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Several common measuring methods of humidity sensor

Humidity measurement technology has been developed for a long time. With the development of electronic technology, modern measurement technology also has rapid development. Humidity measurement can be divided into two or thirty kinds in principle. There are absolute humidity, relative humidity, dew point in the expression of humidity. The ratio of moisture to dry gas (weight or volume), etc. But humidity measurement has always been one of the most famous problems in the field of measurement in the world. Preliminary participants may ignore many factors that must be taken into account in humidity measurement, thus affecting their rational use.

The common methods of humidity measurement are as follows: dynamic method (double pressure method, double temperature method, split method, static method (saturated salt method, sulphuric acid method, dew point method, wet and dry bulb method and various electronic sensor method).

Double pressure method, double temperature method:  based on the principle of thermodynamics, the balance time is longer, the split method is based on the accurate mixing of absolute moisture and absolute dry air. But because the equipment is complex, expensive and time-consuming, it is mainly used as standard metrology, and its measuring precision can reach ±2%RH-±1.5 RHs.

The saturated salt method in the static method:  it is the most common method in the humidity measurement, it is simple and feasible. But the saturated salt method requires the balance of liquid and gas two phases very strictly, and the stability of the environment temperature is higher. It needs to be balanced for a long time. Low humidity points require longer. Especially in the indoor humidity and the bottle humidity difference, each opening needs to balance for 6 hours.

Dew point method:  measuring the temperature when wet air reaches saturation, is the direct result of thermodynamics, and has high accuracy. The accuracy of the precision dew point meter for measurement can reach ±0.2 ℃ or higher. However, the cold mirror dew point meter based on modern optoelectric principle is expensive and is used in conjunction with the standard humidity generator.

Wet and dry bulb method:  it was invented in the 18th century to measure moisture. It has a long history and is most commonly used. The wet and dry bulb method is an indirect method, which 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 be above 2.5m/s. The common dry-wet bulb thermometer simplifies this condition. Therefore, its accuracy is only 5 / 7 RH, which is obviously lower than the electronic humidity sensor. Obviously, the wet and dry bulb is not a static method, so we should not simply think that the measurement accuracy of the two thermometers is equal to that of the hygrometer.

With regard to measurement methods, two points are emphasized:

First, because humidity is a function of temperature, the variation of temperature has a decisive effect on the results of humidity measurement. There is also a certain gradient of temperature in the studio, so the humidity in this space is difficult to be uniform.

Second, because of the large differences in principles and methods, it is difficult to directly calibrate and identify the various methods of measurement, most of them can only be compared indirectly. Therefore, the results of the full wet range (relative humidity 099. 9 RHs) are calibrated between the two methods. Or it is very difficult to calibrate the measurements at all temperatures. For example, a wet and dry bulb hygrometer requires a flow of air with a specified wind speed, while the saturated salt rule requires a strict seal. The best method is to determine the transmission mode and verification regulation according to the national verification system (standard) of humidity measuring instruments.