Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Patent
1991-08-22
1994-02-01
Reynolds, Bruce A.
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
324689, 7333504, H01G 520, H01G 528
Patent
active
052837114
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a capacitive humidity sensor comprising a substrate being a flexible copper laminate, a moisture-impermeable conducting bottom layer etched in said copper laminate as first capacitor plate, a dielectric layer of which the dielectric constant is a function of moisture absorption, and a moisture-permeable conducting top layer as second capacitor plate, said bottom layer and a contact layer making contact with the moisture-permeable top layer and being deposited adjacent each other and isolated from each other on the substrate, the top layer extending both over the dielectric layer and over the contact layer. Such a humidity sensor is disclosed in European patent application 0094266.
In such a sensor, which in fact consists of an electrical capacitor, the humidity is determined by measuring the change in dielectric constant of the dielectric medium, and therefore the change in capacitance value, due to the absorption of water molecules by the dielectric material. If, for example, the material of a dry dielectric layer has a dielectric constant of 3.5 and water approximately has one of 80, it will be clear that the change in capacitance value due to moisture absorption may be appreciable. In order that moisture or water can be absorbed by the dielectric layer, the top layer has to be moisture-permeable.
This known sensor has a substrate on which a copper layer as first capacitor plate is deposited. A moisture-impermeable insulating layer is provided on top of the first capacitor plate. Said insulating layer serves as a barrier layer and prevents resistance loss across the dielectric medium at high relative humidity, or even short circuiting between the plates regardless of the structure of the dielectric medium. The dielectric layer mentioned is sufficiently porous to absorb moisture.
Now in practice it appears that the use of such a sensor is strictly dependent on well-defined precalibration techniques under dry and moist atmospheric conditions before using it for the consumer market and for industrial application.
The object of the invention is to eliminate the above mentioned problem and to provide a cheap sensor which is most robust and which can be easily calibrated after manufacture.
SUMMARY OF THE INVENTION
According to the invention this is achieved in a capacitive humidity sensor of the type mentioned above in that the top layer comprises a solid printed grid pattern of conducting ink, and a row of small conducting ink areas at at least one side of the grid pattern.
In an advantageous embodiment of the above sensor at least between the bottom layer and the dielectric layer a moisture-impermeable insulating layer or barrier layer is provided, whereby in one case the row of small ink areas is provided above the insulating layer and in another case two rows of small ink areas on either side are provided of which one is above the insulating layer and the other is above the dielectric layer.
In this embodiment according to the invention, an exceptionally cheap sensor for the consumer market is obtained which is also suitable for industrial applications. This sensor according to the invention can be calibrated simply, both under dry and moist atmospheric conditions. For calibration in the case of a dry reference environment, several of the small areas of the row above the barrier layer are connected to the main grid by means of small pads of conducting ink in order to obtain the required fixed (dry) capacitance value. For further calibration in the case of a humid reference environment, small areas of the row above the dielectric or active layer are connected to the main grid in order to adjust the required delta capacitance value. For this calibration, it is assumed that it is easier to add than to subtract a capacitance value. Such a subtraction may, however, also be achieved by laser trimming. After trimming, the conducting ink tracks can be hardened. The advantages of this "printing" method are that the exact printing geometry can be defined and t
REFERENCES:
patent: 4438480 (1984-03-01), Chambaz et al.
patent: 4564882 (1986-01-01), Baxter et al.
Flucon B.V.
Reynolds Bruce A.
Switzer Michael D.
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