What is called absolute air humidity. What does relative air humidity depend on?

The weight, or more precisely the mass, of water vapor contained in 1 m3 of air is called absolute air humidity. In other words, this water vapor density in the air. At the same temperature, air can absorb a very certain amount of water vapor and reach a state of complete saturation. in the state of its saturation is called.

moisture capacity The moisture capacity of air increases sharply with increasing temperature. Magnitude ratio absolute air humidity at a given temperature to the value of its moisture capacity at the same temperature is called.

relative air humidity To determine temperature and relative air humidity They use a special device - a psychrometer. The psychrometer consists of two thermometers. The ball of one of them is moistened using a gauze cover, the end of which is lowered into a vessel with water. The other thermometer remains dry and shows the ambient temperature. A wet thermometer shows a lower temperature than a dry thermometer, since the moisture from the gauze requires a certain amount of heat. The wet bulb temperature is called cooling limit . The difference between the readings of dry and wet bulb thermometers is called.

psychrometric difference There is a certain relationship between the magnitude of the psychrometric difference and the relative one. The greater the psychrometric difference at a given air temperature, the less relative humidity air and the more moisture the air can absorb. When the difference is equal to zero, the air is saturated and further evaporation of moisture in such air.

not happening

= g/m 3

φ = (f×100)/fmax

What is dew point

Back forward Attention! Slide previews are for informational purposes only and may not represent all the features of the presentation. If you are interested this work

• , please download the full version. provide concepts of air humidity ;
• develop student independence;
• thinking; ability to draw conclusions; development of practical skills when working with physical equipment; show

practical application and importance of this physical quantity. .

Lesson type: lesson on learning new material

• Equipment: For frontal work
• : a glass of water, a thermometer, a piece of gauze; threads, psychrometric table.

for demonstrations: psychrometer, hair and condensation hygrometers, pear, alcohol.

During the classes

I. Review and check homework

1. Formulate a definition of the processes of vaporization and condensation.

2. What types of vaporization do you know? How are they different from each other?

3. Under what conditions does liquid evaporation occur?

4. What factors does the rate of evaporation depend on?

5.What is the specific heat of vaporization?

6. What is the amount of heat supplied during vaporization spent on?

7. Why is hi-fi food easier to tolerate?

8. Is the internal energy of 1 kg of water and steam at a temperature of 100 o C the same?

9. Why does water in a bottle tightly closed with a stopper not evaporate? II. Learning new things

material Water vapor in the air, despite the huge surfaces of rivers, lakes, and oceans, is not saturated; the atmosphere is an open vessel. The movement of air masses leads to the fact that in some places in this moment

evaporation of water prevails over condensation, while in others it is the other way around.

Atmospheric air is a mixture of various gases and water vapor. The pressure that water vapor would produce if all other gases were absent is called partial pressure (or elasticity)

water vapor. The density of water vapor contained in the air can be taken as a characteristic of air humidity. This quantity is called absolute humidity

[g/m3].

Knowing the partial pressure of water vapor or absolute humidity does not tell you how far the water vapor is from saturation. To do this, introduce a value showing how close water vapor at a given temperature is to saturation -

relative humidity. Relative humidity is called the ratio of absolute air humidity

to the density 0 of saturated water vapor at the same temperature, expressed as a percentage.

P is the partial pressure at a given temperature;

P 0 - saturated vapor pressure at the same temperature;

Absolute humidity;

The pressure and density of saturated steam at different temperatures can be found using special tables.

When cooling humid air at constant pressure, its relative humidity increases; the lower the temperature, the closer the partial pressure of vapor in the air is to the pressure of saturated vapor.

Temperature t, to which the air must be cooled so that the steam in it reaches a state of saturation (at a given humidity, air and constant pressure) is called dew point.

Pressure of saturated water vapor at air temperature equal to dew point, is the partial pressure of water vapor contained in the atmosphere. When the air cools to the dew point, vapor condensation begins : fog appears, falls dew. The dew point also characterizes air humidity.

