Increased humidity in the atmospheric air in percent. Absolute and relative humidity

Career and finance 19.07.2019

Career and finance

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

Humidity is a very important parameter for humans. environment, because our body reacts very actively to its changes. For example, such a mechanism for regulating the functioning of the body 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 disturbed, which leads to violations of thermoregulation. At low humidity, the processes of evaporation of moisture prevail over the processes of condensation and the body loses too much fluid, which can lead to dehydration.

The value of humidity is important not only for humans and other living organisms, but also for the flow technological processes. For example, due to the known property of water to conduct electricity, 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 criterion for evaluating weather conditions that everyone knows from weather forecasts. It is worth noting that if we compare the humidity at different times of the year in the usual for us 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.

Main Features humid air are:

density of water vapor in air;
relative humidity air.

Air is a compound gas, it contains many different gases, including water vapour. To estimate its amount in the air, it is necessary to determine what mass the water vapor has in a certain allocated volume - this value characterizes the density. The density of water vapor in air is called absolute humidity.

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

Designationabsolute humidity : (as well as the usual notation for density).

Unitsabsolute humidity: (in SI) or (for the convenience of measuring the small amount 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 in which the indicated mass of vapor is contained, .

On the one hand, the absolute humidity of the air 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 feels not the mass content of water in the air, but its content relative to the maximum possible value.

To describe this perception, a quantity such as relative humidity.

Definition.Relative humidity- a value showing how far the steam is from saturation.

That is, the value of relative humidity, in simple terms, 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:

Notation:

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 contains the absolute humidity, with which we are already familiar, and the density of saturated vapor at the same temperature. The question arises, how to determine the last value? For this, there are special devices. We'll consider condensinghygrometer(Fig. 4) - a device that serves to determine the dew point.

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

Rice. 4. Condensation hygrometer ()

Easily evaporating liquid, for example, ether, is poured inside the container of the device, a thermometer (6) is inserted and air is pumped through the container using a pear (5). As a result of increased air circulation, intensive evaporation of the ether begins, the temperature of the container decreases because of this, and dew appears on the mirror (4) (droplets of condensed vapor). At the moment when dew appears on the mirror, the temperature is measured using a thermometer, and 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 a useful fact that with an increase in the dew point value, the value of the corresponding saturated vapor density also increases. In other words, the warmer the air, the more 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 fat-free hair to change its length with changes in air humidity (with increasing humidity, the length of the hair increases, with a decrease, it decreases), which allows measuring relative humidity. The hair is stretched over a metal frame. The change in the length of the hair is transmitted to the arrow moving along the scale. It should be remembered that the hair hygrometer gives inaccurate relative humidity values, and is used mainly for domestic purposes.

More convenient to use and accurate is such a device for measuring relative humidity as a psychrometer (from other Greek ψυχρός - “cold”) (Fig. 6).

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

On the case of the device, as a rule, the so-called psychrometric table is also depicted (Table 2). Using this table, the relative humidity of the ambient air can be determined from the temperature value indicated by the dry bulb and the temperature difference between the dry bulb and the wet bulb.

However, even without such a table at hand, you can roughly determine the amount of humidity using the following principle. If the readings of both thermometers are close to each other, then the evaporation of water from a 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 damp tissue prevails over condensation and the air is dry and the humidity is low.

Let's turn to the tables that allow you to determine the characteristics of air humidity.

Temperature,	Pressure, mm rt. Art.	steam density,

Tab. 1. Density and pressure of saturated water vapor

Once again, we note that, as mentioned earlier, the value of the density of saturated vapor increases with its temperature, the same applies to the pressure of saturated vapor.

Tab. 2. Psychometric table

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

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

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

Bibliography

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

Internet portal "dic.academic.ru" ()
Internet portal "baroma.ru" ()
Internet portal "femto.com.ua" ()
Internet portal "youtube.com" ()

Homework

Often from TV screens or from radio speakers we hear about air pressure and humidity. But few people know what their indicators depend on and how one or another of their values affects the human body.

Means and methods for determining

To determine the saturation of air with water vapor, special devices are used: psychrometers and hydrometers. August's psychrometer is a bar with two thermometers: wet and dry.

The first is wrapped in a cloth soaked in water, which, when evaporated, cools its body. Based on the readings of these thermometers, the tables determine the relative humidity of the air. There are many different hydrometers, their work can be based on weight, film, electric or hair, as well as a number of other principles of operation. AT last years integrated measurement sensors have gained popularity. Hydrostats are used to check accuracy.

