In the industrial and agricultural production, meteorology, environmental protection, national defense, scientific research, aerospace and other departments, it is often necessary to measure and control the environmental humidity. The control of environmental temperature and humidity and the monitoring and analysis of the moisture content of industrial materials have become one of the more common technical conditions. However, in conventional environmental parameters, humidity is the most difficult parameter to measure accurately. This is because measuring humidity is much more complicated than measuring temperature. Temperature is measured independently and humidity is affected by other factors (atmospheric pressure, temperature). In addition, the calibration of humidity is also a problem. The humidity calibration equipment produced abroad is very expensive.
First, the definition of humidity
In the measurement method, it is defined that the humidity is defined as the "amount of the object state." The relative humidity in daily life refers to relative humidity, which is expressed as RH%. In summary, the amount of water vapour (water vapour pressure) in the gas (usually air) is the percentage of saturated vapour (saturated vapour pressure) in the same atmosphere as the air.
Humidity has had a close relationship with life long ago, but it is more difficult to express it in terms of quantity. Humidity can be expressed in terms of absolute humidity, relative humidity, dew point, ratio of moisture to dry gas (weight or volume), and so on.
Second, the humidity measurement method
Humidity measurement is divided into two to thirty different types in principle. However, humidity measurement has always been one of the famous problems in the world metrology field. A seemingly simple value, in depth, involves a rather complex physics-chemistry theory analysis and calculation. The beginner may ignore many factors that must be noted in the humidity measurement, and thus affect the reasonable use of the sensor.
Common methods of humidity measurement include: dynamic method (double pressure method, dual temperature method, diverting method), static method (saturated salt method, sulfuric acid method), dew point method, wet and dry ball method and electronic sensor method.
1 The double pressure method and double temperature method are based on the principle of thermodynamics P, V and T balance. The equilibrium time is longer. The split flow method is based on the accurate mixing of absolute moisture and absolute dry air. Due to modern measurement and control methods, these equipments can be made very precise. However, due to the complicated and expensive equipment, the operation is time-consuming and labor-consuming. It is mainly used as a standard measurement, and its measurement accuracy can reach ±2% RH or more.
2 The saturated salt method in the static method is the most common method for humidity measurement and is simple and easy to implement. However, the saturated salt method has strict requirements on the balance between liquid and gas phases, and has a high requirement for the stability of the ambient temperature. Used to wait for a long time to balance, low humidity requirements longer. Especially when the humidity in the room and the difference in humidity inside the bottle are large, it takes 6 to 8 hours for each opening.
3 The dew point method measures the temperature at which humid air reaches saturation. It is a direct result of thermodynamics, with high accuracy and a wide measuring range. Precision dew point metering accuracy can reach ±0.2°C or even higher. However, the cold mirror type dew point meter using the modern light-electricity principle is expensive and often used in combination with a standard humidity generator.
4 Wet and dry ball method, this is the method of measuring humidity that was invented in the 18th century. Has a long history and is most commonly used. The wet and dry ball method is an indirect method that uses the wet-and-dry ball equation to calculate the humidity value. The equation is conditional: that is, the wind speed must be 2.5 m/s or more in the vicinity of the wet bulb. The commonly used wet and dry bulb thermometer simplifies this condition, so its accuracy is only 5 to 7% RH. Dry and wet bulbs do not belong to the static method. Do not simply think that increasing the measurement accuracy of two thermometers is equivalent to increasing the humidity. Accuracy of measurement.
5 Electronic Humidity Sensor Method
Electronic humidity sensor products and humidity measurement belong to the industry that started in the 1990s. In recent years, considerable progress has been made in the field of humidity sensor research and development at home and abroad. Humidity sensors are rapidly developing from simple moisture sensors to integrated, intelligent, multi-parameter detection, which has created favorable conditions for the development of a new generation of humidity measurement and control systems, and has also brought humidity measurement technology to a new level.
Third, the choice of humidity measurement program
There are two main types of modern humidity measurement programs: wet and dry ball moisture measurement, and electronic humidity sensor moisture measurement. Compare the two options below so that customers can choose the right humidity measurement method.
Dry and wet bulb hygrometer features:
As early as the 18th century, humans invented wet and dry hygrometers. The accuracy of wet and dry hygrometers also depends on the accuracy of dry and wet bulbs. The hygrometer must be ventilated: only gauze water jackets and water quality. When the wind speed meets certain requirements, the required accuracy can be achieved. The wet and dry hygrometer accuracy is only 5% to 7% RH.
Wet and dry ball wet method uses indirect measurement method. By measuring the temperature of dry ball and wet bulb, the humidity value is calculated. Therefore, there is no strict limit to the use temperature. Measuring humidity in high temperature environment will not damage the sensor.
The maintenance of wet and dry ball moisture measurement method is quite simple. In actual use, it is only necessary to regularly add water to the wet bulb and replace the wet ball gauze. Compared with electronic humidity sensors, the wet and dry bulb moisture measurement method does not cause problems such as aging and accuracy degradation. Therefore, wet and dry bulb moisture measurement method is more suitable for use in high temperature and harsh environment.
Electronic humidity sensor features:
The electronic humidity sensor has developed rapidly in recent decades, especially in the last 20 years. Humidity sensor production factory must use the standard humidity generator to calibrate each branch before leaving the factory. The accuracy of the electronic humidity sensor can reach 2% to 3% RH.
In actual use, due to the influence of dust, oil, and harmful gases, the use of a long time will cause aging, and the accuracy will decline. The annual drift of the humidity sensor is generally about ± 2%, or even higher. Under normal circumstances, the manufacturer will indicate that the effective time for one calibration is 1 year or 2 years, and the calibration will need to be re-calibrated.
The accuracy level of the electronic humidity sensor must be combined with its long-term stability. Generally speaking, the long-term stability and service life of the electronic humidity sensor is not as good as that of a wet and dry bulb humidity sensor.
Humidity sensors use semiconductor technology and therefore require ambient temperature for use. Exceeding their specified operating temperature will damage the sensor.
Therefore, the electronic moisture sensor humidity measurement method is more suitable for use in clean and room temperature applications.
Fourth, the humidity sensor selection considerations
1. Select measurement range
Just like the measurement of weight and temperature, selecting the humidity sensor first determines the measurement range. In addition to the meteorological and scientific research departments, the measurement of temperature and humidity generally does not require the measurement of full-humidity (0-100% RH).
2. Selecting Measurement Accuracy Measurement accuracy is the most important indicator of a humidity sensor. Each increase of ― percentage points is the last step for the humidity sensor, and even the last level. Because to achieve different accuracy, the difference in manufacturing costs
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