Air humidity can be determined with special instruments.

1. Condensation hygrometer

It is used to determine the dew point. This is the most accurate way to change relative humidity.

2. Hair hygrometer

Its action is based on the properties of fat-free human hair With and lengthen with increasing relative humidity.

It is used in cases where great accuracy is not required in determining air humidity.

3. Psychrometer

Typically used in cases where fairly accurate and rapid determination of air humidity is required.

The value of air humidity for living organisms

At a temperature of 20-25°C, air with a relative humidity of 40% to 60% is considered most favorable for human life. When the environment has a temperature higher than the human body temperature, increased sweating occurs. Excessive sweating leads to cooling of the body. However, such sweating is a significant burden for a person.

Relative humidity below 40% at normal air temperatures is also harmful, as it leads to increased loss of moisture in organisms, which leads to dehydration. Particularly low indoor air humidity in winter time; it is 10-20%. At low air humidity it occurs rapid evaporation moisture from the surface and drying of the mucous membrane of the nose, larynx, and lungs, which can lead to deterioration of well-being. Also, at low air humidity in external environment Pathogenic microorganisms persist longer, and more static charge accumulates on the surface of objects. Therefore, in winter, residential areas are humidified using porous humidifiers. Plants are good humidifiers.

If the relative humidity is high, then we say that the air damp and suffocating. High air humidity is depressing because evaporation occurs very slowly. The concentration of water vapor in the air in this case is high, as a result of which molecules from the air return to the liquid almost as quickly as they evaporate. If sweat evaporates from the body slowly, then the body cools very little, and we do not feel very comfortable. At 100% relative humidity, evaporation cannot occur at all - under such conditions, wet clothes or damp skin will never dry.

From your biology course you know about the various adaptations of plants in arid areas. But plants are also adapted to high humidity air. So, the birthplace of Monstera is humid equatorial forest Monstera “cries” at a relative humidity close to 100%; it removes excess moisture through holes in the leaves - hydathodes. In modern buildings, air conditioning is used to create and maintain an air environment in enclosed spaces that is most favorable for people’s well-being. At the same time, temperature, humidity, and air composition are automatically regulated.

Air humidity is of exceptional importance for the formation of frost. If the humidity is high and the air is close to saturation with vapor, then when the temperature drops, the air may become saturated and dew will begin to fall. But when water vapor condenses, energy is released (the specific heat of vaporization at a temperature close to 0 ° C is 2490 kJ/kg), therefore, the air at the soil surface when dew forms will not cool below the dew point and the likelihood of frost will decrease. The probability of freezing depends, firstly, on the speed of the temperature drop and,

Secondly, from air humidity. It is enough to know one of these data to more or less accurately predict the probability of frost.

Review questions:

1. What is meant by air humidity?
2. What is absolute air humidity called?
3. What formula expresses the meaning of this concept? In what units is it expressed?
4. What is water vapor pressure?
5. What is relative humidity?
6. What formulas express the meaning of this concept in physics and meteorology? In what units is it expressed?

What instruments are used to determine air humidity? What is a person’s subjective sensation of air humidity? Having drawn a picture, explain the structure and principle of operation of hair and condensation hygrometers and psychrometers.

Laboratory work No. 4 "Measuring relative air humidity"

Goal: learn to determine relative air humidity, develop practical skills when working with physical equipment.

Equipment: thermometer, gauze bandage, water, psychometric table

for demonstrations: psychrometer, hair and condensation hygrometers, pear, alcohol.

Before completing the work, it is necessary to draw students’ attention not only to the content and progress of the work, but also to the rules for handling thermometers and glass vessels. It must be recalled that the entire time the thermometer is not used for measurements, it must be in its case. When measuring temperature, the thermometer should be held by the top edge. This will allow you to determine the temperature with the greatest accuracy.

