Measuring and testing – Gas analysis – Moisture content or vapor pressure
Reexamination Certificate
2000-12-18
2002-09-17
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas analysis
Moisture content or vapor pressure
C324S663000, C324S664000, C324S689000, C324S361000, C324S224000, C324S224000, C324S427000, C324S076490
Reexamination Certificate
active
06450026
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a new technology for producing capacitive sensors, more particularly for carrying out relative humidity measurements. It will be seen that the technology and embodiments described within the scope of the present invention may be applied to the production of sensors which may be used for carrying out other measurements.
Several methods for producing capacitive sensors are known as well as the use of such sensors for carrying out humidity measurements.
The technology used here consists in producing two plane electrodes, electrodes between which a material is placed, the dielectric characteristics of which are changed by the presence (or absence) of humidity in the ambient air. The change in characteristics of the dielectric causes a variation of the capacity of the thereby produced condenser, so that the output signal gives information on the air's humidity content. Processing this output signal does not pose any particular problem, for example this is achieved with a R-C or R-L-C circuit.
According to a prior art technique, one of the electrodes is produced by depositing a metal layer on an insulating substrate and the other one by depositing a thin metal layer on the dielectric. In a commonly used embodiment, vacuum metallizing is performed which gives a very low metal thickness. Indeed, to have a capacitive. sensor function as a humidity sensor, the humid air which is to be characterized must absolutely be able to penetrate into the inside of the condenser in order to influence the dielectric. Consequently, it is realized that a particularly thin porous electrode should be produced. If this electrode is made of metal, and taking into account the atomic structure of metals, the limit between electrical continuity and imperviousness is reached for thicknesses of the order of {fraction (1/100)} micron, and this, whichever metal is used (generally, chromium, nickel, or gold). In addition to a relative brittleness of the sensor, this embodiment involves many disadvantages; in particular, making a reliable connection between this particularly thin electrode and the measurement circuit is extremely delicate. Besides, it is seen that presently produced capacitive sensors require a succession of careful and delicate operations; first, metal is coated on a substrate and this on two distinct areas, one area serves as a first electrode and the other one is used as a contact for the second electrode. The whole is then covered by a dielectric film which is interrupted at the contact area provided, for the second electrode. Finally, the second electrode is put into place, i.e. by depositing a metal film on a thickness of the order of one hundredth of a micron. Of course, an adjustment of the useful surface of at least one electrode is required in order to obtain the desired capacity for a given moisture content. Different methods have been suggested for achieving this post-calibration; particularly, French patent FR-2 687 834 (CORECI) is to be mentioned which provides a plurality of bridges capable of being disconnected as well as British patent GB-2 213 323 (VAISALA) which recommends adjusting the useful surface of an electrode by insulating it through the substrate by a laser process without altering the dielectric. In every case, the method is the same; a capacitive sensor is produced according to the above described method, taking a maximum of precautions, then its performances are measured and absolutely necessary adjustments are made in order to obtain a usable response.
In any case, capacitive sensors produced according to the above described methods, tend to behave like “sponges”, i.e. their constituent materials are impregnated with moist air (notably at the substrate/metal interface of the first electrode) so that they exhibit a certain hysteresis and their response may be erroneous in minutes or hours following their exposure to an atmosphere saturated with humidity, or even their immersion in a liquid.
The imperfect and unsatisfactory character of the capacitive sensor for measurement of humidity is now particularly convincing: the sensors are particularly brittle, their accuracy is of the order of 10% on the measured value, they are not interchangeable, they are subject to saturation and their manufacturing process is both long, delicate,and expensive.
Another type of capacitive sensor for measuring humidity is described in document US-5177 662. This sensor comprises two porous electrodes produced as a porous polymer layer which has been made conductive by inclusion of conductive particles, such as carbon particles. However, the effective resistance of these electrodes is of the order of 15,000 &OHgr;.
The used dielectric is an absorbing polymer film with a thickness of about 10 &mgr;m, such as polyimide or polyparabanic acid.
This sensor, therefore, has an entirely porous structure.
SUMMARY OF THE INVENTION
The invention is directed to producing sensors which overcome the aforementioned drawbacks.
An object of the present invention is to produce a capacitive sensor for measuring humidity whereby manufacture thereof is simplified, especially at the level of assembling the electrodes and the dielectric and fixing the contacts for tapping information.
Another object of the invention is to allow for provision of particularly robust and reliable capacitive sensors which notably are not subject to deterioration or alteration of their characteristics as a result of freezing-thawing cycles or after immersion.
Another object of the invention is to allow for production of flexible sensitive elements, adaptable to supports of skewed or curved shape.
An additional object of the invention is to allow for production of independent sensitive elements directly connectible to passive or active measurement circuits.
Another object of the invention is to provide sensors exhibiting manufacturing tolerances for the basic values of the order of 0.25%, allowing for interchangeability without recalibration with a metrological tolerance of the order of 1%.
An additional object of the invention is to provide capacitive sensors which exhibit retention of their performances and instantaneous desaturation after a saturation phase (100% R.H.) maintained for a long period.
The final object of the invention is to provide humidity sensors for which the performances(accuracy, robustness, reproducibility, reliability)/price ratio is enhanced very substantially as compared to existing technologics. This should allow access to mass production applications such as domestic appliances, air-conditioners, automobiles) or computer peripherals whilst maintaining high metrological performance and reliability.
All these objects, as well as others which will be apparent in the following, are achieved through a capacitive sensor for measuring humidity which includes two electrodes separated by a dielectric material, characterized in that one of the electrodes is formed by means of non-porous metal foil. Advantageously, the other electrode is formed by a porous any but metal material positioned as a thick layer and made conductive by inclusion of a plurality of electrically conductive particles. In reference to the prior art, it was seen above that if they were made of metal, these electrodes had to meet two criteria which may be difficultly compatible with one another; on the one hand, they should exhibit the lowest possible pressure drop (porosity) in order to bring the material forming the dielectric (polymer) into equilibrium with the surrounding gas; on the other hand, they should provide electric continuity. These results are only obtained by vacuum-depositing a metal in a thickness which is relatively large for providing electric continuity and sufficiently thin in order to avoid that the electrode behaves as a continuous film preventing any gas exchange, which would preclude use of the condenser as a humidity sensor. According to the present invention, this contradiction is resolved in that one of the electrodes is formed by a non-porous metal foil and the o
Larkin Daniel S.
Sughrue & Mion, PLLC
LandOfFree
Capacitive sensors for measuring humidity and method of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Capacitive sensors for measuring humidity and method of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Capacitive sensors for measuring humidity and method of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2880463