After entering 2 1 century, especially after China's entry into WTO, domestic products are facing great challenges. All walks of life, especially traditional industries, urgently need to apply electronic technology and automatic control technology for transformation and upgrading. For example, in the textile industry, temperature and humidity are important factors affecting the quality of textiles, but the methods of temperature and humidity measurement and control in textile enterprises are still very rough and backward. Most of them still use wet-bulb hygrometers and dry-bulb hygrometers, and manually observe and adjust valves and fans. The control effect can be imagined. This is basically the case in the pharmaceutical industry. In the food industry, based on experience, few people use humidity sensors. It is worth mentioning that with the development of agriculture to industrialization, many farmers realize that it is necessary to get rid of backward traditional farming and breeding methods and adopt modern science and technology to meet the challenge of imported agricultural products and enter foreign markets. More and more greenhouses have been built around the country to plant anti-season vegetables and flowers; It is more and more urgent for aquaculture to monitor the environment; A large number of temperature-regulating cold storage buildings provide a broad market for temperature and humidity measurement and control technology. China has introduced more than 40 advanced large-scale greenhouses from the Netherlands, Israel and other countries, with high degree of automation and high cost. China is gradually digesting and absorbing related technologies, generally starting with temperature regulation, lighting regulation and ventilation control; The second step is automatic temperature and humidity control and CO2 measurement and control. In addition, a large number of national grain reserve projects put forward requirements for temperature and humidity measurement and control technology.

However, at present, in the field of humidity testing, the performance of most humidity sensors can only be used in the usual temperature environment. Most domestic humidity sensors, including many foreign humidity sensors, will frown in applications that need to measure humidity in special environments! For example, the above textile printing and dyeing industry, food industry, high temperature resistant material industry, etc. It is necessary to measure humidity at high temperature. In general, the spindle drying temperature in printing and dyeing industry can reach 120 degrees Celsius or above; In the food industry, the baking temperature of food can reach about 80-200 degrees Celsius; High temperature resistant materials, such as ceramics, can be filtered and dried to above 200 degrees Celsius. In these cases, ordinary humidity sensors are difficult to measure.

Polymer capacitive humidity sensors are usually made on insulating substrates made of glass, ceramics, silicon and other materials by screen printing or vacuum coating, and then humidity-sensitive glue is coated on electrodes by dipping or other methods to make capacitive elements. In the atmospheric environment with different relative humidity, the capacitance of the humidity sensor changes regularly because the humidity sensitive film absorbs water molecules, which is the basic mechanism of the humidity sensor. The temperature characteristics of polymer capacitor elements are affected by temperature, not only the dielectric constant ε of polymer as medium and the dielectric constant ε of adsorbed water molecules, but also the geometric size of the elements is affected by the thermal expansion coefficient. According to Debye's theory, the dielectric constant ε of liquid is a dimensionless constant related to temperature and frequency. The ε of water molecule is 78.36 at t = 5℃ and 79.63 at t = 20℃. The relationship between organic matter ε and temperature varies from material to material, and does not completely follow the proportional relationship. In some temperature regions, ε increases with the increase of T, while in some temperature regions, ε decreases with the increase of T.. In the analysis of humidity-sensitive mechanism of polymer humidity-sensitive capacitive elements, most documents think that the dielectric constant of polymer is small, for example, the dielectric constant of polyimide is 3.0-3.8 at low humidity. While that dielectric constant of wat molecules is ten of times that of polymer ε. Therefore, the dielectric constant of the water-absorbing heterogeneous layer after moisture absorption is greatly improved due to the dipole distance of water molecules, which is determined by the additivity of the composite dielectric constant of multiphase media. Due to the change of ε, the capacitance c of humidity-sensitive capacitive element is proportional to the relative humidity. It is difficult to establish the full humidity range linearity of humidity sensing characteristics in the design and manufacturing process. As a capacitor, the thickness d of polymer dielectric film and the effective area s of flat capacitor are also related to temperature. The change of medium geometry caused by temperature change will affect C value. The average thermal expansion coefficient of polymers can reach orders of magnitude. For example, the average thermal expansion coefficient of nitrocellulose is 108x 10-5/℃. With the increase of temperature, the thickness d of dielectric film increases, which has a negative contribution to c; However, the expansion of the humidity-sensitive film increases the adsorption of water by the medium, which is a positive contribution to C. It can be seen that the temperature characteristics of the humidity-sensitive capacitor are dominated by many factors, and the temperature drift is different in different humidity ranges. It has different temperature coefficients in different temperature regions; Different humidity-sensitive materials have different temperature characteristics. In a word, the temperature coefficient of polymer humidity sensor is not a constant, but a variable. Therefore, in general, the sensor manufacturer can linearize the sensor in the range of-10-60 degrees Celsius to reduce the influence of temperature on the humidity sensor.