The first temperature measurements should be taken with a dry bulb thermometer. This temperature in the classroom will not change during operation.

To measure temperature with a wet thermometer, it is better to use a piece of gauze as a cloth. Gauze absorbs very well and moves water from the wet edge to the dry one.

Using a psychrometric table, it is easy to determine the relative humidity value.

Let t c = h= 22 °C, t m = t 2= 19 °C. Then t = tc- 1 Ш = 3 °C.

Using the table we find the relative humidity. In this case it is 76%.

For comparison, you can measure the relative humidity outside. To do this, a group of two or three students who have successfully completed the main part of the work can be asked to carry out similar measurements on the street. This should take no more than 5 minutes. The resulting humidity value can be compared with the humidity in the classroom.

The results of the work are summarized in conclusions. They should note not only the formal meanings of the final results, but also indicate the reasons that lead to errors.

III. Problem solving

Since this laboratory work Quite simple in content and small in volume, the rest of the lesson can be devoted to solving problems on the topic being studied. To solve problems, it is not necessary that all students begin to solve them at the same time. As work progresses, they can receive assignments individually.

The following simple tasks can be suggested:

It's cold autumn rain outside. In what case will laundry hanging in the kitchen dry faster: when the window is open or when it is closed? Why?

Air humidity is 78%, and the dry bulb reading is 12 °C. What temperature does the wet bulb thermometer show? (Answer: 10 °C.)

The difference in the readings of dry and wet thermometers is 4 °C. Relative humidity 60%. What are the dry and wet bulb readings? (Answer: t c -l9°С, t m= 10 °C.)

Homework

• Repeat paragraph 17 of the textbook.
• Task No. 3. p. 43.

Student reports about the role of evaporation in the life of plants and animals.

Evaporation in plant life

For the normal existence of a plant cell, it must be saturated with water. For algae it is a natural consequence of the conditions of their existence; for land plants it is achieved as a result of two opposite processes: absorption of water by the roots and evaporation. For successful photosynthesis, chlorophyll-bearing cells of land plants must maintain the closest contact with the surrounding atmosphere, which supplies them with the carbon dioxide they need; however, this close contact inevitably leads to the fact that the water saturating the cells continuously evaporates into the surrounding space, and the same solar energy that supplies the plant with the energy necessary for photosynthesis, absorbed by chlorophyll, contributes to the heating of the leaf, and thereby intensifies the process of Evaporation.

Very few, and, moreover, poorly organized plants, such as mosses and lichens, can withstand long interruptions in water supply and endure this time in a state of complete drying. Of the higher plants, only some representatives of rocky and desert flora are capable of this, for example, sedge, common in the sands of the Karakum Desert. For the vast majority of dead plants, such drying out would be fatal, and therefore their water outflow is approximately equal to its inflow.

To imagine the scale of water evaporation by plants, let’s give the following example: in one growing season, one flowering of sunflower or corn evaporates up to 200 kg or more of water, i.e. a large barrel! With such energetic consumption, no less energetic water extraction is required. For this purpose (the root system, the size of which is enormous, counts the number of roots and root hairs for winter rye gave the following amazing figures: there were almost fourteen million roots, the total length of all roots was 600 km, and their total surface was about 225 m 2. On these the roots had about 15 billion root hairs with total area at 400 m 2.

The amount of water consumed by a plant during its life largely depends on the climate. In a hot, dry climate, plants consume no less, and sometimes even more, water than in a more humid climate; these plants have a more developed root system and less developed leaf surfaces. Plants in damp, shady tropical forests and the banks of water bodies use the least amount of water: they have thin, wide leaves and weak root and vascular systems. Plants in arid areas, where there is very little water in the soil and the air is hot and dry, have various methods of adaptation to these harsh conditions. Desert plants are interesting. These are, for example, cacti, plants with thick fleshy trunks, the leaves of which have turned into spines. They have a small surface with a large volume, thick covers, little permeable to water and water vapor, with a few, almost always closed stomata. Therefore, even in extreme heat, cacti evaporate little water.