Humidity is the amount of water vapor in the atmosphere. This characteristic largely determines the well-being of many living beings, and also affects the weather and climatic conditions on our planet. For normal operation human body it must be within a certain range, regardless of the air temperature. There are two main characteristics of air humidity - absolute and relative:

Absolute humidity is the mass of water vapor contained in one cubic meter of air. The absolute humidity unit is g/m3. Relative humidity is defined as the ratio of the current and maximum values of absolute humidity at a certain air temperature.
Relative humidity is usually measured in%. As the temperature increases, the absolute humidity of the air also increases from 0.3 at -30°C to 600 at +100°C. The relative humidity depends mainly on climatic zones Earth (middle, equatorial or polar latitudes) and seasons (autumn, winter, spring, summer).

There are auxiliary terms for determining humidity. For example, moisture content (g/kg), i.e. weight of water vapor per kilogram of air. Or the temperature of the "dew point", when the air is considered to be completely saturated, i.e. its relative humidity is 100%. In nature and refrigeration technology, this phenomenon can be observed on the surfaces of bodies whose temperature is lower than the dew point temperature in the form of water droplets (condensate), frost or frost.

Enthalpy

There is also such a thing as enthalpy. Enthalpy is a property of a body (substance) that determines the amount of energy stored in its molecular structure, which is available for conversion into heat at a certain temperature and pressure. But not all energy can be converted into heat, because. part of the internal energy of the body remains in the substance to maintain its molecular structure.

Moisture calculation

Simple formulas are used to calculate humidity values. So, absolute humidity is usually denoted p and defined as

p = m aq. steam / V air

where m water. steam - mass of water vapor (g)
V air - the volume of air (m 3) in which it is contained.

The generally accepted notation for relative humidity is φ. Relative humidity is calculated using the formula:

φ \u003d (p / p n) * 100%

where p and p n are the current and maximum values of absolute humidity. The value of relative humidity is most often used, since the state of the human body is largely affected not by the weight of moisture in the volume of air (absolute humidity), but rather by the relative water content.

Humidity is very important for the normal functioning of almost all living beings and, in particular, for humans. Its value (according to experimental data) should be in the range from 30 to 65%, regardless of temperature. For example, low humidity in winter (due to the small amount of water in the air) leads to drying out of all mucous membranes in a person, thereby increasing the risk colds. High humidity, on the contrary, worsens the processes of thermoregulation and sweating through the skin. This creates a feeling of suffocation. In addition, maintaining air humidity is an important factor:

for carrying out many technological processes in production;
operation of mechanisms and devices;
safety from destruction of building structures of buildings, interior elements made of wood (furniture, parquet, etc.), archaeological and museum artifacts.

Enthalpy calculation

Enthalpy is the potential energy contained in one kilogram of moist air. Moreover, in the equilibrium state of the gas, it is not absorbed and is not emitted into the external environment. The enthalpy of moist air is equal to the sum of the enthalpies of its constituent parts: absolutely dry air, as well as water vapor. Its value is calculated according to the following formula:

I = t + 0.001(2500 +1.93t)d

Where t is the air temperature (°С) and d is its moisture content (g/kg). Enthalpy (kJ/kg) is a specific quantity.

Wet bulb temperature

The wet bulb temperature is the value at which the process of adiabatic (constant enthalpy) saturation of air with water vapor takes place. To determine its specific value, an I - d diagram is used. First, a point corresponding to a given state of air is applied to it. Then an adiabatic ray is drawn through this point, crossing it with the saturation line (φ = 100%). And already from the point of their intersection, the projection is lowered in the form of a segment with a constant temperature (isotherm) and the temperature of the wet bulb is obtained.

The I-d diagram is the main tool for calculating / plotting various processes associated with a change in the state of air - heating, cooling, dehumidification and humidification. Its appearance greatly facilitated the understanding of the processes occurring in systems and units for air compression, ventilation and air conditioning. This diagram graphically shows the complete interdependence of the main parameters (temperature, relative humidity, moisture content, enthalpy and partial pressure of water vapor) that determine the heat-humidity balance. All values are specified at a specific value atmospheric pressure. Usually it is 98 kPa.

The diagram is made in the system of oblique coordinates, i.e. the angle between its axes is 135°. This contributes to an increase in the zone of unsaturated moist air (φ = 5 - 99%) and greatly facilitates the graphic drawing of the processes occurring with the air. The diagram shows the following lines:

curvilinear - humidity (from 5 to 100%).
straight lines - constant enthalpy, temperature, partial pressure and moisture content.

