RenhotecIC Humidity Sensor Knowledge

 

  The hygroscopic substance that can be used to manufacture the humidity sensor of the dehumidifier must meet the basic condition that the humidity-resistance (or capacitance) characteristic is reversible, and should have good repeatability. Humidity-sensitive elements made of these substances are equipped with appropriate circuits to form corresponding humidity measuring instruments. People usually call hygrometers based on measuring the resistance or capacitance changes of hygroscopic substances as humidity sensors.

 

  Humidity sensors for dehumidifiers mainly include lithium chloride humidity sensors, alumina humidity sensors, carbon and ceramic humidity sensors, and those made of polymer films and various inorganic compound crystals, such as lithium niobate, cadmium sulfide, and sodium chloride. Resistive humidity sensor, etc. The following is a brief introduction to various humidity sensors.

 

1. Lithium chloride humidity sensor

(1) Resistive lithium chloride hygrometer

 

The first lithium chloride electric humidity sensor based on the principle of resistance-humidity characteristics was developed by F.W.Dunmore of the American Bureau of Standards. This kind of element has a series of advantages such as high precision, simple structure, low price, and suitable for measurement and control of normal temperature and humidity.

 

The measurement range of the lithium chloride element is related to the lithium chloride concentration and other components of the humidity sensitive layer. The effective humidity range of a single element is generally within 20% RH. For example, the humidity range corresponding to the concentration of 0.05% is about (80-100)%RH, and the corresponding range of the concentration of 0.2% is (60-80)%RH, etc. It can be seen that to measure a wide range of humidity, components with different concentrations must be used in combination. The number of components of the hygrometer combination that can be used for full-scale measurement is generally 5. The measurable range of the lithium chloride hygrometer using the component combination method is usually (15-100)%RH. Some foreign products claim that the measurement range can reach ( 2 to 100) %RH.

 

(1) Dew point lithium chloride hygrometer

 

The dew point lithium chloride hygrometer was first developed by the Forboro company in the United States, and a lot of research work has been done in my country and many other countries. This type of hygrometer is similar in form to the above-mentioned resistive lithium chloride hygrometer, but the working principle is completely different. In short, it works by using the saturated water vapor pressure of lithium chloride saturated aqueous solution as the temperature changes.

 

2. Carbon humidity sensor

          The carbon humidity sensor was first proposed by E.K.Carver and C.W.Breasefield in the United States in 1942. Compared with the commonly used sounding elements such as hair, casing and lithium chloride, the carbon humidity sensor has fast response, good repeatability, The advantages of no erosion effect and narrow hysteresis loop are therefore attractive. In the early 1970s, my country's meteorological department carried out the research and development of carbon and humidity sensitive elements, and achieved positive results. The measurement uncertainty is not more than ±5%RH, the time constant is 2 to 3s at positive temperature, and the hysteresis is generally 7 % or so, the specific resistance stability is also good.

 

3. Alumina hygrometer

        The outstanding advantages of the alumina sensor are that the volume can be very small (for example, the humidity sensor used in the sonde is only 90 μm thick and 12 mg in weight), the sensitivity is high (the lower limit of measurement can reach -110 °C dew point), and the response speed is fast (generally at 0.3 s to 3s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing procedure, and so on. In addition, it is also suitable for measuring moisture in liquids. The above characteristics are desired in some measurement fields in industry and meteorology. Therefore it is considered to be one of several acceptable sensors for upper atmosphere detection. It is precisely because of these characteristics that people have a strong interest in this method. However, it is a pity that although professionals in many countries have made unremitting efforts to improve the performance of sensors, they have not been able to explore the process conditions for producing products with stable quality and improve the stability of performance and other important issues related to practical use. Make a major breakthrough. So far, sensors have generally only been used under specific conditions and within a limited range. In recent years, this method has come to prominence in industry for low frost point measurements.

 

4. Ceramic humidity sensor

           In the field of humidity measurement, low humidity and high humidity and their measurement under low temperature and high temperature conditions are still a weak link so far, and the humidity measurement technology under high temperature conditions is the most backward. In the past, the ventilated psychrometer was almost the only method that could be used under this temperature condition, and this method also had various problems in actual use, which was unsatisfactory. On the other hand, with the advancement of science and technology, there are more and more occasions that require the measurement of humidity at high temperatures, such as the humidity measurement and control of many industrial processes involving process conditions and quality control, such as cement, metal smelting, and food processing. Therefore, since the 1960s, many countries have begun to develop humidity sensors suitable for measurement under high temperature conditions.

 

         Considering the conditions of use of the sensor, it is natural for people to focus on certain inorganic substances that are both water-absorbing and high-temperature resistant. Practice has proved that ceramic elements not only have moisture-sensitive properties, but also can be used as temperature-sensing elements and gas-sensing elements. These characteristics make it very likely to become a promising multifunctional sensor. Terahi, Fukushima, Nitta and others have taken a fairly successful step in this regard. They developed multifunctional sensors called "Wet Porcelain-Type II" and "Wet Porcelain-Type III" in 1980.

 

         The former can measure and control temperature and humidity, and is mainly used in air conditioners, while the latter can be used to measure humidity and various organic vapors such as alcohol, and is mainly used in food processing.

 

         The above types are several types of sensors that are widely used. In addition, there are other humidity sensors developed according to different principles, which will not be introduced here.

 

Humidity sensors are widely used in the field of dehumidifiers, and the accuracy of the humidity sensors of dehumidifiers has become a topic of most concern to the people.