GB2034896A

GB2034896A - A moisture sensor

Info

Publication number: GB2034896A
Application number: GB7934452A
Authority: GB
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1978-10-12
Filing date: 1979-10-04
Publication date: 1980-06-11
Also published as: NL7907316A; SE7908187L; IT7926432A0; DE2844443A1; FR2438838A1; IT1123821B

Abstract

A moisture sensor, especially for the determination of the moisture content of the drying air of a laundry drier comprises at least two discrete electrical conductor tracks (4, 5), which are arranged on an insulating material carrier (6) and coated by a moisture-absorbing covering layer (8) having a variable electrical resistance. <IMAGE>

Description

SPECIFICATION Moisture sensor The present invention relates to a moisture sensor.

DE-AS 11,225,1 13 disclosesdiscloses a moisture- sensitive resistance for determining air moisture, which comprises a temperature-sensitive resistor, which is exposed to a moist air stream and which by an additional heater is preheated to a higher temperature for example above the maximum drying air temperature of a hot air drier. For the attainment of a high measuring sensitivity, it is possible to compensate for the effect of the temperature of the moist air on the moisturedependent resistance element by the use of a further unheated, temperature-sensitive resistance element disposed in a bridge branch. In case of hot conductors with weekly inclined characteristic, so-called cold conductors may be used and these are heatable to the proximity of their Curie point.

By altering the moisture content and therewith the cooling effect of the exhaust air stream, the Curie point is in operation exceeded or fallen below and a switching process is thereby initiated. These independent switching-off devices require relatively expensive components and circuits.

Journal "Strompraxis" 1976, volume 6, page 18 discloses an electronic moisture measurement apparatus in a laundry drier, which is set to the conductivity of the laundry. Drying programmes "cupboard-dry, ironing-moist and airing" are preselectable through a selector switch and the desired residual moisture is preselectable through an additional switch. The laundry during the circulation touches two electrodes, between which a direct voltage is present. The one electrode forms the drum while the other electrode protrudes through the filling opening into the drum. In accordance with the water content of the laundry, the current circuit between the electrodes is closed.Arising signals are amplified by means of electronic components and transmitted to a switching device which stores the measurement signals in a self-holding circuit for a certain time in order to avoid too early a switching-off of the heater and of the drum drive.

Disadvantages of this measuring equipment are an unfavourable electrical connection with the electrodes coming into contact with the laundry and the circumstance that the conductance measurment with decreasing moisture of the laundry is set limits by the then required highresistance components.

According to the present invention there is provided a moisture sensor comprising a carrier of electrically insulating material, at least two discrete electrically conductive tracks disposed on the carrier, and a moisture-absorbing layer covering the tracks to provide an electrical path of variable resistance therebetween.

The moisture sensor may comprise a corrosionresistant layer covering the tracks and disposed between the tracks and the moisture absorbing layer.

The tracks may be plated with a noble metal such as gold.

The tracks may comprise portions which intercomb with each other.

The moisture-absorbing layer may comprise a water-insoluble acrylic dye and a hardening material.

The carrier may comprise a ceramic plate member.

The carrier may comprise glass fibre impregnated with an epoxy resin.

The tracks may be disposed on each major face of the carrier.

The moisture sensor may be used to determine the moisture content of air conducted from a laundry drier, the moisture sensor being disposed with a further variable-resistance moisture sensor in a bridge circuit for comparison of the resistance of the moisture sensor with the resistance of the further moisture sensor.

On the use of a moisture sensor embodying the present invention in a laundry drier, a drying programme may be started after preselection of a desired residual moisture through a selector switch of the laundry drier. Thereupon, the switched-on heater starts to heat up the moist laundry and in that case to evaporate water. A timing member of a circuit device inhibits the moisture control during this starting phase until the drying air issuing from the drum has attained a high degree of moisture and it is ascertained that the moisture-sensitive sensor has attained a resistance value which is smaller than the resistance value corresponding to the set residual moisture.The inertia of the moisture sensitive sensor can in that case be small when the covering layer receiving the water molecules bridges over the spacings of the conductor track configurations and the spacings of the conductor tracks are designed to be small. The resistance value of the moisture-sensitive sensor becomes less with increasing air moisture of the drying air and its resistance value again rises with reducing air moisture until it becomes of such a high resistance with air moisture approaching zero that it attains a resistance value which for example, corresponds to the degree of drying "cupboarddry".