Other plants of the desert zone (camel thorn, steppe alfalfa, wormwood) have thin leaves with wide open stomata, which vigorously assimilate and evaporate, due to which the temperature of the leaves is significantly reduced. Often the leaves are covered with a thick layer of gray or white hairs, representing a kind of translucent screen that protects plants from overheating and reduces the intensity of evaporation.

Many desert plants (feather grass, tumbleweed, heather) have hard, leathery leaves. Such plants can tolerate long-term wilting. At this time, their leaves curl into a tube, with the stomata located inside it.

Evaporation conditions change dramatically in winter. Roots cannot absorb water from frozen soil. Therefore, due to leaf fall, the evaporation of moisture by the plant is reduced. In addition, in the absence of leaves, less snow lingers on the crown, which protects plants from mechanical damage.

The role of evaporation processes for animal organisms

Evaporation is the most easily controlled method of reducing internal energy. Any conditions that make mating difficult disrupt the regulation of heat transfer from the body. So, leather, rubber, oilcloth, synthetic clothing makes it difficult to regulate body temperature.

Sweating plays an important role in the thermoregulation of the body; it ensures the constancy of the body temperature of a person or an animal. Due to the evaporation of sweat, internal energy decreases, thanks to which the body cools down.

Air with a relative humidity of 40 to 60% is considered normal for human life. When the environment has a temperature higher than the human body, then enhanced occurs. Abundant sweating leads to cooling of the body, helps to work in conditions high temperature. However, such active sweating is a significant burden for a person! If at the same time the absolute humidity is high, then living and working becomes even harder (humid tropics, some workshops, for example dyeing).

Relative humidity below 40% at normal air temperatures is also harmful, as it leads to increased loss of moisture from the body, which leads to dehydration.

Some living creatures are very interesting from the point of view of thermoregulation and the role of evaporation processes. It is known, for example, that a camel can go without drinking for two weeks. This is explained by the fact that it uses water very economically. A camel hardly sweats even in forty-degree heat. Its body is covered with thick and dense hair - the wool saves from overheating (on the back of a camel on a sultry afternoon it is heated to eighty degrees, and the skin under it is only up to forty!). Wool also prevents the evaporation of moisture from the body (in a shorn camel, sweating increases by 50%). A camel never, even in the most intense heat, opens its mouth: after all, from the mucous membrane of the oral cavity, if you open your mouth wide, you evaporate a lot of water! The camel's breathing rate is very low - 8 times a minute. Thereby less water leaves the body with air. In hot weather, however, his breathing rate increases to 16 times per minute. (Compare: under the same conditions, a bull breathes 250 times, and a dog - 300-400 times per minute.) In addition, the camel’s body temperature drops at night to 34°, and during the day, in the heat, it rises to 40-41°. This is very important for saving water. The camel also has a very interesting device for storing water for future use. It is known that fat, when it “burns” in the body, produces a lot of water - 107 g from 100 g of fat. Thus, if necessary, a camel can extract up to half a hundredweight of water from its humps.

From the point of view of economy in water consumption, American jerboa jumpers (kangaroo rats) are even more amazing. They never drink at all. Kangaroo rats live in the Arizona desert and chew seeds and dry grass. Almost all the water that is in their body is endogenous, i.e. produced in cells during the digestion of food. Experiments have shown that from 100 g of pearl barley, which was fed to kangaroo rats, they received, after digesting and oxidizing it, 54 g of water!

In the thermoregulation of birds big role air sacs play. In hot weather, moisture evaporates from the inner surface of the air sacs, which helps cool the body. II connection with this bird in hot weather opens its beak. (Katz //./> Biophysics in physics lessons. - M.: Education, 1974).

n. Independent work

Which amount of heat released complete combustion 20 kg coal? (Answer: 418 MJ)

How much heat will be released during the complete combustion of 50 liters of methane? Take the methane density to be 0.7 kg/m3. (Answer: -1.7 MJ)

On a cup of yogurt it is written: energy value 72 kcal. Express the energy value of the product in J.

