Air humidity, that is, the content of water vapor in the air, is used to characterize the dryness of the atmosphere. At a certain temperature, the less water vapor contained in a certain volume of air, the drier the air; the more water vapor, the more humid the air. In weather forecasting, the concept of relative humidity is more commonly used, which reflects the possibility of rain or fog. In medicine, the relationship between air humidity and breathing is very close: at a certain humidity, oxygen is easier to enter the blood through the alveoli.
The relative humidity at which the human body feels comfortable is below 70%; when the relative humidity reaches 45% to 55%, it feels most comfortable. Too high humidity affects the perspiration function of regulating body temperature, and people will feel stuffy.
So, how is humidity measured?
The earliest moisture measuring instrument
The earliest moisture measuring instrument in my country appeared in the Western Han Dynasty, called a balance hygrometer, which appeared more than 1600 years earlier than foreign moisture measuring instruments. The structure of this hygrometer is very simple: it is suspended at the midpoint of the column with a wooden pole of uniform thickness, and objects with different moisture absorption capacities are hung on both ends. It looks like a balance.
Balance Hygrometer
The book “Huainanzi” written by Liu An, the king of Huainan in the Western Han Dynasty, once recorded balance-type hygrometers, describing the use of feathers and charcoal to measure humidity to predict rainfall.
The book also pointed out the principle of moisture measurement: because carbon has strong hygroscopicity, it is also easy to become dry, and it is easy to produce quality changes with the change of air humidity. “Dry, so carbon is light; wet, so carbon is heavy”. It can be described in scientific language as that the quality of feathers or charcoal is less affected by temperature and remains basically unchanged. Hang feathers or charcoal on both ends of the balance to keep it in a balanced state. When the humidity changes, the quality of the charcoal changes with the humidity, but the feathers do not change significantly, so the balance loses balance. After many measurements, the humidity can be roughly known from the size of the tilt angle of the balance.
During the Three Kingdoms period, Meng Kang recorded in detail the structure of the balance-type hygrometer: hanging iron and charcoal on both ends of the balance to balance the balance. In winter, the air is relatively dry, the carbon end is upturned, and the iron end is lowered; in summer, the air is relatively humid, the carbon end is lowered, and the iron end is upturned.
These two records about balance-type hygrometers are not all. There are many similar records in ancient books. The other end of the balance-type hygrometer is not the same. There are iron and charcoal as well as feathers and charcoal. There are also people who use soil and charcoal, in short, they can’t do without charcoal.
Because this kind of balance-type hygrometer has the advantages of simplicity, ease of manufacture, and acceptable sensitivity, it is still used in some rural areas. However, this type of hygrometer also has the disadvantages of being too large and not accurate.
In the Ming and Qing Dynasties, a deer tendon-type hygrometer appeared: a 2-foot-long deer gut was clamped on the top of the shelf, and there was a pointer disc underneath. The change in air humidity would cause the disc to rotate, which can be accurately obtained. humidity. Western scholars believe that this deer tendon hygrometer is the prototype of modern hygrometer.
In Europe, it was not until the middle of the 16th century that someone used the intestines of birds and beasts to make strings to measure the humidity of the atmosphere. The skins of birds and beasts have the most obvious effect on drying dampness, so people use tendons to make instruments, such as the deer tendon hygrometer mentioned earlier.
The ancients had a coup for measuring wetness
In the final analysis, the invention of the hygrometer originated from the ancients’ observation of the rules of sunny and rainy weather. There are many proverbs and native methods about predicting rain or shine, many of which are the crystallization of scientific experience.
In the Tang Dynasty, there are records such as “Those who naturally grow water on the wall, the sky will rain heavily” and “Ishigami Jin runs out the liquid and will rain for several days.” In the Northeast, there are proverbs such as “the water tank wears skirts, the heavy rain pours”, “the salt turns water, the sky will flood” and so on.
The length of the guqin strings will change with the change of the external humidity, which will cause obvious changes in the tone of the guqin. Guqin therefore became the prototype of a hygrometer.
Xu Guangqi, a scientist in the Ming Dynasty, quoted an agricultural proverb in his “Nongzheng Quanshu”: “Put the willow green on the eaves, and the peasants can look forward to the sunshine; the peasants are good to be sweet when the willows are inserted into the eaves.” The “zuojiao” refers to wine making . This agricultural proverb is translated into the vernacular: If the willow branches that are folded and inserted into the eaves remain evergreen, it means that the air is humid and the water in the willow leaves is not easy to evaporate, so the weather will not clear; if the willow leaves are yellow, it is because the air is dry. , The water in the willow leaves evaporates quickly, the weather will clear up, and the temperature will rise, just in time for winemaking.
Since the Yuan Dynasty, the folks also have a method of “weighing water”, which can be used to predict sunny and rainy. This method is to fill a bottle of water every night and weigh it the next morning. If the weight is still heavy, it means that the water evaporates slowly and the weather is easy to rain. On the contrary, it shows that the air is very dry and it is not easy to rain.
It has been found that the most important factor affecting sunny and rainy is atmospheric humidity, which is clearly recorded in the book “Huainanzi” in the Western Han Dynasty, that is, “wet and easy to rain”. Therefore, the ancients paid much attention to the phenomenon that was affected by atmospheric humidity, and the Warring States Guqin was the prototype of the hygrometer.