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings in which: Fig. 1 shows a moisture sensor, in plan view with a partially removed covering layer, Fig. 2 shows a schematic sectional illustration through the layers of the moisture sensor, and Fig. 3 shows a circuit diagram for the use of the moisture sensor in a drier control, by means of a bridge circuit.

A moisture sensor 1, schematically illustrated in the Figs. 1 and 2 has two connecting tags 2 and 3 at the ends of two discrete conductor tracks 4 and 5, which for example intermesh in comblike manner. The tracks 4 and 5 are disposed on a plate-shaped insulating material carrier 6 of ceramic of synthetic material. For corrosion protection, the conductor tracks 4 and 5 are rhodium-plate or carry a conductive layer 7 of noble metal such as for example gold. Provided as a covering of the conductor tracks 4 and 5 is a water-absorbing covering layer 8, the conductivity of which with increasing degree of moisture likewise increases and bridges over the conductor track spacings with a resistance value dependent on the degree of moisture.The covering layer 8 for example comprises a fatique-resistant, transparent acrylic dye with the addition of a hardener, which absorbs water without dissolving.

In use of the moisture sensor in a laundry drier, one moisture-sensitive sensor 1 acted on by moist drying air issuing from for example a drum of the drier and one moisture-insensitive fixed resistor 9 are arranged in a bridge circuit with resistors 10 and 11, wherein for example the one resistor 10 can form a balancing resistance, while the other resistor 1 likewise formed by a moisturesensitive sensor, detects the room air moisture as comparison with the sensor 1. A voltage source 13 supplies the bridge voltage. A timing member 14, which in the starting phase of the drying process inhibits the control voltage of the bridge, is connected to the bridge circuit. An in itself known amplifier circuit connected behind the timing member 14 and a discriminator 15 or window discriminator serves for the comparison of target and actual values of the moisture-sensitive sensor 1 representing the degree of moisture of the drying air by its resistance value relative to the moisture-sensitive sensor 1 1, which detects the degree of moisture of the room air and states the desired degree of drying of laundry. The voltage of the discriminator 15 is transmitted through a relay 12, a triac, thyristor or the like into the heavy current circuit for the termination of the drying process.

Claims

1. A moisture sensor comprising a carrier of electrically insulating material, at least two discrete electrically conductive tracks disposed on the carrier, and a moisture-absorbing layer covering the tracks to provide an electrical path of variable resistance therebetween.

2. A moisture sensor as claimed in claim 1, comprising a corrosion-resistant layer covering the tracks and disposed between the tracks and the moisture absorbing layer.

3. A moisture sensor as claimed in either claim 1 or claim 2, wherein the tracks are plated with a noble metal.

4. A moisture sensor as claimed in claim 3, wherein the tracks are plated with gold.

5. A moisture sensor as claimed in any one of the preceding claims, wherein the tracks comprise portions which intercomb with each other.

6. A moisture sensor as claimed in any one of the preceding claims, wherein the moistureabsorbing layer comprises a water-insoluble acrylic dye and a hardening material.

7. A moisture sensor as claimed in any one of the preceding claims, wherein the carrier comprises a ceramic plate member.

8. A moisture sensor as claimed in any one of claims 1 to 6, wherein the carrier comprises glass fibre impregnated with an epoxy resin.

9. A moisture sensor as claimed in any one of the preceding claims, wherein the tracks are disposed on each major face of the carrier.

10. A moisture sensor as claimed in any one of the preceding claims, when used to determine the moisture content of air conducted from a laundry drier, the moisture sensor being disposed with a further variable-resistance moisture sensor in a bridge circuit for comparison of the resistance of the moisture sensor with the resistance of the further moisture sensor.

1 1. A moisture sensor substantially as hereinbefore described with reference to the accompanying drawings.