Moisture meter, humidity sensor

Wednesday, 17 January 2007

Moisture meters or humidity sensors are electrical instruments for determining the moisture content. Humidity or moisture is a universal parameter of common environments and its control is recognized to be very important in a variety of fields, such as industries related to precision manufacturing, fiber, food, and electronics industries. Moisture is undesirable whether it appears in houses, textiles, packaging materials, electronic appliances, chemicals, dry foodstuffs, etc. Our industrialized society produces increasingly refined products, many of which are highly sensitive to moisture. Knowing the moisture content of materials is often very important. For example, soil moisture is fundamentally important to activities in agriculture, forestry, hydrology, and civil engineering. The moisture content of grains and like material has a significant impact on the market value of the grain. If grains have too high of moisture content they will sell for less than those in an appropriate moisture range. Therefore, an accurate and precise means of testing moisture content in grain will help farmers monitor their crops. With the aid of monitoring, farmers may dry their grain until the preferred moisture content is achieved. Knowing the moisture content of soil would enable farmers to tailor their activities to achieve crop yield optimization. Soil moisture detection has been used for monitoring and evaluating construction sites, landscape sites, mining operations, forest areas, flood control areas, and farming districts. Moisture content measurements are important to sampling grain water content, field water content, and storage water content. Automatically detecting the presence of moisture on a surface has many applications. Windshield moisture can occur as rain, snow, ice, frost, fog, and the like on the windshield outer surface. Moisture may also occur as frost or fog on the windshield inner surface. The accumulation of moisture on transparent materials can obstruct a person's view through the material. Motor vehicles have long been equipped with motor-driven windshield wipers for clearing the moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance one's vision through the windshield. The ability to detect moisture on an automotive vehicle windshield frees the vehicle operator from the distraction of having to locale controls such as wipers and defoggers when driving conditions change. The penetration of moisture into hermetically sealed electronic packages or the permeation of moisture through plastic in non-hermetically sealed packages, especially in the presence of small amounts of ionic contaminants, is known to cause device or circuit failure. The failure mechanisms are generally moisture and ion-induced chemical corrosion of a layer of thin film metallization, of aluminum, nichrome, or copper for example. These impurities cause electrochemical phenomena such as metal migration that catastrophically bridge and short out closely spaced conductors. Another common and very important application of humidity sensor is in industrial or residential heating, ventilation and air conditioning (HVAC) systems. In such systems the humidity sensor provides a measure of air humidity in a controlled space or supply system, which is used by the overall HVAC control for managing the air handling equipment to maintain humidity within preselected ranges.

Because of the importance of knowing the moisture content of materials, various techniques have been developed to measure it. Three electronic techniques of measuring moisture are capacitive sensing, resistance sensing, and time domain reflectometry. The capacitance type humidity sensor is based on variation of dielectric constants by hygroscopic property of an organic material such as polyimide. A capacitance type humidity sensor detects humidity by measuring the change in the electrostatic capacity of an element corresponding to the ambient humidity. Capacitive moisture meters deliver a capacitance value in dependence on the air humidity in the area around the measuring element. That capacitance value can be measured by means of an electronic evaluation system. A current moisture value is ascertained with the measured capacitance value, using further parameters such as temperature and comparative parameters. Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time. Capacitive moisture sensors are typically made by depositing several layers of material on a substrate material. A resistance type humidity sensor detects relative humidity by measuring the change in the resistance of an element corresponding to the ambient humidity. Most of the resistance type humidity sensors include an electrolytic, polymeric, or metallic oxide sensor element. An electrolytic sensor element, which has become the most predominant type of humidity sensors, is made by forming a layer of moisture-lyzable electrolyte on an insulating moisture-absorbing substrate. The capacitance type humidity sensors typically do not exhibit a satisfactory linear relationship between the capacitance and humidity, and an external circuit is required to overcome this disadvantage. This increases the manufacturing cost of capacitance-change type humidity sensors. Thus, the resistance type humidity sensors, which generally exhibit a linear relationship between the resistance and humidity, appear to have been the preferred choice. Resistive moisture sensors are used for instance in order to report moisture or raindrops on the windshield of a motor vehicle so as to control the windshield wiper automatically or semiautomatically. For this purpose, conductive paths are applied to the windshield or to some other suitable place so that the resistance between the conductive paths decreases with the amount of moisture present. In time domain reflectometry electromagnetic pulses are impressed upon a transmission line that includes a sensor probe portion. That probe portion is comprised of at least two conductors that are imbedded in the soil whose moisture is to be sensed such that the soil is between the two conductors. Disturbances in the transmission line cause reflections of the impressed pulses. By monitoring the reflections from the input end of the sensor probe and the distal end of the sensor probe, and by knowing the length of the sensor probe, the velocity of the pulse along the sensor probe can be determined. Since that velocity depends on the permittivity of the soil between the sensor probe conductors, which in turn depends upon the moisture content of the soil, the soil moisture can be found determined.

Solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors, for example, are fabricated using semiconductor processes. Humidity sensors are but one class of semiconductor-based sensors finding many industrial applications. In a semiconductor-based system, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Polymeric films have been used as a humidity-sensing element. Polymer-based humidity-sensing elements can generally be classified into two categories: capacitance-type and impedance-type. The former typically involves more complicated circuit design and manufacturing process, and thus is more expensive, than the latter. An impedance-type electric humidity-sensing element changes its electrical impedance as the humidity of the surrounding environment changes, and the measured impedance is converted into humidity readings. One type of sensor uses a humidity-sensitive film to form a variable capacitor. The film has capacitor electrodes formed on either side thereof, by vacuum deposition of gold or the like. These conductive areas separated by the film form a capacitor, whose capacitance varies as a function of humidity. A capacitive moisture sensor, which detects humidity based on a change of capacitance between two detection electrodes provided on a semiconductor substrate, has two detection electrodes, which oppose each other, on a first insulation film formed on a surface of a semiconductor substrate. The detection electrodes are covered with a second insulation film and are further covered with a moisture sensitive film thereon. In addition, a reference portion, having a reference capacitance which does not change even when humidity changes, is provided on the semiconductor substrate. The detection electrodes and a circuit element portion including the reference portion construct a switched capacitor (SC) circuit which converts a change of capacitance between the detection electrodes to a voltage signal and outputs it. Accordingly, humidity can be detected based on a difference between the reference capacitance and a capacitance of the detection electrodes, which changes according to humidity.

Moisture meters exist today in various forms. Generally, a moisture meter includes a sensor section having a sensor element, an electronics section having electronics, and and a sensor readout. The electronics in the electronics section must be connectable to the sensor element in the sensor section. Humidity sensors of various types are well known in industry and are utilized in many applications wherein it is necessary to measure or control humidity. There are complex moisture meters used by agriculture and gardening professionals as part of a larger weather monitoring or irrigation system. Such moisture meters generally are used to record soil moisture along with a collection of other weather related data to detect trends to aid in making decisions affecting crop yield. Moisture meters are sometimes part of large scale irrigation systems used with golf courses or other large properties for the purpose of water management. Moisture sensors are often used in conjunction with sprinkler controls to regulate the flow of water in proportion to the watering needs of grass and plants. Automotive windshield moisture sensors have been introduced for automating the operation of windshield wipers. One such type of moisture sensor is based on electro-optical detection of raindrops by sensing changes in the total internal reflection of a light beam reflected off the front glass-air interface. An alternative method of sensing moisture on the windshield surface relies on the relatively large dielectric constant of water as it affects the capacitance between a set of conductive electrodes deposited on the windshield. Moisture sensors based on this method are integral with the windshield and are potentially less expensive and less conspicuous than optical moisture sensors. Moisture meters are generally placed at a measuring point and contain a sensor element which registers the desired process quantity and changes it into an electric value. The electric value is sent to an electronic device, which further processes and converts this information into an electric output signal, which makes possible a display and/or the regulation of the measurement result.

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