Note: Descriptions are shown in the official language in which they were submitted.
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CaJ~acitive hy~rometer and its ~roductionJ~ocess
BACKGROUND OF THE INVENTION
__ _ _ _ _
The present invention relates to a hygrometer
functioning by means of a capacitive me~hod, as
well as to a process for producing this hygrometer.
A known means for measuring the humidity of
the air consists of using a capacitor, whose dielectric
absorbs a quantity of water which is a function of
the relative humidity of the air and measuring the
dielectric constant thereof.
) For such an instrument to function correctly,
at least one of the electrodes of the capacitor must
be permeable to water, have a low electriOE~ resistance
and be relatively insensitive to corroslon.
In existing sensors, the compromise between
these three requirements is generally brought about
by the use of ~ vacuum-deposited ultra-thin gold
coating (approx. 100 ~). Although the first two
conditions are generally satisfactorily fulfilled
(good electrical conductivity of the gold and good
permeability because the coating is very thin~
such capacitors have a poor corrosion resistance.
Thus, a gold thickness of 100 ~ can be relatively
rapidly destroyed by pollutants, particularly sulphur-
based pollutants (S02, H2S04) and the life of theinstrument is reduced.
A conventional means for solving this problem
consists of prote$ting the gold coating by adding filters,
e.g. in the form of the sheet of cellophane (trademark).
However, this unfortunately has the effect of increasing the
sensor response time and therefore reducing its sensitivity.
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BRIEF SUMMARY OF THE INVENTION
The present inven~ion relates to a capacitive
hygrometer having both a good response time and a
good corrosion resistance.
According to the main feature of the invention,
the hygrometer is of the type constituted by a
capacitor having a dielectric material whose
dielectric constant varies as a function of the
quantity of water absorber, the conductive faces
, of the ca~acitor being connected to means for measuring
) the dielectric constant and has on one of its conductive
faces and in the dielectric numerous fissures bringing
the dielectric material into direct contact with the
atmosphere, whose relative humidity is to be measured.
However, the electrical continuity of the fissured
conductive face is maintained.
Due to the presence of these fissures, the
dielectric is in direct contact with the atmosphere
whose relative humidity is to be measured and is no
(~ ) longer separated therefrom by a thin metal,coating
to which is optionally added a filter. Thus~ the
operating speed of the hygrometer is signific~ntly
improved.
Moreover, the fissured electrode need no
.
longer comprise a very thin metal coating, because
it is scored to a significant extent. It can in
fact be formed by one or more thicker layers of one
or more materials with a greater resistance (e.g.
a chromîum layer), which makes it substantially
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insensitive to corrosion and increases the service
life of the instrument.
The invention is described in greater detail
hereinafter relative to non-limitative embodiments
and the attacheddrawings, wherein show:
Fig 1 a diagrammatic perspective view of a hygrometer
according to the invention.
Fig 2 a plan view illustrating a process for producing
a hygrometer according to the invention.
Fig 3 a sectional view along line A-A of Fig 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig 1 shows capacitor 1 comprising a first
continuous and unfissured electrode 2. This electrode
carries dielectric 3, which can e.g. be a cellulose
ester and then the other electrode 4. Electrode 4
and dielectric 3 are scored by numerous fissures 5.
For reasons of clarity, the scale of the latter has
deliberately been increased inthe drawing, but in
actual fact they can be microscopic fissures which
are invisible to the naked eye. Finally, two conductor
wires 6, 7, connected to electrodes 2, 4 respectively
connect the capacitor to measuring means 8 permitting
- the determination of the dielectric constant thereof.
The fissured electrode 4 is constituted by
a metal which is relatively insensitive to corrosion,
such as chromium. In the presently described system,
the humidity of the air does not have to traverse
a thin metal coating and fissured elecbode 4 may be
impermeable and even very thick. Such hygrometers
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have been made w;th chromium thicknesses varying
between 200 and 5000 ~, no significant difference
in the response times having been detected. For
the same reason, it is possible to further improve
the corrosion resistance by adding to the chromium
layer a precious metal layer, e.g. of platinum or
gold, whilst further improving the life of the
hygrometer.
The capacitor is produced by the deposition
by evaporation onto the dielectric layer of a known
metal so as to be tensionally stressed when deposited
in the form of a thin coating and which contracts
in such a way as to produce a large number of cracks.
More specifically, a polymer layer, e.g. a cellulose
ester serving as the dielectric is placed in an
enclosure in which the vacuum has been formed. A
metal is evaporated in the same enclosure and
deposited on the dielectric, the experimental conditions
being adjustable as a function of the desired thickness.
