Simply put, the humidity controller is mainly composed of a humidity sensor and a humidifier.
Humidity sensor principle (basic knowledge)
1. Humidity and dew point of the atmosphere
[1]. Absolute Humidity and Relative Humidity The atmosphere on the earth's surface is made up of 78% nitrogen, 21% oxygen and a small part of carbon dioxide, water vapor, and other inert gases. Because the water and animals and plants on the ground will evaporate, so the ground is constantly generating moisture, so that the amount of water vapor in the atmosphere is constantly changing. Since the process of evaporation and condensation of water is always accompanied by heat absorption and heat release, the amount of water vapor in the atmosphere will not only affect the humidity of the atmosphere, but also make the air appear humid or dry. The degree of dryness and humidity of the atmosphere is usually expressed by the density of water vapor in the atmosphere. That is expressed in grams of water vapor per 1m3 of the atmosphere, which is called the absolute humidity of the atmosphere.
To directly measure the water vapor density of the atmosphere, the method is relatively complicated. Theoretical calculations show that under normal temperature conditions, the atmospheric water vapor density is very close to the atmospheric water vapor pressure. Therefore, the water vapor density of the atmosphere can be defined as the pressure of the water vapor in the atmosphere, and it is also called the absolute humidity of the atmosphere. It is represented by the symbol D, and the commonly used unit is mmHg. .
In many phenomena related to atmospheric humidity, such as the end of crop growth cotton yarn and people's feelings, etc., there is no direct relationship with the absolute humidity of the atmosphere, mainly related to the degree of the water vapor in the atmosphere from the saturation state related. For example, it is also the absolute humidity of 6mmHg. If it is far from the saturated water vapor pressure (31.38mmHg) at noon in the hot summer, it makes people feel dry. If it is in the early winter evening, because the water vapor pressure is close to the saturated water vapor at that time Pressure (18.05mmHg) makes people feel moist. Therefore, the percentage of the absolute humidity of the atmosphere and the saturated water vapor pressure at the current temperature is usually called the relative humidity of the atmosphere, that is, H-relative humidity
D——The absolute humidity of the atmosphere (mmHg)
Ds——Saturated water vapor pressure (mmHg) at the temperature
The above formula shows that if the pressure of water vapor in the atmosphere is equal to the saturated water vapor pressure at the current temperature, then the relative humidity of the atmosphere is equal to 100% RH.
[2]. Dew point Decreasing the temperature can turn unsaturated water vapor into saturated water vapor. Dew point refers to the temperature that must be lowered to turn the unsaturated water vapor originally contained in the atmosphere into saturated water vapor. Therefore, as long as the dew point can be measured, the absolute humidity of the atmosphere at that time can be checked through some data tables.
When unsaturated water vapor in the atmosphere comes into contact with objects with lower temperatures, it will cause the unsaturated water vapor in the atmosphere to reach or approach saturation, condensing into water droplets on these objects. This phenomenon is called condensation. Condensation is good for crops, but harmful to electronic products.
2. Classification of humidity sensor
Water is an extremely strong electrolyte. Water molecules have a large electric dipole moment and a large positive electric field near the hydrogen atom, so it has a large electron affinity, making water molecules easily adsorb on the surface of the solid and penetrate into the solid. Humidity sensors made with the characteristics of water molecules are called water molecule affinity sensors. The humidity sensor that has nothing to do with the affinity of water molecules is called a non-water molecule affinity sensor. Humidity sensors used in modern industry are mostly water-molecule affinity sensors, which convert changes in humidity into changes in impedance or capacitance values ​​and output.
For example, the principle of the mechanical hygrometer: the humidity sensor of the mechanical pointer hygrometer, which has a hair type, is also made of a metal hairspring coated with a polymer hydrophilic plastic material. The principle is the same as that of the mechanical humidity sensor, and both belong to the variable length type. The displacement caused by the length change is used to drive the pointer shaft to move the pointer on the dial to realize the humidity measurement function. Therefore, it has the same weakness as the mechanical humidity sensor, that is, the response is slow, and the lag time can be as long as one or twenty minutes. More than 10%, the annual drift value is large, unstable, prone to aging and deterioration, affecting life. Although it cannot be accurately and reliably measured, the displayed value can only be used as a reference, but because of its low price, it can work without electrical signals.
If the humidity control value of a control position of the mechanical humidity sensor is defined, the control error can be up to ± 15%, and the maximum error can be generated between the ± 5% display error of the mechanical pointer humidity meter Accumulated error of ± 20%.
Although the products of mechanical humidity sampling and control display have these deficiencies, they are also used by less demanding users due to their low price.