Below the curve φ \u003d 100%, the air is completely saturated with moisture, which is in it in the form of a liquid (water) or solid (hoarfrost, snow, ice) state. It is possible to determine the state of air at all points of the diagram, knowing any two of its parameters (out of four possible). Graphic construction of the process of changing the state of air is greatly facilitated with the help of an additionally plotted pie chart. It shows the values of the heat-humidity ratio ε at different angles. This value is determined by the slope of the process beam and is calculated as:

where Q is the heat (kJ/kg) and W is the moisture (kg/h) absorbed or released from the air. The value of ε divides the entire diagram into four sectors:

ε = +∞ … 0 (heating + humidification).
ε = 0 … -∞ (cooling + humidification).
ε = -∞ … 0 (cooling + dehumidification).
ε = 0 … +∞ (heating + dehumidification).

Humidity measurement

Measuring instruments for determining relative humidity values are called hygrometers. Several methods are used to measure air humidity. Let's consider three of them.

For relatively inaccurate measurements in everyday life, hair hygrometers are used. In them, the sensitive element is a horse or human hair, which is installed in a steel frame in a taut state. It turned out that this hair in a fat-free form is able to sensitively respond to the slightest changes in the relative humidity of the air, changing its length. As the humidity increases, the hair lengthens, and as it decreases, on the contrary, it shortens. The steel frame, on which the hair is fixed, is connected to the arrow of the device. The arrow perceives the change in the size of the hair from the frame and rotates around its axis. At the same time, it indicates the relative humidity on a graduated scale (in %).
For more accurate thermotechnical measurements during scientific research, condensation-type hygrometers and psychrometers are used. They measure relative humidity indirectly. The condensation type hygrometer is made in the form of a closed cylindrical container. One of its flat covers is polished to a mirror finish. A thermometer is installed inside the container and some low-boiling liquid, such as ether, is poured. Then, with a manual rubber diaphragm pump, air is pumped into the container, which begins to circulate intensively there. Because of this, the ether boils, lowers the temperature (cools) the surface of the container and its mirror, respectively. Drops of water condensed from the air will appear on the mirror. At this point in time, it is necessary to record the readings of the thermometer, which will show the temperature of the "dew point". Then, using a special table, the corresponding density of saturated steam is determined. And according to them, the value of relative humidity.
A psychrometric hygrometer is a pair of thermometers mounted on a base with a common scale. One of them is called dry, it measures the actual air temperature. The second is called wet. The wet bulb temperature is the temperature that humid air takes when it reaches a saturated state and maintains a constant air enthalpy equal to the initial one, i.e. this is the limiting temperature of adiabatic cooling. At the wet bulb thermometer, the ball is wrapped in a batiste cloth, which is immersed in a container of water. On the fabric, water evaporates, which leads to a decrease in air temperature. This cooling process continues until the air around the balloon is completely saturated (i.e. 100% relative humidity). This thermometer will show the "dew point". On the scale of the device there is also a so-called. psychrometric table. With its help, according to the dry bulb and the temperature difference (dry minus wet), the current value of relative humidity is determined.

Humidity control

Humidifiers are used to increase humidity (humidify the air). Humidifiers are very diverse, which is determined by the method of humidification and design. According to the method of humidification, humidifiers are divided into: adiabatic (nozzle) and steam. In steam humidifiers, water vapor is formed when water is heated on the electrodes. As a rule, steam humidifiers are most often used in everyday life. In ventilation and central air-conditioning systems, humidifiers of both steam and nozzle types are used. In industrial ventilation systems, humidifiers can be placed both directly in the ventilation units themselves, and as a separate section in the ventilation duct.

Most effective method removal of moisture from the air is carried out using compressor-based refrigeration machines. They dehumidify the air by condensing water vapor on the cooled surface of the evaporator heat exchanger. Moreover, its temperature should be below the "dew point". The moisture collected in this way is removed by gravity or with the help of a pump to the outside through the drainage pipe. Exist various types and appointments. By type, dehumidifiers are divided into monoblock and with a remote condenser. According to their purpose, dryers are divided into:

household mobile;
professional;
stationary for swimming pools.

The main task of dehumidification systems is to ensure the well-being of people inside and the safe operation of structural elements of buildings. It is especially important to maintain the level of humidity in rooms with increased moisture release, such as swimming pools, water parks, baths and SPA complexes. The air in the pool high humidity due to intensive processes of evaporation of water from the surface of the bowl. Therefore, excess moisture is the determining factor for. Excess moisture, as well as the presence of aggressive media in the air, such as chlorine compounds, have a devastating effect on the elements of building structures and interior decoration. Moisture condenses on them, causing mold growth or corrosion damage to metal parts.