The calorific value of the daily diet for schoolchildren your age is about 1.2 MJ.

1) Is 100 g of fatty cottage cheese, 50 g of wheat bread, 50 g of beef and 200 g of potatoes sufficient for you? Required additional data:

• fat cottage cheese 9755;
• wheat bread 9261;
• beef 7524;
• potatoes 3776.

2) Is consuming 100 g perch per day enough for you, 50 g fresh cucumbers, 200 g grapes, 100 g rye bread, 20 g sunflower oil and 150 g ice cream.

Specific heat of combustion q x 10 3, J/kg:

• perch 3520;
• fresh cucumbers 572;
• grapes 2400;
• rye bread 8884;
• sunflower oil 38900;
• creamy ice cream 7498. ,

(Answer: 1) Approximately 2.2 MJ consumed - enough; 2) Consumed To 3.7 MJ is enough.)

When preparing for lessons, you spend about 800 kJ of energy within two hours. Will you regain your energy if you drink 200 ml of skim milk and eat 50 g of wheat bread? The density of skim milk is 1036 kg/m3. (Answer: Approximately 1 MJ consumed is enough.)

Water from the beaker was poured into a vessel heated by the flame of an alcohol lamp and evaporated. Calculate the mass of burnt alcohol. Heating of the vessel and losses due to heating the air can be neglected. (Answer: 1.26 g.)

• What amount of heat will be released during the complete combustion of 1 ton of anthracite? (Answer: 26.8. 109 J.)
• What mass of biogas must be burned to release 50 MJ of heat? (Answer: 2 kg.)
• How much heat will be released during the combustion of 5 liters of fuel oil? Raft ness take fuel oil equal to 890 kg/m 3. (Answer: approximately 173 MJ.)

On the box of chocolates it is written: calorie content 100 g 580 kcal. Express the nilor content of the product in J.

Study the labels of different food products. Write down the energy I, with what is the value (calorie content) of products, expressing it in joules or k-Yuries (kilocalories).

When riding a bicycle in 1 hour, you spend approximately 2,260,000 J of energy. Will you restore your energy levels if you eat 200 g of cherries?

IN this lesson The concept of absolute and relative air humidity will be introduced, terms and quantities associated with these concepts will be discussed: saturated steam, dew point, instruments for measuring humidity. During the lesson we will get acquainted with the tables of density and saturated vapor pressure and the psychrometric table.

For humans, humidity is a very important parameter. environment, because our body reacts very actively to its changes. For example, a mechanism for regulating the functioning of the body, such as sweating, is directly related to the temperature and humidity of the environment. At high humidity, the processes of evaporation of moisture from the surface of the skin are practically compensated by the processes of its condensation and the removal of heat from the body is disrupted, which leads to disturbances in thermoregulation. At low humidity, moisture evaporation processes prevail over condensation processes and the body loses too much fluid, which can lead to dehydration.

The amount of humidity is important not only for humans and other living organisms, but also for the flow of technological processes. For example, due to known property water conducts electric current; its content in the air can seriously affect the correct operation of most electrical appliances.

In addition, the concept of humidity is the most important evaluation criterion weather conditions, which everyone knows from weather forecasts. It is worth noting that if we compare humidity at different times of the year in our usual climatic conditions, then it is higher in summer and lower in winter, which is associated, in particular, with the intensity of evaporation processes at different temperatures.

The main characteristics of humid air are:

1. density of water vapor in the air;
2. relative humidity.

Air is a composite gas and contains many different gases, including water vapor. To estimate its amount in the air, it is necessary to determine what mass water vapor has in a certain allocated volume - this value is characterized by density. The density of water vapor in the air is called The density of water vapor contained in the air can be taken as a characteristic of air humidity. This quantity is called.

