. For a given stone at a given temperature, there are
relations (not one-to-one in the case of hysteresis) between water content and
suction on one hand, and between water content and relative humidity on the other.
In both cases, it will be interesting to distinguish whether the considered value is
imposed or only measured. In section 9.2.4.1, techniques are described where
suction or relative humidity are imposed on a stone specimen. In other devices (for
example the tensiometer according Bear [BEA 72]), the intrinsic suction of a soil is
measured. In the same way, by drilling a channel of sufficient length into a stone
and setting up a hygrometric probe, the relative humidity of air enclosed in the