The length of the guqin strings will change with the change of the external humidity, which will cause obvious changes in the tone of the guqin. Guqin has therefore become the prototype of a hygrometer.
The correct way to wrap the wet ball cloth. The picture on the right shows the cut wet ball gauze.
Ventilated wet and dry table
As early as the Warring States period, people knew that “changes in wind and rain can be known by sound.” The same was said in the later “Huainanzi”. In this regard, the Eastern Han Dynasty scientist Wang Chong pointed out in his “Lun Heng”: “The strings are slow” is a sign of rain. This is a very correct experience. Because when it is about to rain, the air humidity will increase significantly, and the tensioned strings will absorb moisture and become long and slack, so the pitch becomes lower. The change in the length of the strings with atmospheric humidity is very small and not easily noticeable, but the change in pitch caused by this is very obvious. Therefore, it is very sensitive and ingenious to forecast the weather through the change of tone, and it is also very ingenious. The principle of the hanging string hygrometer has been conceived in it.
The so-called hanging string hygrometer is a hygrometer made according to the principle that the length of a certain “hanging string” changes with humidity. In this way, the Guqin from the Warring States Period played the role of this kind of hygrometer.
Wet and dry bulb method and “Asman”
In the Centennial Meteorological Exhibition Hall in Yingkou, Liaoning Province, there is a ventilated psychrometer made in Japan in 1939. This kind of ventilated wet and dry meter was first manufactured by German R. Assmann in 1887, so it is collectively referred to as “Assmann”.
”Assman” is suitable for measuring air temperature and humidity during field observation. It is equipped with two identical thermometers. The bulb of the thermometer is in the center of the double-layer anti-radiation sleeve. The outer surface of the sleeve is electroplated with chromium with good reflectivity. Floor. The inner wall of the sleeve is painted black to absorb the radiation from the inner layer of the cover and prevent it from being reflected to the sensor. The upper part of the instrument is a ventilator, which is powered by a clockwork or a micro-motor. When the ventilator blades rotate, air will be sucked in from the sleeve port where the thermometer bulb is located, and flowed to the blades through the fork pipe and the main pipe. Drain the instrument.
The correct way of dressing the ball cloth. The picture on the right shows the cut wet ball gauze.
Psychrometer
In fact, the principle of measuring humidity “Asman” and louvers box of wet and dry bulb thermometer is the same. We understand the humidity measurement principle of the wet and dry bulb thermometer, and also understand the humidity measurement principle of “Assman”.
If you have the opportunity to go to a meteorological observatory, you can often see the weather shutters. Two thermometers with exactly the same structure are hung vertically in the louver box: the bulb on the east side is exposed to the air to measure the temperature and is called a “dry bulb thermometer”; the bulb on the west side is wrapped with distilled water. The moistened gauze is called a “wet bulb thermometer”. Together, these two thermometers can measure the relative humidity of the air.
Its working principle is: the moisture in the wet ball gauze continuously evaporates to the surrounding air and takes away heat, so that the temperature near the thermometer drops; and the moisture evaporation rate is related to the moisture content of the surrounding air-the lower the relative humidity of the air, The faster the moisture evaporates, the lower the wet bulb temperature. It can be seen that there is a certain functional relationship between the air humidity and the temperature difference between the dry and wet bulbs. The wet and dry bulb thermometer uses this relationship to determine the relative humidity of the air.
The accuracy of measuring air humidity has a lot to do with the accuracy of the wet bulb temperature indication. To make the wet bulb temperature display accurate, the wet bulb surface must have good evaporation and heat exchange. Usually use gauze with good water absorption to wrap the wet bulb thermometer, and pay attention to keeping the gauze clean, and use pure distilled water to moisten the gauze.
In the meteorological department, the ball part of the wet bulb thermometer is uniformly bandaged with allotted gauze in a prescribed manner. The gauze is generally changed once a week, and it can be changed at any time when the gauze is easy to get dirty in sandy weather.
When the weather turns cold, some wet ball gauze will freeze. At this time, take the water cup back indoors from the louver box to prevent freezing and cracking. At the same time, cut the gauze on the wet ball two or three centimeters below the ball, and restore the original length of the gauze until the weather becomes warmer and the wet ball no longer freezes.
During the icing of the wet ball, water can not be supplied with a water cup, and the ball gauze must be moistened before each observation. This step is called “melting ice”. The purpose of ice melting is to make the gauze on the wet ball have enough moisture or ice coat to keep the surface of the wet ball in good evaporation condition to obtain the correct wet ball indication. It should be noted that when the temperature (indicated by the dry bulb temperature gauge) is below -10°C, stop observing the wet bulb temperature and use a hair hygrometer or hair hygrometer to measure the humidity.
The temperature and humidity measured by the dry and wet bulb thermometer in the louver box are certainly accurate, but if we want to go to farmland in the wild or observe meteorological elements in the greenhouse, the louver box cannot be taken away. In this case, “Asman” can show its talents. Compared with the louver box, “Asman” is small and exquisite, easy to carry, and people still use it today.