Definition.Absolute air humidity- the amount of moisture contained in one cubic meter of air.

Designationabsolute humidity: (as is the usual designation for density).

Unitsabsolute humidity: (in SI) or (for the convenience of measuring small amounts of water vapor in the air).

Formula calculations absolute humidity:

Designations:

Mass of steam (water) in air, kg (in SI) or g;

The volume of air containing the indicated mass of steam is .

On the one hand, absolute air humidity is an understandable and convenient value, since it gives an idea of ​​the specific water content in the air by mass; on the other hand, this value is inconvenient from the point of view of the susceptibility of humidity by living organisms. It turns out that, for example, a person does not feel the mass content of water in the air, but rather its content relative to the maximum possible value.

To describe such perception, the following quantity was introduced: relative humidity.

Definition.Relative humidity– a value indicating how far the steam is from saturation.

That is, the value of relative humidity, in simple words, shows the following: if the steam is far from saturation, then the humidity is low, if it is close, it is high.

Designationrelative humidity: .

Unitsrelative humidity: %.

Formula calculations relative humidity:

Designations:

Water vapor density (absolute humidity), (in SI) or ;

Density of saturated water vapor at a given temperature, (in SI) or .

As can be seen from the formula, it includes absolute humidity, with which we are already familiar, and saturated vapor density at the same temperature. The question arises: how to determine the latter value? For this there are special devices. We'll consider condensinghygrometer(Fig. 4) - a device that is used to determine the dew point.

Definition.Dew point- the temperature at which steam becomes saturated.

Rice. 4. Condensation hygrometer ()

An easily evaporating liquid, for example, ether, is poured into the container of the device, a thermometer (6) is inserted, and air is pumped through the container using a bulb (5). As a result of increased air circulation, intensive evaporation of ether begins, the temperature of the container decreases because of this and dew (droplets of condensed steam) appears on the mirror (4). At the moment dew appears on the mirror, the temperature is measured using a thermometer; this temperature is the dew point.

What to do with the obtained temperature value (dew point)? There is a special table in which data is entered - what density of saturated water vapor corresponds to each specific dew point. It should be noted useful fact, that as the dew point increases, the value of the corresponding saturated vapor density also increases. In other words, the warmer the air, the greater the amount of moisture it can contain, and vice versa, the colder the air, the lower the maximum vapor content in it.

Let us now consider the principle of operation of other types of hygrometers, devices for measuring humidity characteristics (from the Greek hygros - “wet” and metreo - “I measure”).

Hair hygrometer(Fig. 5) - a device for measuring relative humidity, in which hair, for example human hair, acts as an active element.

The action of a hair hygrometer is based on the property of defatted hair to change its length when air humidity changes (with increasing humidity, the length of the hair increases, with decreasing it decreases), which makes it possible to measure relative humidity. The hair is stretched over a metal frame. The change in hair length is transmitted to the arrow moving along the scale. It should be remembered that a hair hygrometer does not give accurate relative humidity values, and is used primarily for domestic purposes.

A more convenient and accurate device for measuring relative humidity is a psychrometer (from the ancient Greek ψυχρός - “cold”) (Fig. 6).

A psychrometer consists of two thermometers, which are fixed on a common scale. One of the thermometers is called a wet thermometer because it is wrapped in cambric fabric, which is immersed in a reservoir of water located on the back of the device. Water evaporates from the wet fabric, which leads to cooling of the thermometer, the process of reducing its temperature continues until the stage is reached until the steam near the wet fabric reaches saturation and the thermometer begins to show the dew point temperature. Thus, the wet bulb thermometer shows a temperature less than or equal to the actual ambient temperature. The second thermometer is called a dry thermometer and shows the real temperature.

On the body of the device, as a rule, there is also a so-called psychrometric table (Table 2). Using this table, you can determine the relative humidity of the surrounding air from the temperature value shown by the dry bulb thermometer and from the temperature difference between the dry and wet bulb bulbs.