The first electrode is constituted by an
anodically oxidized tantalum layer. The very thin
tantalum oxide layer prevents any short-circuit
between the two electrodes, even if there are holes
in the polymer. It forms a very high capacitance in
series with that of the polymer and consequently
in no way reduces the sensitivity of the sensor.
Part of the first electrode is protected from
oxidation for forming contacts. The polymer can be
deposited by immersion in a solution, followed by
drying.
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The standard polymer used is cellulose
acetate butyrate, but a good sensitivity level
has been obtained with other polymers, such as
polyimides or plexiglass (trademark~.
The metal layer which is tensionally s~ressed
for various, not well known reasons, exerts
considerable forces on the polymer. As the latter
is not sufficiently rigid the metal layer fissures,
leading to the fissuring of the polymer layer. ~his
leads to a network of very numerous microscopic
fissures, whose width is a fraction of a micrometer.
~ his network of fissures defines small islands
or islets Or metal on the surface of the capacitor,
the dimensions of the latter being approximately
a few micrometers. However, it hæs been experimentally
observed that there are always a certain number Or
contact points between the various islets and
that the electrical continuity of the fissured
' ) electrode 4 is maintained.
~igs 2 and 3 illustrate an embodiment Or
the production process for a hygrometer according
to the invention.
On an insulating substrate 11 (e.g. Or glass)
is deposited a tantalum layer 12 forming the first
electrode and is anodically oxidized so as to obtain
a surface oxide layer 13. Contacting electrodes are
then deposited, namely a first contacting electrode
14 on first electrode 12 and a second contacting
electrode 16 on substrate 11 alongside the first
tantalum electrode 12. ~he contacting electrodes can
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be of chromium, nickel or gold. In order to ensure
the electrical contact between electrodes 14 and
12 it is either possible to protect electrode 12
against oxidation by means of a resin, or to
perforate the oxide layer before making the bond
or weld.
A polymer layer 15 is then deposited over
the entire surface of substrate 11, so that it
has a certain number of holes 17. In this connection
it is advantageous to rapidly rotate the assembly
by means of an apparatus of the type used for
spreading photosensitive resins onto integrated
circuits. The polymer is then left to dry and then
a thick, porous chromium layer 20 is deposited
thereon. If necessary, it is possible to adjust
! the capacitance of the capacitor by scraping the
polymer and the chromium layer linked therewith.
The holes 17 in the polymer layer ensure that the
chromium provides the electrical contact with the
contacting electrodes 16.
This process has made it possible to obtain
6 x 6mm sensors produced in batches of 49 with an
efficiency of almost 100%.
Rapid calculations make it possible to
estimate the gain in the response time of the
hygrometer according to the invention compared with
a prior art hygrometer.
In a conventional hygrometer, the dielectric
surface in contact with the humidity of the air
(across a thin gold coating) is equal to the surface
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of the capacitor foils, whereas with the hygrometer
according to the invention it is equal to the
surface provided by means of the fissures. If
d is the average distance between fissures, e the
thickness of the polymer and if each ~slet is
likened to a square and accepting that each fissure
completely scores the dielectric, the ratio between
the surfaces in contact with the atmosphere of a
conventional sensor and of the hygrometer according
to the invention is:
d2
4 ed
Values of e ~ d ~ 2~m have been obtained
with the sensors produced. The surface in contact
with the atmosphere is conseqllently multiplied by
4 for a same volume of dielectric material and for
a same surface area of the capacitor foils. In
fact, the tests which have been performed indicate
an even greater gain for the response time. This
shows that a fissured dielectric according to the
invention allows water to penetrate much more
easily than a dielectric which is merely covered by
a ~ery thin metal coating, as in the prior art
hygrometers.
Thus, the hygrometer according to the invention
offers numerous advantages. It is sensitive to the
relative humidity of the air with a lo~ response time.
Moreover, it is relatively insensitive to corrosion
because the fissured electrode no longer has to be
very thin to permit the passage of the atmospheric
humidity. In fact, it can have a significant thickness,
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which increases its life, as well as that of the
hygrometer. In addition, the fissures have a
very limited width (approximately 1000 to 2000 ~)
and the generally larger atmospheric dust particles
cannot penetrate them. This also contributes to the
long service life of the instrument.
Finally, it is obvious that the capacitive
sensor according to the invention can have applications
other than hygrometric measurements. It is merely
necessary to use as the dielectric layer a material
whose dielectric constant varies as a function o~
the magnitudeto be measured.
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