Humidity sensor principle (basic knowledge)
1. Humidity and dew point of the atmosphere
[1]. Absolute Humidity and Relative Humidity The atmosphere on the earth's surface is made up of 78% nitrogen, 21% oxygen and a small part of carbon dioxide, water vapor, and other inert gases. Because the water and animals and plants on the ground will evaporate, so the ground is constantly generating moisture, so that the amount of water vapor in the atmosphere is constantly changing. Since the process of evaporation and condensation of water is always accompanied by heat absorption and heat release, the amount of water vapor in the atmosphere will not only affect the humidity of the atmosphere, but also make the air appear humid or dry. The degree of dryness and humidity of the atmosphere is usually expressed by the density of water vapor in the atmosphere. That is expressed in grams of water vapor per 1m3 of the atmosphere, which is called the absolute humidity of the atmosphere.
To directly measure the water vapor density of the atmosphere, the method is relatively complicated. Theoretical calculations show that under normal temperature conditions, the atmospheric water vapor density is very close to the atmospheric water vapor pressure. Therefore, the water vapor density of the atmosphere can be defined as the pressure of the water vapor in the atmosphere, and it is also called the absolute humidity of the atmosphere. It is represented by the symbol D, and the commonly used unit is mmHg. .
In many phenomena related to atmospheric humidity, such as the end of crop growth cotton yarn and people's feelings, etc., there is no direct relationship with the absolute humidity of the atmosphere, mainly related to the degree of the water vapor in the atmosphere from the saturation state related. For example, it is also the absolute humidity of 6mmHg. If it is far from the saturated water vapor pressure (31.38mmHg) at noon in the hot summer, it makes people feel dry. If it is in the early winter evening, because the water vapor pressure is close to the saturated water vapor at that time Pressure (18.05mmHg) makes people feel moist. Therefore, the percentage of the absolute humidity of the atmosphere and the saturated water vapor pressure at the current temperature is usually called the relative humidity of the atmosphere, that is, H-relative humidity
D——The absolute humidity of the atmosphere (mmHg)
Ds——Saturated water vapor pressure (mmHg) at the temperature
The above formula shows that if the pressure of water vapor in the atmosphere is equal to the saturated water vapor pressure at the current temperature, then the relative humidity of the atmosphere is equal to 100% RH.
[2]. Dew point Decreasing the temperature can turn unsaturated water vapor into saturated water vapor. Dew point refers to the temperature that must be lowered to turn the unsaturated water vapor originally contained in the atmosphere into saturated water vapor. Therefore, as long as the dew point can be measured, the absolute humidity of the atmosphere at that time can be checked through some data tables.
When unsaturated water vapor in the atmosphere comes into contact with objects with lower temperatures, it will cause the unsaturated water vapor in the atmosphere to reach or approach saturation, condensing into water droplets on these objects. This phenomenon is called condensation. Condensation is good for crops, but harmful to electronic products.
2. Classification of humidity sensor
Water is an extremely strong electrolyte. Water molecules have a large electric dipole moment and a large positive electric field near the hydrogen atom, so it has a large electron affinity, making water molecules easily adsorb on the surface of the solid and penetrate into the solid. Humidity sensors made with the characteristics of water molecules are called water molecule affinity sensors. The humidity sensor that has nothing to do with the affinity of water molecules is called a non-water molecule affinity sensor. Humidity sensors used in modern industry are mostly water-molecule affinity sensors, which convert changes in humidity into changes in impedance or capacitance values ​​and output.
For example, the principle of the mechanical hygrometer: the humidity sensor of the mechanical pointer hygrometer, which has a hair type, is also made of a metal hairspring coated with a polymer hydrophilic plastic material. The principle is the same as that of the mechanical humidity sensor, and both belong to the variable length type. The displacement caused by the length change is used to drive the pointer shaft to move the pointer on the dial to realize the humidity measurement function. Therefore, it has the same weakness as the mechanical humidity sensor, that is, the response is slow, and the lag time can be as long as one or twenty minutes. More than 10%, the annual drift value is large, unstable, prone to aging and deterioration, affecting life. Although it cannot be accurately and reliably measured, the displayed value can only be used as a reference, but because of its low price, it can work without electrical signals.
If the humidity control value of a control position of the mechanical humidity sensor is defined, the control error can be up to ± 15%, and the maximum error can be generated between the ± 5% display error of the mechanical pointer humidity meter Accumulated error of ± 20%.
Although the products of mechanical humidity sampling and control display have these deficiencies, they are also used by less demanding users due to their low price.
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