Several common classifications of temperature and humidity sensors

Several common classifications of temperature and humidity sensors

Human survival and social activities are closely related to humidity. With the development of modernization, it is difficult to find a field unrelated to humidity. Due to different application fields, the technical requirements of humidity sensors are also different. From the manufacturing point of view, the same humidity sensor has different material, structure and technology. Its performance and technical index (image accuracy) are very different, so the price is also very different. For users, when choosing humidity sensors, they should first figure out what kind of sensors they need; when their financial resources permit, they should choose what grade of products, and weigh the relationship between "need and possibility", so as not to act blindly. From our contacts with users, we collected several common classifications of humidity sensors.

Humidity sensor is the simplest humidity sensor. There are two kinds of humidity sensors: resistance type and capacitance type.

The characteristic of humidity sensitive resistor is that the substrate is covered with a film made of humidity sensitive material. When the water vapor in the air is adsorbed on the humidity sensing film, the resistivity and resistance value of the element will change. Humidity sensitive capacitors are usually made of polymer film capacitors. The commonly used polymer materials are polystyrene, polyimide, butyric acid acetate fiber, etc. When the ambient humidity changes,

When the dielectric constant of humidity sensitive capacitor changes, its capacitance also changes, and the capacitance change is proportional to the relative humidity. The accuracy of electronic humidity sensor can reach 2-3% RH, which is higher than that of wet and dry bulb. The linearity and anti pollution of humidity sensor are poor. When detecting the environmental humidity, the humidity sensor should be exposed to the environment for a long time, which is easy to be polluted and affect its measurement accuracy and long-term stability. This is not as good as the wet bulb method. The following is a brief introduction to various humidity sensors.

1. Lithium chloride humidity sensor

(1) Resistance type lithium chloride hygrometer

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

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

(2) Dew point lithium chloride hygrometer

Dew point lithium chloride hygrometer was first developed by forboro company in the United States, and then a lot of research work has been done in China and many countries. This kind of hygrometer is similar to the resistance lithium chloride hygrometer, but its working principle is completely different. In short, it works by using the saturated vapor pressure of lithium chloride saturated aqueous solution varying with temperature.

2. Carbon humidity sensor

Carbon humidity sensor was first put forward by E.K. Carver and C.W. breasefield in 1942. Compared with the commonly used radiosonde elements such as hair, casing and lithium chloride, the carbon humidity sensor has the advantages of fast response, good repeatability, no erosion effect and narrow hysteresis loop. In the early 1970s, the meteorological department of our country carried out the research and development of carbon humidity sensor, and achieved positive results. The uncertainty of measurement is less than ± 5% RH, the time constant is 2 ~ 3S at positive temperature, the hysteresis is generally about 7%, and the specific resistance stability is also good.

3. Alumina hygrometer

The outstanding advantages of alumina sensor are that the volume can be very small (for example, the humidity sensor used in radiosonde is only 90 μ m thick and 12 mg weight), high sensitivity (the lower limit of measurement is - 110 ℃ dew point), the response speed is fast (generally between 0.3 s and 3 s), the measurement signal is directly output in the form of electrical parameters, which greatly simplifies the data processing program, etc. In addition, it can also be used to measure moisture in liquid. The above characteristics are exactly what some measurement fields in industry and meteorology hope for. Therefore, it is considered to be one of several kinds of sensors that can be selected for high-altitude atmospheric detection. It is precisely because of these characteristics that people have a strong interest in this method. However, it is a pity that although the professionals in many countries have made unremitting efforts to improve the performance of sensors, they are still exploring the technological conditions for producing stable quality products and improving the performance stability

No major breakthrough has been made. Therefore, up to now, sensors can only be used in specific conditions and limited range. In recent years, this method has been used in the measurement of low frost point in industry.

4. Ceramic humidity sensor

In the field of humidity measurement, the measurement of low humidity, high humidity and low temperature and high temperature is still a weak link, and the humidity measurement technology under high temperature is the most backward. In the past, the ventilated dry wet bulb hygrometer is almost the only method that can be used under this temperature condition. However, there are various problems in the actual use of this method, which is not satisfactory