Brief introduction of temperature and humidity sensor
Temperature and humidity are closely related to people's lives, and attention to temperature and humidity has given birth to the birth of temperature and humidity sensors. Temperature and humidity sensor refers to a device or device that can convert temperature and humidity into electrical signals that can be easily measured and processed. Temperature and humidity sensors on the market generally measure temperature and relative humidity.
Explanation of several concepts about temperature and humidity:
Temperature: A physical quantity that measures the heat and cold of an object. It is one of the seven basic physical quantities in the International System of Units. In production and scientific research, many physical phenomena and chemical processes are carried out at a certain temperature, and people's lives are closely related to it.
Humidity: Humidity has been closely related to life a long time ago, but it is difficult to express it by quantity.
The most commonly used physical quantity to express humidity in daily life is the relative humidity of the air. Expressed by %RH. Relative humidity is closely related to temperature in the derivation of physical quantities. For a certain volume of airtight gas, the higher the temperature, the lower the relative humidity, and the lower the temperature, the higher the relative humidity. Which involves complex thermal engineering knowledge.
Some definitions of humidity:
Relative humidity: In the measurement law, humidity is defined as the "quantity of the state of the object". The humidity referred to in daily life is relative humidity, expressed in RH%. In short, that is, the percentage of saturated water vapor (saturated water vapor pressure) in the gas (usually in the air) when the amount of water vapor (water vapor pressure) is the same as that of the air.
Absolute humidity: refers to the amount of water vapor actually contained in a unit volume of air, generally in grams. Temperature has a direct effect on absolute humidity. Generally, the higher the temperature, the more water vapor is emitted, and the greater the absolute humidity; on the contrary, the absolute humidity is lower.
Saturated humidity: The maximum amount of water vapor that can be contained in the air per unit volume at a certain temperature. If it exceeds this limit, the excess water vapor will condense and become water droplets, and the air humidity at this time will become saturated humidity. The saturated humidity of the air is not fixed, it changes with the change of temperature. The higher the temperature, the more water vapor can be contained in the unit volume of air, and the greater the saturated humidity.
Dew point: refers to the air containing a certain amount of water vapor (absolute humidity). When the temperature drops to a certain level, the water vapor contained in it will reach a saturated state (saturated humidity) and begin to liquefy into water. This phenomenon is called condensation. The temperature at which water vapor begins to liquefy into water is called "dew point temperature" or "dew point" for short. If the temperature continues to drop below the dew point, the super-saturated water vapor in the air will condense into water droplets on the surface of the object. In addition, wind is closely related to the temperature and humidity in the air, and it is also one of the important factors affecting the changes in air temperature and humidity.
Measurement methods
Several common measuring methods of humidity measuring sensors
Humidity measurement technology has a long history. With the development of electronic technology, modern measurement technology has also developed rapidly. Humidity measurement is divided into as many as 20 to 30 in principle. The expression methods of humidity include absolute humidity, relative humidity, dew point, the ratio of moisture to dry gas (weight or volume), and so on. But humidity measurement has always been one of the famous problems in the world of metrology. A seemingly simple value, when studied in depth, involves quite complex physics-chemical theoretical analysis and calculations. The beginner may ignore many factors that must be paid attention to in humidity measurement, thus affecting its reasonable use.
Common humidity measurement methods are: dynamic method (double pressure method, double temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, dry and wet bulb method and various electronic sensor methods.
Here, the dual pressure method and the dual temperature method are based on the thermodynamics P, V, T balance principle, the balance time is longer, and the split method is based on the precise mixing of absolute moisture and absolute dry air. Due to the modern measurement and control methods, these equipment can be made quite precise, but because the equipment is complex, expensive, and time-consuming and labor-intensive, it is mainly used for standard measurement, and its measurement accuracy can reach ±2%RH-±1.5%RH.
The saturated salt method in the static method is the most common method in humidity measurement and is simple and easy to implement. However, the saturated salt method has strict requirements on the balance of liquid and gas phases, and higher requirements on the stability of the ambient temperature. To use it requires a long time to balance, and the low humidity point requires a longer time. Especially when the difference between the indoor humidity and the humidity in the bottle is large, it needs to be balanced for 6 to 8 hours each time it is turned on.
The dew point method measures the temperature when the humid air reaches saturation, which is a direct result of thermodynamics, with high accuracy and wide measurement range. The accuracy of the precision dew point meter used for measurement can reach ±0.2℃ or even higher. However, the cold mirror dew point meter using modern photo-electric principle is expensive, and it is often used in conjunction with a standard humidity generator.
The wet and dry bulb method is a method of measuring humidity invented in the 18th century. It has a long history and is the most commonly used. The wet and dry bulb method is an indirect method. It uses the wet and dry bulb equation to convert the humidity value, and this equation is conditional: that is, the wind speed near the wet bulb must reach 2.5m/s or more. The common dry and wet bulb thermometer simplifies this condition, so its accuracy is only 5~7%RH, which is significantly lower than that of the electronic humidity sensor. Obviously, the wet and dry bulb does not belong to the static method. Don't simply think that as long as the measurement accuracy of the two thermometers is improved, the measurement accuracy of the hygrometer is improved.
This article would like to emphasize two points: First, because humidity is a function of temperature, temperature changes decisively affect the measurement results of humidity. Regardless of the method, accurately measuring and controlling the temperature is the first priority. Note that even if it is a well-insulated constant temperature and humidity box, the temperature in the working room also has a certain gradient. Therefore, the humidity in this space is also difficult to be completely uniform.
Second, due to the large differences in principles and methods, it is difficult to directly calibrate and confirm various measurement methods, and most of them can only be compared by indirect methods. Therefore, it is very difficult to calibrate the measurement results of the whole humidity range (relative humidity 0~100%RH) between the two humidity measurement methods, or to calibrate the measurement results of each point in all temperature ranges. For example, the ventilated wet and dry bulb hygrometer requires flowing air with a specified wind speed, while the saturated salt law requires strict sealing, and the two cannot be compared. The best way is to determine the level by level according to the transmission method and verification procedures stipulated by the country for the verification system (standard) of humidity measurement instruments.