Some problems in selecting humidity sensor

Some problems in choosing humidity sensors

preface

human survival and social activities are closely related to humidity. With the realization of modernization, it is difficult to find a field that has nothing to do with humidity. Due to different application fields, the technical requirements for humidity sensors are also different. From the perspective of manufacturing, the same humidity sensor has different materials, structures and processes. Its performance and technical indicators are very different, so the price is also very different. For users, when choosing a humidity sensor, they should first figure out what kind of sensor they need; What grade of products can you choose with your own financial resources, and weigh the relationship between "need and possibility", so as not to act blindly. From our contacts with users, we feel that the following issues deserve attention

1. select the measurement range

the same as measuring weight and temperature. First, determine the measurement range when selecting the humidity sensor. Except for the meteorological and scientific research departments, those engaged in temperature and humidity measurement and control generally do not need full humidity range (%rh) measurement. In today's information age, sensor technology is closely combined with computer technology and automatic control technology. The purpose of measurement is to control, and the measurement range and control range are collectively referred to as the use range. Of course, for those who don't need a measurement and control system, it's OK to directly choose a universal hygrometer. The following lists the different requirements for temperature and humidity of humidity sensors in some application fields for users' reference (see Table 1). The medium and long-term energy technology development strategy and development plan to 2020 are formulated. Users propose the measurement range to the sensor manufacturer according to their needs, and the manufacturer gives priority to ensuring the stable and consistent performance of the sensor within the user's range of use, so as to obtain a reasonable performance price ratio, which is a mutually reinforcing thing for both parties

2. Select the measurement accuracy

as the measurement range, and the measurement accuracy is the most important indicator of the sensor. Every increase of one percentage point is a step up or even a higher level for the sensor. Because to achieve different accuracy, the manufacturing cost and selling price vary greatly. For example, a cheap imported humidity sensor is only a few dollars, while a full wet range humidity sensor for calibration costs hundreds of dollars, a difference of nearly a hundred times. Therefore, users must tailor their clothes and should not blindly pursue "high, fine and sharp"

manufacturers often give the accuracy of their humidity sensors in sections. For example, the medium and low temperature section (0-80% RH) is ± 2% RH, while the high humidity section (80-100% RH) is ± 4% RH. And this precision is the value at a specified temperature (such as 25 ℃). If the humidity sensor is used at different temperatures, the influence of temperature drift should also be considered in its indication. As we all know, relative humidity is a function of temperature, which seriously affects the relative humidity in a designated space. Every 0.1 ℃ change in temperature. Will produce a humidity change (error) of 0.5% RH. If it is difficult to achieve constant temperature in the application occasion, it is inappropriate to put forward too high humidity measurement accuracy. Because the humidity also fluctuates with the change of temperature, it will lose practical significance to talk about the accuracy of humidity measurement. Therefore, to control humidity, we must first control the temperature, which is the reason why temperature and humidity integrated sensors are often used in a large number of applications, rather than simply humidity sensors

in most cases, if there is no accurate temperature control means, or the measured space is not sealed, the accuracy of ± 5% RH is sufficient. For local spaces that require precise control of constant temperature and humidity, or where humidity changes need to be tracked and recorded at any time, humidity sensors with an accuracy of more than ± 3% RH shall be selected. For the corresponding temperature sensor, its temperature measurement accuracy must be more than ± 0.3 ℃, at least ± 0.5 ℃. The requirement of accuracy higher than ± 2% RH may be difficult for the standard humidity generator to calibrate the sensor, not to mention the sensor itself. According to the article of the humidity room of the national reference material research center, "it is still difficult for the relative humidity measuring instrument to achieve the accuracy of 2% RH even at 20-25 ℃."

3. Consider time drift and temperature drift

almost all sensors have time drift and temperature drift. Since the humidity sensor must be in contact with the water vapor in the atmosphere, it cannot be sealed. This determines that its stability and service life are limited. Generally, the manufacturer will indicate that the effective use time of a calibration is 1 year or 2 years, and will be responsible for recalibration when it expires. Please consider the channel of recalibration in the future when selecting the sensor, and don't be greedy for cheap or superstitious about foreign goods and ignore the after-sales service