However, even without such a table at hand, you can approximately determine the humidity value using the following principle. If the readings of both thermometers are close to each other, then the evaporation of water from the humid one is almost completely compensated by condensation, i.e., the air humidity is high. If, on the contrary, the difference in thermometer readings is large, then evaporation from the wet fabric prevails over condensation and the air is dry and humidity is low.

Let us turn to the tables that allow us to determine the characteristics of air humidity.

Table 1. Density and pressure of saturated water vapor

Let us note once again that, as stated earlier, the value of the density of saturated steam increases with its temperature, the same applies to the pressure of saturated steam.

Table 2. Psychometric table

Let us recall that relative humidity is determined by the value of the dry bulb readings (first column) and the difference between the dry and wet readings (first row).

In today's lesson we learned about an important characteristic of air - its humidity. As we have already said, humidity decreases in the cold season (winter) and increases in the warm season (summer). It is important to be able to regulate these phenomena, for example, if it is necessary to increase humidity, place several reservoirs of water indoors in winter in order to enhance evaporation processes, however, this method will only be effective at the appropriate temperature, which is higher than outside.

In the next lesson we will look at what gas work is and the principle of operation of an internal combustion engine.

Bibliography

1. Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

1. Internet portal “dic.academic.ru” ()
2. Internet portal “baroma.ru” ()
3. Internet portal “femto.com.ua” ()
4. Internet portal “youtube.com” ()

Homework

For quantification Air humidity uses absolute and relative air humidity.

Absolute air humidity is measured by the density of water vapor in the air, or its pressure.

A clearer idea of ​​the degree of air humidity is given by relative humidity B. Relative air humidity is measured by a number showing what percentage the absolute humidity is of the water vapor density necessary to saturate the air at its existing temperature:

Relative humidity can also be determined by vapor pressure, since practically vapor pressure is proportional to its density. Therefore, B can be determined this way: relative humidity is measured by a number showing what percentage the absolute humidity is of the pressure of water vapor saturating the air at its existing temperature:

Thus, relative humidity is determined not only by absolute humidity, but also by air temperature. When calculating relative humidity, the values ​​or must be taken from the tables (see Table 9.1).

Let's find out how changes in air temperature can affect its humidity. Let the absolute humidity of the air be equal to Since the density of saturating water vapor at 22 °C is equal (Table 9.1), then the relative humidity B is about 50%.

Let us now assume that the temperature of this air drops to 10°C, but the density remains the same. Then the relative air humidity will be 100%, i.e. the air will be saturated with water vapor. If the temperature drops to 6 °C (for example, at night), then kg of water vapor will condense from each cubic meter of air (dew will fall).

Table 9.1. Pressure and density of saturating water vapor at different temperatures

The temperature at which the air becomes saturated with water vapor during its cooling process is called the dew point. In the above example, the dew point is Note that with a known dew point, the absolute air humidity can be found from the table. 9.1, since it is equal to the saturation vapor density at the dew point.

One of the most important characteristics compressed air, used in industry, food industry, medicine and other industries, is humidity. This article defines the concept of “air humidity”, provides tables for determining the dew point depending on temperature and relative humidity, values ​​of saturated vapor pressure above the surface of water and ice, values ​​of absolute humidity. And also, a table of correction factors for converting the relative humidity of air saturated relative to water into the relative humidity of air saturated relative to ice.

The most general definition is this: humidity is a measure characterizing the content of water vapor in air (or other gas). This definition, of course, does not pretend to be “science-intensive”, but gives the physical concept of humidity.

