1. Select the same measuring range as measuring weight and temperature. When choosing a humidity sensor, we must first determine the measuring range. Except for meteorological and scientific research departments, temperature and humidity measurement and control generally do not need full humidity range (0- 100%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 users who don't need to engage in measurement and control system, it is enough to choose universal hygrometer directly. The following is a list of different requirements for temperature and humidity of humidity sensors in some application fields for users' reference (see table 1). The user puts forward the measurement range to the sensor manufacturer according to the needs, and the manufacturer gives priority to ensuring the stability and consistency of the sensor performance within the user's use range, and obtains a reasonable cost performance, which is complementary to both parties.
2. The measurement accuracy and measurement range are the same, and the measurement accuracy is the most important index of the sensor. Every percentage increase is a step or even a grade for the sensor. Because to achieve different accuracy, its manufacturing cost is very different, and its selling price is also very different. For example, the imported 1 cheap humidity sensor costs only a few dollars, while the calibration 1 full wet range humidity sensor costs several hundred dollars, a difference of nearly a hundred times. Therefore, users must tailor their clothes, not blindly pursue "high, precise and sharp". Manufacturers usually give the accuracy of humidity sensors in sections. For example, the low temperature range (0-80%RH) is 2% RH, and the high humidity range (80- 100% RH) is 4% RH. And this accuracy is a value at a specific temperature (such as 25℃). If the humidity sensor is used at different temperatures, the indication value should also consider the influence of temperature drift. As we all know, relative humidity is a function of temperature, which seriously affects the relative humidity of a given space. When the temperature changes by 0.65438 0℃. There will be a humidity change (error) of 0.5%RH. If it is difficult to keep the temperature constant in use, it is not appropriate to put forward too high humidity measurement accuracy. Because humidity fluctuates with the change of temperature, it is of no practical significance to talk about humidity measurement accuracy. Therefore, in order to control the humidity, the temperature must be controlled first, which is why the integrated temperature and humidity sensor is often used in a large number, not just the humidity sensor.
In most cases, if there is no precise temperature control means or the measured space is not sealed, the accuracy of 5% RH is enough. For the local space where constant temperature and humidity need to be accurately controlled, or for the occasion where humidity changes need to be tracked and recorded at any time, a humidity sensor with accuracy above 3% RH is selected. For the corresponding temperature sensor, its temperature measurement accuracy must be above 0.3℃, at least 0.5℃. The accuracy is higher than 2% RH, so it is difficult to calibrate the standard humidity generator of the sensor, let alone the sensor itself. The article in the Humidity Room of the National Center for Reference Materials said: "Even at 20-25℃, it is still very difficult for relative humidity measuring instruments to achieve the accuracy of 2%RH."
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(1) The external probe is 2 degrees lower than the built-in temperature of the probe (25 degrees). According to the calculation of 5000 square meters, the electricity bill can be saved every year 1 10,000 yuan.
⑵ Integrated measurement and control, intelligent error correction function, it is more convenient to extend control at the end of the measuring point, reserve enough functions for future upgrading, save the cost of transformation and improve the adaptability of the system.
⑶ Data redundancy-never-disappearing radio waves: data is not deleted after reading, but compressed into redundant chips. When the monitoring host machine stops, the original data can be read from the end of the measuring point many times to ensure the traceability of the data.
⑷ Strong weatherability of the sensor-a mysterious film: The end of the sensor chip is coated with patented polymer film material (which has been recognized by Swiss Shi Sheng Ruian Company as the key improvement to improve the weatherability of its products), which greatly improves the weatherability of the sensor, enables it to work in frozen, refrigerated, high humidity, high temperature and corrosive environments for a long time, and greatly improves the precision life of the sensor.
5]1700-odd enterprises have experience in drug production, storage, transportation and use, accounting for more than 50% of the domestic market, and have sold more than 30,000 sets. After more than 7 years of continuous improvement in practical application, the product has convincing stability and adaptability.
3. Consider time drift and temperature drift.
Almost all sensors have time drift and temperature drift. Because the humidity sensor must be in contact with water vapor in the atmosphere, it cannot be sealed. This determines that its stability and life span are limited. Under normal circumstances, the manufacturer will indicate that the effective use time of 1 calibration is 1 year or 2 years, and will be responsible for recalibration upon expiration. When selecting sensors, users should consider the channels of recalibration in the future, and don't be greedy for cheap or superstitious about foreign goods and ignore after-sales service.
Section 1 has already mentioned the temperature drift. When selecting humidity sensor, we should consider the applied temperature range 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 temperature coefficient α of capacitive humidity sensor is a variable, which varies with the temperature and humidity range. This is because the dielectric coefficients of water and polymer are not synchronized with the change of temperature, and the temperature coefficient α mainly depends on the dielectric coefficients of water and humidity-sensitive materials, so the temperature coefficient of capacitive humidity-sensitive elements is not constant. The temperature coefficient of capacitive humidity sensor is the smallest at normal temperature and moderate humidity, and the temperature drift of moderate and low humidity can be ignored at 5-25℃. However, when it is used in high temperature and high humidity area or negative temperature and high humidity area, it is necessary to consider the influence of temperature drift and make necessary compensation or correction.
As early as the18th century, people invented wet and dry balls and hair hygrometers, and electronic humidity sensors have developed rapidly in recent decades, especially in the last 20 years. The alternation of old and new things is closely related to the change of people's ideas. Because the price of wet and dry bulb and hair hygrometer is still significantly lower than that of humidity sensor, some people do not recognize the price of electronic humidity sensor. Just like when people who are used to using brooms switch to vacuum cleaners, they always feel that it is not good to spend hundreds of dollars to buy a vacuum cleaner. It is better to spend a few dollars to buy a broom, which is psychologically easy to balance.
Because the traditional humidity measurement method is so impressive in people's minds, some people have formed an inherent concept that only the wet and dry bulb hygrometer is accurate. Some users compare the humidity sensor they just bought with the wet and dry bulb hygrometer. If they find that the instructions are different, they will immediately think that the humidity sensor is inaccurate. It should be noted that the accuracy of dry and wet bulb is only 5%-7%RH, which is not only lower than that of electronic humidity sensor, but also depends on the accuracy of dry bulb and wet bulb thermometer. The hygrometer must be ventilated: the gauze water jacket, water quality and wind speed meet certain requirements to achieve the specified accuracy. Before leaving the factory, humidity sensor manufacturers all use standard humidity generators to calibrate one by one, and the most common one is the shunt standard humidity generator. Therefore, it is hoped that users will adopt the same method when they need calibration, and avoid using low-precision instruments to calibrate or compare high-precision sensors.
5. Other considerations:
The humidity sensor is unsealed. In order to protect the accuracy and stability of measurement, it should be avoided to be used in acidic, alkaline and organic solvent atmospheres. Also avoid using 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 in a dead corner where the air is not circulating. If the measured room is too large, multiple sensors should be placed.
Some humidity sensors require high power supply, otherwise the measurement accuracy will be affected, or the sensors will interfere with each other or even fail to work. When in use, a suitable power supply meeting the accuracy requirements should be provided according to the technical requirements.
When the sensor needs to transmit signals over a long distance, attention should be paid to signal attenuation. When the transmission distance exceeds 200m, it is recommended to choose a humidity sensor with frequency output signal.
Due to the dispersion of humidity sensors, both imported and domestic sensors need to be calibrated one by one. Most humidity sensors need to be recalibrated after replacing them, which is especially important for humidity sensors with high measurement accuracy.