Atmospheric humidity and dew point
[1]. Humidity and relative humidity
The Earth's atmosphere is composed of 78% nitrogen, 21% oxygen, and a small amount of carbon dioxide, water vapor, and other inert gases. Because water on the ground and from plants and animals evaporate, water is constantly being generated on the ground, causing the amount of water vapor in the atmosphere to fluctuate. Since the evaporation and condensation of water always involve heat absorption and release, the amount of water vapor in the atmosphere not only affects atmospheric humidity but also causes the air to appear humid or dry. The degree of dryness or wetness of the atmosphere is usually expressed by the density of water vapor in the atmosphere, that is, the number of grams of water vapor contained in 1 m³ of atmosphere; this is called atmospheric humidity.
Directly measuring atmospheric water vapor density is quite complex. However, theoretical calculations show that under normal temperature conditions, atmospheric water vapor density is very close to the pressure of water vapor in the atmosphere. Therefore, atmospheric water vapor density can also be defined as the pressure of water vapor contained in the atmosphere, and is also called atmospheric humidity, denoted by the symbol D, with the commonly used unit being mmHg.
Many phenomena related to atmospheric humidity, such as crop growth, yarn breakage, and human sensations, are not directly related to atmospheric humidity itself, but rather to the proximity of atmospheric water vapor to saturation. For example, a humidity level of 6 mmHg can feel dry at midday in the hot summer because it is far from the saturation vapor pressure (31.38 mmHg), while it feels humid in the early winter evening because the vapor pressure is close to the saturation vapor pressure (18.05 mmHg). Therefore, the percentage of atmospheric humidity to the saturation vapor pressure at the given temperature is usually referred to as relative humidity.
[2]. Dew point
Lowering the temperature can convert unsaturated water vapor into saturated water vapor. The dew point is the temperature at which the unsaturated water vapor in the atmosphere must be lowered to become saturated water vapor. Therefore, as long as the dew point can be measured, the humidity of the atmosphere at that time can be found through some data tables.
When unsaturated water vapor in the atmosphere comes into contact with a cooler object, the unsaturated water vapor reaches or nearly reaches saturation, condensing into water droplets on that object. This phenomenon is called condensation. Condensation is beneficial to crops but harmful to electronic products.
II. Classification of Humidity Sensors
Water is an extremely strong electrolyte. Water molecules have a large electric dipole moment and a very large positive electric field near hydrogen atoms, resulting in a strong electron affinity that allows water molecules to easily adsorb onto solid surfaces and penetrate into the solid's interior. Humidity sensors that utilize this property of water molecules are called water affinity sensors. Humidity sensors that do not rely on water affinity are called non-water affinity sensors. Most humidity sensors used in modern industry are water affinity sensors; they convert changes in humidity into changes in impedance or capacitance as their output. Figure 1 is a schematic diagram of humidity sensor classification.