temperature drift has been mentioned in the previous section. When selecting humidity sensor, the temperature variation range of the application shall be considered to see whether the selected sensor can work normally at the specified temperature and whether the temperature drift exceeds the design index. Users should be reminded that the capacitive humidity sensor can input the basic information of the experiment and the temperature coefficient of the sensor in the upper part of the page α Is a variable, which varies with the temperature and humidity range. This is because the permittivity of water and polymer can always provide us with reliable data with temperature, and the change of temperature coefficient is asynchronous α It mainly depends on the dielectric coefficient of water and humidity sensitive materials, so the temperature coefficient of capacitive humidity sensor is not constant. The temperature coefficient of capacitive humidity sensor is the smallest in the normal temperature and medium humidity sections. When the temperature is ℃, the temperature drift in the medium and low humidity sections can be ignored. However, when used in high temperature and high humidity areas or negative temperature and high humidity areas, the influence of temperature drift must be considered and necessary compensation or correction must be made

field Department temperature (℃) temperature (%rh)

textile spinning factory 23 60

textile factory 18 85

pharmaceutical factory 10 mainly includes cylindrical force sensor, spoke force sensor, s double hole sensor 10 beam sensor and other types ~ 30 50 ~ 60

operating room 23 ~ 26 50 ~ 60

light industry printing factory 23 ~ 27 49 ~ 51

cigarette factory 21 ~ 24 55 ~ 65

match factory 18 ~ 22 50

electronic semiconductor 2230 ~ 45

computer room 20 ~ 30 40 ~ 70

communication cable aeration -10 ~ 30 0 ~ 20

food and beer fermentation 4 ~ 8 50 ~ 70

agricultural seed cultivation 15 ~ 40 40 ～ 75

artificial greenhouse 5 ～ 404 0 ~ 100

Storage Fruit freezing-3 ~ 5 80 ~ 90

underground vegetable cellar-3 ~ -1 70 ~ 80

preservation of cultural relics 16 ~ 1850 ~ 55

note: the scope of use in different fields (%rh/℃)

4. The relationship with traditional humidity measurement methods

as early as the 18th century, humans invented dry and wet balls and hair hygrometers, while electronic humidity sensors have developed rapidly in recent decades, especially in the past 20 years. The alternation of old and new things is closely related to the change of people's ideas. As the price of dry and wet balls and hair hygrometers is still significantly lower than that of humidity sensors, some people do not recognize the price of electronic humidity sensors. It's just like when people who are used to brooms switch to vacuum cleaners, they always feel that it's not worth spending hundreds of yuan to buy a vacuum cleaner. It's easier to balance their psychology than spending a few yuan to buy a broom

because the traditional method of humidity measurement is too impressive in people's minds, some people have formed the inherent concept that only the wet and dry bulb hygrometer is accurate. Some users use wet and dry bulb hygrometers to compare the newly purchased humidity sensors. If they find that the indicated values are different, they immediately think that the humidity sensor is inaccurate. It should be noted that the accuracy of the dry and wet bulb is only 5% to 7% RH, which is not only lower than the electronic humidity sensor, but also depends on the accuracy of the dry bulb and wet bulb thermometers themselves; The hygrometer must be in a ventilated state: the specified accuracy can be achieved only when the gauze jacket, water quality and wind speed meet certain requirements. Humidity sensor manufacturers should use standard humidity generator to calibrate one by one before products leave the factory, and the shunt standard humidity generator is most commonly used for calibration. Therefore, users are expected to adopt the same method when they need calibration, and avoid using instruments with low accuracy to calibrate or compare sensors with high accuracy

5. Other precautions

humidity sensor is non sealing. In order to protect the accuracy and stability of measurement, it should be avoided to use it in acidic, alkaline and organic solvent containing atmosphere. Also avoid using it in dusty environment. In order to correctly reflect the humidity of the space to be measured, it is also necessary to avoid placing the sensor too close to the wall or at a dead corner where the air is not circulating. If the measured room is too large, multiple sensors should be placed

some humidity sensors have high requirements for power supply, otherwise the measurement accuracy will be affected, or the sensors will interfere with each other and even fail to work. When in use, provide appropriate power supply that meets the accuracy requirements according to the technical requirements

when the sensor needs long-distance signal transmission, pay attention to the attenuation of the signal. When the transmission distance is more than 200m, it is recommended to select the humidity sensor with frequency output signal

due to the certain dispersion of humidity sensors, both imported and domestic sensors need to be debugged and calibrated one by one. Most of them need to be re calibrated after replacing humidity sensors, especially for humidity sensors with high measurement accuracy