To quantify the “humidity” of gases, the following characteristics are most often used:

• partial pressure of water vapor (p)- pressure that water vapor included in atmospheric or compressed air would have if it alone occupied a volume equal to the volume of air at the same temperature. The total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual components of this mixture .
• relative humidity- is defined as the ratio of the actual air humidity to its maximum possible humidity, i.e. relative humidity shows how much moisture is still missing for condensation to begin under given environmental conditions. A more “scientific” formulation is this: relative humidity is a value defined as the ratio of the partial pressure of water vapor (p) to the saturated vapor pressure at a given temperature, expressed as a percentage.
• dew point temperature(frost), is defined as the temperature at which the partial pressure of steam saturated relative to water (ice) is equal to the partial pressure of water vapor in the characterized gas. That is, this is the temperature at which the process of moisture condensation begins. Practical significance dew point is that it shows what maximum amount of moisture can be contained in the air at a specified temperature. Indeed, the actual amount of water that can be held in a constant volume of air depends only on temperature. The concept of dew point is the most convenient technical parameter. Knowing the value of the dew point, we can safely say that the amount of moisture in a given volume of air will not exceed a certain value.
• absolute humidity, defined as the mass content of water per unit volume of gas. this is a value showing how much water vapor is contained in a given volume of air, this is the most general concept, it is expressed in g/m3. At very low gas humidity, a parameter such as moisture content, the unit of measurement of which is ppm (parts per million - parts per million). This is an absolute value that characterizes the number of water molecules per million molecules of the entire mixture. It does not depend on temperature or pressure. This is understandable; the number of water molecules cannot increase or decrease with changes in pressure and temperature.

Dependences of saturated vapor pressure over a flat surface of water and ice on temperature, obtained theoretically based on the Clausius-Clapeyron equation and verified with the experimental data of many researchers, are recommended for meteorological practice by the World Meteorological Organization (WMO):

ln p sw =-6094.4692T -1 +21.1249952-0.027245552 T+0.000016853396T 2 +2.4575506 lnT
ln p si = -5504.4088T -1 - 3.5704628-0.017337458T+ 0.0000065204209T 2 + 6.1295027 lnT,

where p sw is the saturated vapor pressure above a flat water surface (Pa);
p si - saturated vapor pressure over a flat ice surface (Pa);
T - temperature (K).

The given formulas are valid for temperatures from 0 to 100ºC (for p sw) and from -0 to -100ºC (for p si). At the same time, WMO recommends the first formula for negative temperatures for supercooled water (up to -50ºC).

It is obvious that these formulas are quite cumbersome and inconvenient for practical work, therefore, in calculations it is much more convenient to use ready-made data compiled in special tables. Below are some of these tables.

Table 1. Definitions of dew point depending on temperature and relative humidity

Table 2. Values ​​of saturated vapor pressure above a flat surface of water (p sw) and ice (p si).

Table 3. Values ​​of saturated vapor pressure above a flat water surface (p sw).

Table 4. Values ​​of absolute gas humidity with relative humidity of water 100% at different temperatures.

Let's give an example of using the above tables in practical activities: with a capacity of 10 m 3 /min, it “sucks” 10 cubic meters per minute atmospheric air.

Let's find the amount of water contained in 10 cubic meters of atmospheric air with parameters temperature +25 ° C, relative humidity 85%. According to Table 4, air with a temperature of +25 °C and one hundred percent humidity contains 23.04 g/m 3 of water. This means that at 85% humidity, one cubic meter of air will contain 0.85 * 23.04 = 19.584 g of water, and ten - 195.84 g.

As air is compressed, the volume it occupies will decrease. The reduced volume of compressed air at a pressure of 6 bar can be calculated based on the Boyle-Mariotte law (the air temperature does not change significantly):

P1 x V1 = P2 x V2

V2 = (P1 x V1) / P2

Where P1- atmospheric pressure equal to 1.013 bar;
V2= (1.013 bar x 10 m 3)/ (6 + 1.013) bar = 1.44 m 3.

That is, 10 cubic meters of atmospheric air, during the compression process, “turned” into 1.44 m 3 of compressed air, with an excess pressure of 6 bar, at the outlet of the compressor.