Dew point hygrometers and dew sensors

ES 2 270 618 T3

DESCRIPTION

Dew point hygrometers and humidity sensors.

Field of the invention

The present invention concerns dew point hygrometers for determining the dew point of gas and dew sensors for determining neutral dew condensation. More specifically the present invention concerns hygrometers based on capacitance change.

Background of the invention

The hygrometers of the state of the art used in agriculture, especially in greenhouses, normally operate according to one of the following three types. The first is a psychrometer that requires a very high degree of maintenance. The second is a relative humidity meter whose principle of operation is based on capacity change, which is very troublesome to determine relatively high humidity. Inexpensive instruments based on capacitance measurement are unsafe since the reading changes over time regardless of changes in the moisture content of the air. The third type of sensors are optical mirror-based dew point hygrometers, which are very expensive and unsuitable for routine work in greenhouse conditions since they require constant cleaning of the mirror surface.

Another device for detecting humidity is a dew sensor (as opposed to a dew point hygrometer). This sensor in fact mimics moisture condensation on a natural surface such as leaves, without sensor temperature control, and thus is in fact very reliable since it directly mimics the natural process of condensation. Several measurements have shown a good correlation between leaf temperature and dew sensor temperature at night. Thus, such a sensor is suitable for use in detecting water condensation on various surfaces, such as leaves in greenhouses, warning when an air humidity is too high, a situation that occurs for example at night and that is a cause of many plant diseases. Such a warning can trigger various drying mechanisms to lower the humidity in the greenhouse again.

In fact, at night the dew sensor is more suitable to use than dew point hygrometers in greenhouses, since it reflects the real and natural situation of condensation on the surfaces, which is a better indicator of the condition of the leaves than the moisture content detected by dew point hygrometers.

Conventional mist sensors such as those used in greenhouses typically employ a separate pair of electrical wires, between which the resistance drops from about 19 megohms to 3 megohms when the dew bridges the two wires. But such devices are electrically noisy and their sensitivity can change depending on the precipitation of salts on the sensor. Another problem with such devices is that they generally require high tension and the exposure of conductors and guides whose characteristics make them vulnerable to corrosion over time. Corrosion is especially rapid in the presence of high voltage used in the 1 to 20 volt range.

US Patent 4,948,263 refers to a dew point sensor for a dew point measuring device for measuring the dew point of water vapor in gases, which comprise a sensor surface that is exposed to the gas that must be measured and on which, by cooling, the water vapor condenses at the dew point temperature. Mounted on the surface of the sensor are two electrode structures comprising electrode parts which are arranged in a uniform interval parallel to each other and which are covered with a moisture insensitive insulating sheath. The dew point temperature range is determined by measuring the impedance or capacitance between the two electrode structures. The distance between the parts of the electrode, arranged parallel to each other, of the two electrode structures is of the order of magnitude of the diameter of the largest condensation droplet that is formed when reaching the dew point temperature, or smaller than said diameter, and the thickness of the insulating sheath is small compared to the distance between the parts of the electrode.

US Patent 4,626,774 relates to a dew point measuring instrument having a capacitive dew point sensor which is cooled by a cooling device to the dew point temperature measured by a temperature sensor. A phase measurement circuit measures the phase angle of the impedance of the capacitive dew point sensor. The measured phase angle is used as a gauge for dew point sensor contamination.

US Patent 5,402,075 refers to a capacitive humidity sensor that includes insulating means; capacitance means including a sensing capacitor with a plurality of separate capacitive sensor leads mounted with the insulating means for exposure to the atmosphere; and first and second electrodes mounted with insulating means away from the separate capacitive sensor leads; means for applying a periodic input current through the first and second electrodes; and means for detecting a change in capacitance between the first and second electrodes indicative of moisture bridging in at least two of the conductors of the capacitive sensor.

WO 96/05506 describes a dew point sensor, comprising a miniaturized capacitor with isolated electrodes on the surface of a chip, on which water is condensed between the capacitor plates.

Summary of the invention

In a first embodiment of the invention called the "capacitive dew sensor embodiment" the present invention concerns a capacitive dew sensor, where the problem of solid contaminants (salts, dust, etc.) deposited on the surface is significantly reduced. outer surface of the sensor and corrosion of the electrode using a layer of water as one of the capacitor electrodes.

The sensor includes two electrode structures separated from each other by an insulator and cables, connecting each of the electrodes to a measurement circuit. The first structure of the electrode is a conductive plate mounted on a surface of the insulator. The first electrode is isolated from the ambient atmosphere

ES 2 270 618 T3 by a coating that does not allow the penetration of water and electrolytes through it. An example of such a coating is lacquer. The second structure of the electrode in fact is not formed, a priori, in the sensor, but is formed when a layer of water (including some electrolytes that are naturally present in the atmosphere) condenses or precipitates on the external exposed surface. of the insulator (that is, the surface that is not in contact with said first electrode). A measurement circuit is connected through wires to apply a current to both the first and second electrode structures to detect a change in capacitance between them.

According to said first embodiment, it is preferable that the external exposed surface of the insulator on which the second electrode is formed, (which is the water layer), is rough, to accelerate the appearance of dew condensation and to increase the sensitivity. sensor. Furthermore, said rough surface eliminates the influence of contamination by salts on the sensitivity of the sensor, since only an essentially continuous layer of water (not single water droplets that do not constitute a continuous layer) forms the second structure of the electrode.

The sensitivity of the dew sensor can be changed by varying the roughness of the external insulating surface, on which the water condenses. For example, different peelable insulators with different roughness can be used.

The sensitivity of the dew sensor can also be changed by changing the infrared emission of the insulating outer surface, for example by changing the material, color or roughness of the surface.

The influence of contamination by salts can be eliminated using a network (made of conductive or insulating material) The measurement circuit measures the resistance between two or more leads that connect the circuit to the second and the first electrodes, when the stratum of water forming the first electrode. The resistance is used as a gauge for contamination of the sensor by salts.

A printed circuit board, for example 0.5 mm thick and 59 X 54 mm area, can be used for the realization of the sensor.

The present invention will now be described in more detail with reference to some non-limiting drawings and examples.

Brief description of the drawings

Fig. 1 shows a capacitive dew sensor according to the first embodiment of the invention; Y

Fig. 2 shows a schematic representation of the electrical circuit of Fig. 1.

Detailed description of the invention

Fig. 1 shows a capacitive dew sensor 106 according to the first embodiment of the invention. The sensor includes an insulator 107, a first structure of the electrode 108 isolated from the ambient atmosphere by a protective layer such as lacquer, which protective layer does not allow moisture and electrolytes to penetrate through it. The sensor comprises a second electrode structure 109 formed by a layer of water that condenses on the external exposed surface of the insulator 107. Water is conductive due to the fact that it contains salts that are naturally present in air. The sensor is connected to the measurement circuit (not shown) by wires 110 and 111. When water is present as shown in Fig. 1, the electrode 109 is formed, and as a result the capacitance changes slightly from 0.1 to 160 picofarads and said change in capacitance can be measured. Alternatively, it is possible to add a net covering the outer surfaces of the insulator 107 to reduce the effect of airborne salts on the sensitivity of the sensor.

Fig. 2 illustrates the schematic equivalent circuit of the capacitive sensor created by the first electrode structure 108 'with constant area and the second electrode structure 109' created by the water stratum, with variable area where 110 'and 111' are the connection of cables to the measurement circuit (not shown).