What is a humidity sensor ? This is easy to understand; it belongs to the category of sensors and is mainly used to record changes in humidity in the air in real time. For example, the scrolling displays on high-speed trains and buses typically show the interior and exterior temperatures, which requires temperature sensors. Installing temperature sensors inside and outside the vehicle allows us to know the temperature inside and outside the passenger compartment. Humidity sensors can also be used in vaccine cold chain transportation, smart agriculture, pharmaceuticals, engines, air conditioning, and many other fields. So, how do humidity sensors work? What are some domestic brands of humidity sensors? How accurate are Xiaomi humidity sensors? Below, the editors at Xianji.com will answer these questions one by one.
Humidity sensor working principle
Working principle of capacitive humidity sensor
A capacitive humidity sensor mainly consists of two parts: a humidity-sensitive capacitor and a conversion circuit. It comprises a glass substrate, a lower electrode, a humidity-sensitive material, and an upper electrode. The two lower electrodes, the humidity-sensitive material, and the upper electrode form two capacitors connected in series.
Humidity-sensitive materials are polymeric materials whose dielectric constant changes with the relative humidity of the environment. When the ambient humidity changes, the capacitance of the humidity-sensitive element changes accordingly; that is, when the relative humidity increases, the capacitance increases, and vice versa (the capacitance is typically between 48 and 56 pF). The sensor's conversion circuit converts the change in capacitance into a change in voltage. Corresponding to a change in relative humidity from 0 to 100% RH, the sensor's output exhibits a linear change from 0 to 1 V.
Working principle of resistive lithium chloride humidity sensor
The measurement range of lithium chloride elements is related to the lithium chloride concentration and other components of the humidity-sensitive layer. The effective humidity sensing range of a single element is generally within 20%RH. For example, a concentration of 0.05% corresponds to a humidity sensing range of approximately (80-100)%RH, and a concentration of 0.2% corresponds to a range of (60-80)%RH, etc. Therefore, to measure a wider humidity range, elements of different concentrations must be combined. Hygrometer combinations suitable for full-range measurements typically use 5 elements. Lithium chloride hygrometers using element combinations typically have a measurable range of (15-100)%RH, although some foreign products claim a measurement range of (2-100)%RH.
How accurate is Xiaomi's humidity sensor? Only those who have bought it can truly answer this question. Here's the answer:
Mi_46353790: I bought a temperature and humidity sensor during the Mi Fan Festival. Last night, I configured it with the Xiaomi Smart Home gateway according to the instructions, and the thermometer was also attached to the wall, away from metal. However, I found that the readings were seriously inaccurate. Last night at 7 pm, after installation, the temperature displayed in my home was 28 degrees Celsius! In Qingdao in April, the indoor temperature is at most around 15 degrees Celsius. The Xiaomi humidity sensor's readings are seriously inaccurate.
sunjialin: I've been using the Zigbee version of the temperature and humidity sensor for about half a year. I've found that the results it gives are extremely inaccurate. Compared with other mechanical and electronic temperature and humidity meters, the temperature readings are about 1-1.5 degrees Celsius higher, and the humidity readings are even more outrageous, with a deviation of about 30%. The 0.3-degree deviation advertised is probably just the test result of that component; the deviation will be much larger when used in Xiaomi devices.
What should I do if my Xiaomi humidity sensor is inaccurate?
Odinmax: If you bought the regular version of the Xiaomi humidity sensor, I suggest replacing it with a pressure-sensitive humidity sensor. I bought the one with barometric pressure and put it on the balcony. After using it for two weeks, I found it to be accurate. I compared the data from the local weather app and the built-in weather app on my phone.
Xiaoxiao Maker: Try changing the battery. The temperature and humidity monitoring chip inside is also a world-renowned product, so it won't cause the problem you're experiencing.
Mi_697318749: I use an Aqara thermometer and hygrometer, and its accuracy is quite good. The difference between it and the temperature recorder I bought for a few hundred yuan is within 0.1°C. Cheap electronic or mechanical thermometers are unreliable in terms of accuracy. You can find a high-precision one to compare.
The humidity error range of a typical temperature and humidity meter is generally within 7%RH. If you need high accuracy, then purchase an industrial-grade temperature and humidity sensor (temperature and humidity transmitter), which typically has a measurement accuracy of 2-5%RH. Claims that accuracy is guaranteed to be within 1%RH are likely misleading.
What are some domestic brands of humidity sensors?
Currently, domestic temperature and humidity sensor brands on the market include Risym, Huakong, Chuanjing, Aosong Electronics, Anpuhua, Yimeng, Yangxin, Qiaotian, and Jiuchunjian, among others. Risym, Aosong Electronics, and Jiuchunjian are the most popular. Jiuchunjian Technology's independently developed temperature sensors have been widely used in high-speed rail projects. Currently, tens of thousands of Jiuchunjian temperature sensors are used in high-speed rail, including over a thousand individual temperature and humidity sensors.
Domestic brands of temperature and humidity meters include: Yipin Boyang, Kejian, Zhenli, Meideshi, Oudashi, Minggao, Pinyi, Yimeite, Shentuo, Deli, Baoyi, Duolico, Hegao, Xiaobaixiong, etc.
Imported humidity sensor brands include: Galltec, Honeywell, E+E, Vaisala, Siemens, Dwyer, Rotronic, Michell, Veris, and KIMO.
The above is an introduction to the working principle, domestic brands, and accuracy of humidity sensors. Humidity sensors can effectively monitor environmental humidity and have important applications in food preservation, textile setting machines, air purifiers, central machine rooms, and tobacco fermentation. How to choose a humidity sensor? From a manufacturing perspective, even among humidity sensors, differences in materials, structure, and processes result in significant differences in performance and technical specifications (such as accuracy), leading to substantial price variations. Therefore, selection should be based on actual application and budget.
