Physical bases of freezing point measurement using ... - Boschung

Proceedings of the 9 th SIRWEC Conference 15-17 March 1998, Luleå, Sweden 279
in [Franks 1972], those of pure ice in [Hobbs 1974, Jaccard 1959, Ruepp 1973, Franks 1982]
and those of aqueous electrolyte solutions in [Horvath 1985, Franks 1973]. Data are given in
[Zaytsev and Aseyev 1992, Barthel et al. 1995].
4. DISCUSSION
4.1 Utilization of active and passive probes
As we have seen, the first goal of the active probe is to measure the freezing
temperature. However it allows also to measure the surface temperature Tr of the road. In that
case, stabilization between cycles would have to be performed. It is interesting to note that the
surface temperature of the road could be difficult to measure with only one probe. As shown
by Scharsching [Scharsching 1991] its value is not homogeneous on all the surface of the
road. The location of the probe has to be chosen with care.
It is also possible to use in the same vicinity all together an active an a passive probe,
the first one measuring Ts and the second one 7 Tr. The last possible configuration is to use
only a passive probe, in conjunction with meteorological sensors (relative humidity,
precipitation, etc.).
4.2 Salt content
For the road's winter maintenance executive, the knowledge of the freezing temperature,
as important it could be, is nevertheless not sufficient. Due to precipitation, streaming or
evaporation, the amount of water on the pavement, and then the concentration of chemical
and the freezing temperature, may vary. The parameter which can be used to characterize the
protection rate of the road is the salt content. As it will be shown, it can be deduced from the
freezing temperature and the thickness of the solution. The example of spreading solid
sodium chloride as anti-icing treatment will be considered.
The quantity of salt spread by the service vehicle can be chosen to give a salt content s
comprised between 5 and 40 g/m 2 . After the salting, the mass mNaCl of salt laying on an area A
of the pavement is given by :
m = s⋅
A . (6)
NaCl
If salt is diluted in a certain amount mH 2 O of water (in order of simplification, the salt
content on the area A is kept constant), the weight concentration c of NaCl will be :
mNaCl
c =
m + m
NaCl H2O m
=
m
NaCl
tot
, (7)
7 The passive probe has to be placed far enough from the active one in order to be not influenced by it.