Dehumidification talks about relative humidity

In the process of purchasing a dehumidifier, we have several important reference indicators, dehumidification capacity, temperature and humidity, etc., one of which is relative humidity, which can be expressed as the ratio (percentage) of the water vapor pressure and saturated vapor pressure of the air at the same temperature. Relative humidity is a difficult concept to understand. Here's an explanation of it.

At standard air pressure, a cubic meter of water vapor in the air is extracted and its weight is weighed to obtain a humidity. It is expressed as the water content (g/m3) of a cubic meter of air under standard air pressure, which is the popular way of understanding. If you want to know more about relative humidity, you can check Baidu Encyclopedia "Relative Humidity". To give you a better understanding of this concept, here are some examples!

When measuring 10 degrees Celsius according to the data, the air energy contains 7.7 grams of water (calculated by Vaisala's humidity calculation software, hereinafter referred to as "Vaisala value"). At this point, the air is at its maximum absolute humidity, which is what we call saturation. At 20 degrees Celsius, the air saturation is 14.7 g/m3 (Vaisala). Therefore, if one cubic meter of air in a sealed container contains 9 grams of water vapor at 20 degrees Celsius, its saturation is 9 g/m3. If 3 grams of water are added to the container, the evaporation of the water increases the absolute humidity inside the container to 12 g/m3. If you add another 5 grams of water, 2.7 grams of water will evaporate and another 2.3 grams of water will remain at the bottom of the container, because at 20 degrees Celsius, only 14.7 grams of water vapor can be contained in one cubic meter of air. When there are only 9 grams of water vapor in the container, the relative humidity of the air in the container is 9/14.7=61.2%

Relative humidity depends on air temperature. If no excess water vapor is added, the relative humidity will drop as the temperature rises. So, when the container is heated to 25 degrees Celsius, the table shows that at this temperature, one cubic meter of air can contain 20 grams of water (Vaisala value), and the relative humidity decreases: 9/20 = 45% If the air temperature in the container drops to 15 degrees Celsius, the relative humidity will rise even if more water is not added. At 15 degrees Celsius, only 10.8 grams of water (Vaisala value) can be contained in one cubic meter of air, so the relative humidity is 9/10.8 = 83%.

If the air is cooled to 8 degrees Celsius, the water vapor inside the container is saturated and its relative humidity rises to 100 percent. As the air cools further, small water droplets form on the walls of the container, because the air must condense some of the water vapor. The temperature at which water vapor condensation begins (i.e., the temperature at which the air reaches saturation) is called the dew point. For example, indoors in winter, air circulates to the window edge, where the temperature is low enough for the air to be cooled below its dew point and water droplets appear on the window.