The high-quality products of foreign manufacturers mainly use polyamide resin. The product structure is summarized as follows: vacuum evaporation of gold-plated electrodes on borosilicate glass or sapphire substrate, then spraying a planar humidity-sensitive film in the form of humidity-sensitive dielectric material (as mentioned above), and then evaporation of gold-plated electrodes on the film. The capacitance of the humidity sensor is proportional to the relative humidity, and the linearity is about 2%. Although the humidity measurement performance is ok, the temperature resistance and corrosion resistance are not ideal. In the industrial field, the service life, temperature resistance, stability and corrosion resistance need to be further improved.

Ceramic humidity sensor is a new type of sensor developed vigorously in recent years. The advantages are high temperature resistance, humidity lag, fast response speed, small volume and convenience for mass production. However, due to the porous material, which has a great influence on dust and frequent daily maintenance, it often needs to be cleaned by electric heating, which easily affects the product quality and humidity. The poor linearity in low humidity and high temperature environment, especially the short service life and poor long-term reliability, is an urgent problem for this kind of humidity sensor.

At present, in the development and research of humidity sensor, resistance humidity sensor should be the most suitable for humidity control. Its representative product, lithium chloride humidity sensor, has many important advantages such as stability, temperature resistance and long service life. Lithium chloride humidity sensor has more than 50 years of production and research history, and there are many product types and manufacturing methods, all of which apply the advantages of lithium chloride humidity sensitive liquid, especially the strongest stability.

Lithium chloride humidity sensitive device belongs to electrolyte humidity sensitive material. Among many humidity-sensitive materials, lithium chloride electrolyte humidity-sensitive liquid first attracted people's attention and was used to manufacture humidity-sensitive devices. The equivalent conductance of lithium chloride electrolyte humidity-sensitive liquid decreases with the increase of solution concentration. Electrolyte is dissolved in water to reduce the water vapor pressure on the water surface.

The substrate structure of lithium chloride humidity sensor is divided into columnar and dressing-like, and the humidity-sensitive liquid and gold electrode with lithium chloride polyvinyl alcohol coating as the main components are the three components of lithium chloride humidity sensor. Over the years, product manufacturing has been continuously improved and product performance has been continuously improved. The unique long-term stability of lithium chloride humidity sensor is irreplaceable by other humidity sensitive materials, and it is also the most important performance of humidity sensor. In the process of product production, the preparation of humidity-sensitive mixture and strict control of process are the keys to maintain and exert this characteristic.

In China, JUCSAN is engaged in the research and production of temperature and humidity sensor products relying on large scientific research units such as National Institute of Metrology, Institute of Automation of Chinese Academy of Sciences and Institute of Chemical Engineering. Choose lithium chloride humidity-sensitive materials as the main direction to produce lithium chloride humidity-sensitive sensors and related transmitters, automatic instruments and other products. While absorbing the successful experience of this technology at home and abroad, we strive to overcome the weaknesses of traditional products and make substantial progress. The product uses Al2O3 and SiO2 ceramic substrates as substrates, which greatly reduces the substrate area, and adopts special technology to greatly improve the moisture resistance and adhesion. Five dressing electrodes made of 9% industrial pure gold were sintered on the substrate by sintering process. The humidity-sensitive mixed solution of lithium chloride is mixed with new product additives and inherent components. After special aging and coating process, the service life and long-term stability of humidity-sensitive substrate have been greatly improved, especially the temperature resistance has reached -40℃- 120℃. The unique process of combining various humidity sensors is adopted. The humidity sensitivity range of the sensor is 1%RH-98%RH, and the measurement performance is below 15%RH. Both the drift curve and the humidity sensing curve have reached a good linearization level, which makes humidity compensation easy to realize and easy to ensure the humidity measurement accuracy in a wide temperature range. The closed system of circulating cooling device is adopted, and the measured gas is sampled first, and then cooled to ensure the absolute humidity is constant, so that the temperature tolerance range of the probe is increased to about 600℃, which greatly enhances the humidity measurement function at high temperature. Successfully solved the problem of "high temperature and high humidity measurement" in the field of humidity measurement. At present, JCJ200W, a separate high-temperature temperature and humidity sensor, which can directly measure the environmental humidity in the range of 150 degrees without using any devices, has been successfully applied to wood drying, high-low temperature test chambers and other systems. At the same time, JCJ200Y products can withstand high temperatures as high as 600 degrees, and have been successfully applied to spindle automatic drying system, food automatic baking system, special ceramic material automatic drying system, export large-scale drying machinery, etc. Good results have been achieved, which fills the gap of high temperature and high humidity measurement in the field of automatic control in China and lays a certain foundation for the industrialization process in China.