For these reasons, the recommended value of relative humidity inside the pool should be maintained in the range of 50 - 60%. Building structures, in particular walls and glazed surfaces of the pool room, should be additionally protected from moisture falling on them. This can be realized by supplying a stream of fresh air to them, and always in the direction from the bottom to the top. From the outside, the building must have a layer of highly effective thermal insulation. To achieve additional benefits, we strongly recommend the use of a variety of dehumidifiers, but only in combination with optimally calculated and selected

For a person, the value of humidity is a very important parameter of the environment, since our body reacts very actively to its changes. For example, such a mechanism for regulating the functioning of the body 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 disturbed, which leads to violations of thermoregulation. At low humidity, the processes of evaporation of moisture prevail over the processes of condensation and the body loses too much fluid, which can lead to dehydration.

The value of humidity is important not only for humans and other living organisms, but also for the flow of technological processes. For example, due to the known property of water to conduct electricity, 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 criterion for evaluating weather conditions, which is known to all from weather forecasts. It should be noted that if we compare the 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:

density of water vapor in air;
relative humidity.

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

Designationabsolute humidity: (as well as the usual notation for density).

Unitsabsolute humidity: (in SI) or (for the convenience of measuring the small amount 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 in which the indicated mass of vapor is contained, .

To describe this perception, a quantity such as relative humidity.

Definition.Relative humidity- a value showing 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:

Notation:

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

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

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

Rice. 4. Condensation hygrometer ()

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.

More convenient to use and accurate is such a device for measuring relative humidity as a psychrometer (from other Greek ψυχρός - “cold”) (Fig. 6).

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

Let's turn to the tables that allow you to determine the characteristics of air humidity.

Temperature,	Pressure, mm rt. Art.	steam density,

Tab. 1. Density and pressure of saturated water vapor

Once again, we note that, as mentioned earlier, the value of the density of saturated vapor increases with its temperature, the same applies to the pressure of saturated vapor.

Tab. 2. Psychometric table

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

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

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

Bibliography

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

Internet portal "dic.academic.ru" ()
Internet portal "baroma.ru" ()
Internet portal "femto.com.ua" ()
Internet portal "youtube.com" ()

Homework

In the air, characterized by a number of quantities. The water evaporated from the surface when they are heated enters and concentrates in the lower layers of the troposphere. The temperature at which the air reaches saturation with moisture for a given water vapor content and unchanged is called the dew point.

Humidity is characterized by the following indicators:

Absolute humidity(lat. absolutus - complete). It is expressed as the mass of water vapor in 1 m of air. It is calculated in grams of water vapor per 1 m3 of air. The higher , the greater the absolute humidity, since more water changes from liquid to vapor when heated. During the day, the absolute humidity is higher than at night. The indicator of absolute humidity depends on: in polar latitudes, for example, it is up to 1 g per 1 m2 of water vapor, at the equator up to 30 grams per 1 m2 in Batumi (, coast) the absolute humidity is 6 g per 1 m, and in Verkhoyansk ( , ) - 0.1 grams per 1 m The vegetation cover of the area largely depends on the absolute humidity of the air;

Relative Humidity. This is the ratio of the amount of moisture in the air to the amount that it can hold at the same temperature. Relative humidity is calculated as a percentage. For example, the relative humidity is 70%. This means that air contains 70% of the amount of vapor that it can hold at a given temperature. If a daily course absolute humidity is directly proportional to the course of temperatures, then relative humidity is inversely proportional to this course. A person feels good when equal to 40-75%. Deviation from the norm causes a painful state of the body.

The air in nature is rarely saturated with water vapor, but always contains some amount of it. Nowhere on earth has a relative humidity of 0% been recorded. On the meteorological stations humidity is measured using a hygrometer device, in addition, recorders - hygrographs are used;

The air is saturated and unsaturated. When water evaporates from the surface of the ocean or land, the air cannot hold water vapor indefinitely. This limit depends on . Air that can no longer hold moisture is called saturated. From this air, at the slightest cooling, water droplets in the form of dew begin to stand out. This is because water, when cooled, changes from a state (vapor) to a liquid. Air above a dry and warm surface usually contains less water vapor than it could at a given temperature. Such air is called unsaturated. When it is cooled, water is not always released. The warmer the air, the greater its ability to absorb moisture. For example, at a temperature of -20°C, the air contains no more than 1 g/m of water; at a temperature of + 10°C - about 9 g/m3, and at +20°C - about 17 g/m