What Is a Psychrometer?

Wet and dry bulb hygrometer is also called wet and dry meter. The use of water to absorb heat to cool down, and the speed of evaporation (that is, how much to cool down) is based on the principle of the relative humidity of the air at that time. Its structure is to use two thermometers, one of which is covered with white gauze in the ball, and the other end of the gauze is immersed in a water tank, that is, the gauze is often kept wet by capillary action, which is a wet ball. Another thermometer that is not covered with gauze and exposed in the air is called a dry bulb (a dry bulb means the temperature of the air temperature).

If the amount of water vapor in the air is not saturated, the surface of the wet bulb will continuously evaporate water vapor and absorb the heat of vaporization, so the temperature indicated by the wet bulb is lower than that shown by the dry bulb. The drier the air (ie, the lower the humidity), the faster the evaporation, the continuous absorption of the heat of vaporization, the temperature shown by the wet bulb decreases, and the difference from the dry bulb increases. Conversely, when the amount of water vapor in the air is saturated, the water no longer evaporates and does not absorb the heat of vaporization. The temperatures shown by the wet and dry bulbs will be equal. When in use, the wet and dry meter should be placed at a height of 1.2 to 1.5 meters from the ground. Read the temperature difference indicated by the dry and wet balls, and the relative humidity of the air can be found out by the comparison table attached to the hygrometer. Because the speed of moisture evaporation of the gauze wrapped by the wet ball is not only related to the relative humidity of the air at that time, but also to the air circulation speed. Therefore, the comparison table attached to the wet and dry bulb thermometer is only applicable to the specified

For example: Let the dry bubble temperature be 22 and the wet bubble temperature be 16

22 can be found in the column of dry bulb temperature shown in the table first;

Find the approximate value of 16 in the humidity temperature table of the line;

The value of the header corresponding to the vertical column of the approximate value is 54. The relative humidity is: 54%.

As early as the 18th century, humans invented the wet and dry bulb hygrometer. The accuracy of the wet and dry bulb hygrometer also depends on the accuracy of the two dry bulb and wet bulb thermometers; the hygrometer must be in a ventilated state: only the gauze water jacket and water quality When the wind speed meets certain requirements, the specified accuracy can be achieved. The accuracy of the wet and dry bulb hygrometer is only 5% to 7% RH.

In industrial and agricultural production,

Wet and dry bulb thermometers consist of thermometers of the same size and shape. The dry bulb thermometer is used to measure the air temperature; the ball of the other thermometer is wrapped with gauze, and one end of the gauze is introduced into a water cup, which is called a wet bulb thermometer. The indications of wet bulb thermometers are usually lower than the dry bulb temperature. According to the indications of the dry bulb and wet bulb thermometers, the absolute humidity, relative humidity, saturation difference, and dew point temperature of the air can be found by using the humidity check table. ^[2]

The accuracy of using a wet and dry meter to determine air humidity depends to a large extent on the accuracy of the wet and dry meter coefficient A. The value of A is a constant that changes very little. However, according to the experimental results, the factors affecting the wet and dry surface coefficient A are: (1) A value changes greatly with wind speed, the value is larger at small wind speeds, and decreases rapidly with increasing wind speeds. When the wind speed exceeds 3m / s, the value of A tends to be constant, so when measuring humidity with a wet bulb thermometer, a stable ventilation of more than 3m / s should be applied; (2) the A value of different types of thermometers is different, but at high The difference is small at wind speed; (3) The smaller the characteristic diameter d of the element, the smaller the A value changes with the wind speed, that is, it can reach a constant value earlier at low wind speed. The reading of a general glass thermometer can be estimated to ± 0.1 ° C. Assuming that any thermometer in the dry or wet bulb misreads 0.1 ° C, an error will occur in calculating the relative humidity. Therefore, wet and dry bulb thermometers are not suitable for measuring atmospheric humidity at low temperatures. According to China's current regulations, the use of wet and dry bulb thermometers should be stopped at -10 ° C. ^[2]

There are two main types of modern humidity measurement schemes: dry-wet bulb wet method, electronic humidity sensor wet method. The two solutions are compared below so that customers can choose the humidity measurement method that suits them.

The maintenance of the wet-wet-bulb method is quite simple. In actual use, you only need to regularly add water to the wet-bulb and replace the wet-bulb gauze. Compared with the electronic humidity sensor, the dry-wet-bulb method does not cause problems such as aging and accuracy degradation. Therefore, the dry-wet-ball humidity measurement method is more suitable for use in high-temperature and harsh environments.

Features of electronic humidity sensor:

Electronic humidity sensors have developed rapidly in recent decades, especially in the last 20 years. Humidity sensor manufacturers use standard humidity generators to calibrate one by one before the products leave the factory. The accuracy of electronic humidity sensors can reach 2% to 3% RH.

In actual use, due to the effects of dust, oil pollution and harmful gases, over a long period of use, aging will occur, the accuracy will decline, and the annual drift of the humidity sensor is generally around ± 2% or even higher. Under normal circumstances, the manufacturer will indicate that the valid use time of a single calibration is 1 year or 2 years, and recalibration is required when it expires.

The accuracy level of electronic humidity sensors should be judged in conjunction with their long-term stability. Generally speaking, the long-term stability and service life of electronic humidity sensors are not as good as those of wet and dry bulb humidity sensors.

The humidity sensor uses semiconductor technology, so it has requirements on the ambient temperature used. Exceeding its specified operating temperature will cause damage to the sensor.

Therefore, the electronic humidity sensor method is more suitable for use in clean and normal temperature occasions. ^[3]