Relative humidity
Air humidity can generally be understood as the degree of humidity of the air, which can be divided into absolute humidity and relative humidity.
The humidity of the air can be expressed by the density of vapor contained in the air, that is, the mass of vapor contained in a unit volume of air. Since it is difficult to directly measure the density of water vapor in the air, and the pressure of water vapor increases with the increase of water vapor density, the pressure of water vapor in the air is usually used to represent the air humidity, that is, the absolute humidity of the air.
We're all familiar with relative humidity, which can be learned from weather forecasts—one reason why is that relative humidity is an indicator of people's comfort. For example, people sweat because their bodies regulate their temperature. When the human body sweats, water evaporates, which is a cooling process. The higher the relative humidity, the smaller the volatilization, and the human body will not cool down quickly. When the temperature is high, say 32°C (90°F), you will feel more uncomfortable at 90% relative humidity than at 40%.
The concept of relative humidity is introduced to represent the distance of water vapor in the air from saturated state to saturated state.
The meaning of relative humidity is: at a certain atmospheric temperature, the ratio of the amount of water vapor contained in the quantitative air to the maximum amount of water vapor contained in the same amount of air at that temperature. Relative humidity is usually expressed as a percentage
In the coatings industry, relative humidity has clear limits, and many codes and standards stipulate that the relative humidity should not exceed 85%. When the relative humidity is lower than 85%, water vapor condensation will generally not occur on the surface of the steel, and the coating quality can be guaranteed.
When unsaturated air is cooled, its relative humidity increases because the cooler the air, the less water vapor it can hold. Conversely, the higher the temperature, the greater the maximum amount of water vapor that can be contained.
The rotary wet and dry bulb hygrometer is one of the commonly used hygrometers in coating inspection. Used to test ambient air temperature (dry bulb temperature). The wet bulb temperature is more practical because it is closer to the work site. These data are then used to calculate dew point temperature and relative humidity. A psychrometer consists of two identical tubular thermometers, one of which is covered with distilled water. The thermometer with a protective cover is called a wet bulb thermometer, and the other is called a dry bulb thermometer. The dry bulb represents the air temperature. Water evaporates from the wet jacket, causing potential heat loss and producing a wet bulb reading
The faster the water evaporates, the lower the humidity and dew point temperature. The gyratory psychrometer saturates the sheath with distilled water and shakes the psychrometer quickly for about 40 seconds. Then read the wet bulb temperature. Repeat this process (spin and read, no additional wetting) until the temperature stabilizes. Record when wet bulb count is constant. After the wet bulb reading is stable, read the dry bulb value again. Record the dry bulb value. If used frequently near sandblasting or painting job sites, replace the jacket if it becomes dirty or it will produce inaccurate readings.
Be careful when the temperature is below 0C (32F). Hand-held and fan-operated psychrometers are unreliable due to freezing of water. If the temperature is very low, a direct reading instrument should be used for testing.
