Consider the device and principle of operation Psychrometer- more accurate instrument for measuring air humidity. The psychrometer has two thermometers: dry and wet. They are so called because the end of one of the thermometers is in the air, and the end of the second is lined with a piece of gauze immersed in water (see Figure). Evaporation of water from the surface of the gauze leads to a decrease in its temperature. The second, "dry" thermometer shows the usual air temperature. The temperature measured by the psychrometer is to translate into the value of the relative humidity of the air in the table (see below).
| Dry thermometer, ° C | The difference of thermometer readings, ° C | ||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
| Relative humidity,% | |||||||||
| 18 | 91 | 82 | 73 | 65 | 56 | 49 | 41 | 34 | 27 |
| 20 | 91 | 83 | 74 | 66 | 59 | 51 | 44 | 37 | 30 |
| 22 | 92 | 83 | 76 | 68 | 61 | 54 | 47 | 40 | 34 |
| 24 | 92 | 84 | 77 | 69 | 62 | 56 | 49 | 43 | 37 |
Consider an example. Suppose the temperature is 20 ° C in the room, and the wet thermometer shows 15 ° C. That is the difference in the thermometers of 5 ° C. In the table, on the line "20", moving to the column "5". We read there: 59. Consequently, relative humidity The air in the room where the psychrometer hangs is 59%.
If in the air where a psychrometer is located, little water vapor, then the vaporization from the surface of Marli is intensively. According to the formula Q \u003d Rm (see § 6-g), it is consumed by the heat "selected" by the water on the marla, and it is cooled according to the formula $ Q \u003d C \\ Cdot M \\ Cdot \\ Delta T ^ O $ (see § 6-c). therefore a wet thermometer shows a smaller temperature than dry. If the air is so humid that the water pairs contained in it is saturated, then there will be no evaporation of water from the surface of the gauze. Consequently, both thermometers will show equal temperatures, and this will mean that the relative air humidity is 100%.
Check how you learned the material:
- The purpose of this paragraph is to consider ...
- Air humidity is important not only for human health, but also for ...
- Why is it important that water steam contained in the air is not (almost) saturated?
- We introduced new physical quantity Should show ...
- The relative humidity of the air is calculated by the ratio of the density of water vapor in the air to ...
- Hair hygrometer is ...
- The change in relative humidity in the hygrometer reacts ...
- The hygrometer allows (capable) to measure the relative humidity of the air, since ...
- Ease of using a hygrometer for humidity measurements is that its arrow ...
- Instead of a hygrometer, a psychrometer is often used as ...
- Why is the right thermometer of a psychrometer usually shows a lower temperature?
- A specially composed so-called psychrometric table applies to ...
- If the air temperature in the room is 30 ° C, and on a humid thermometer 25 ° C, then ...
- With what condition evaporating water from the surface of Marli is fast?
- Wet marl, and with it and the right thermometer is cooled, because ...
- With what condition both thermometers will show equal temperatures?
One of the very important indicators in our atmosphere. It can be both absolute and relative. As measured absolute humidity And what formula need to apply for this? You can find out about it by reading our article.
Air humidity - what is it?
What is humidity? This is the amount of water contained in any physical body or environment. This indicator directly depends on the nature of the medium or substance, as well as on the degree of porosity (if we are talking about solid bodies). We will talk about the specific form of humidity in this article - about air humidity.
From the course of chemistry, we all well know that the atmospheric air consists of nitrogen, oxygen, carbon dioxide and some other gases, which make up no more than 1% of the total mass. But besides these gases, air also contains water vapor and other impurities.
Under the humidity of the air understand the amount of water vapor that this moment (and B. this place) Contained in air mass. At the same time, meteorologists allocate two of its values: it is absolute and relative humidity.
Air humidity is one of the most important characteristics of the atmosphere of the Earth, which affects the character. local weather. It is worth noting that the magnitude of humidity atmospheric air Nonodynakova - both in a vertical section and in a horizontal (latom). So, if the relative indicators of air humidity (in lower layer The atmosphere) make up about 0.2-0.5%, then in tropical - up to 2.5%. Next, we find out what is absolute and relative humidity. Also consider which difference exists between these two indicators.
Absolute Humidity: Definition and Formula
Translated from Latin, the word Absolutus means "full". Based on this, the essence of the concept of "absolute air humidity" becomes obvious. This value that shows how many grams of water vapor is actually contained in one cubic meter specific air mass. As a rule, this indicator is denoted by the Latin Literary F.
G / m 3 is a unit of measurement in which absolute humidity is calculated. The formula for its calculation is as follows:
In this formula, the letter M is indicated by the mass of water vapor, and the letter V is the volume of the specific air mass.
The magnitude of absolute humidity depends on several factors. First of all, this air temperature and the nature of the advection processes.
Relative humidity
Now consider what relative air humidity. it relative valuewhich shows how much moisture is contained in air relative to the maximum possible amount of water vapor in this air mass at a specific temperature. The relative humidity of air in percent (%) is measured. And it is precisely this percentage that we can often learn in weather forecasts and weather engineers.
It is also worth mentioning about such an important concept as a dew point. This phenomenon of the maximum possible saturation of the air mass of water vapor (the relative humidity of this moment is 100%). In this case, the excess moisture is condensed, and formed precipitation, fog or clouds.
Air humidity measurement methods
Women know that to detect the increase in humidity in the atmosphere can be using their lush hairstyles. However, there are other, more accurate, methods and technical devices. Such are the hygrometer and psychrometer.
The first hygrometer was created in the XVII century. One of the types of this device is just based on the properties of the hair to change its length with changes in environmental humidity. However, today there are e-hygrometers. A psychrometer is a special device in which there is a wet and dry thermometer. By the difference of their indicators and determine the humidity of the air at a particular point in time.
Air humidity as an important ecological indicator
It is believed that the relative humidity of air 40-60% is considered to be optimal for the human body. Humidity indicators are highly affected by the perception of air temperature. So, with low moisture, it seems to us that the air is much colder than in reality (and on the contrary). That is why in tropical and equatorial latitudes Our planet travelers are so hard to experience the heat and heat.
Today there are special humidifiers and dehumidifiers that help a person to regulate air humidity in closed rooms.
Finally...
Thus, the absolute humidity of the air is the most important indicatorwhich gives us an idea of \u200b\u200bthe state and features air mass. At the same time, you need to be able to distinguish this value from relative humidity. And if the latter shows the proportion of water vapor (in percent), which is present in the air, the absolute humidity is the actual amount of water vapor in grams in one cubic air meter.
Often from TV screens or from the speakers of radios, we hear about the pressure and humidity of the air. But few know why their indicators depend on and how certain their values \u200b\u200baffect the human body.
To determine air saturation with water vapors, special devices are used: psychoometers and hydrometers. The psychrometer of August is a bar with two thermometers: wet and dry.
The first wrapped with a cloth moistened in water, which during evaporation cools its housing. Relying on the testimony of these thermometers, the relative humidity of the air determine the tables. 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 action. IN last years Integral measurement sensors have gained popularity. In order to check the accuracy of hydrostates are used.
Pressure saturated vapor Water grows greatly with increasing temperature. Therefore, with an isobarical (that is, constant pressure) Cooling the air with a constant concentration of steam occurs the moment (dew point) when the pair is saturated. At the same time, the "extra" steam is condensed in the form of fog, dews or ice crystals. The processes of saturation and condensation of water vapor play a huge role in the physics of the atmosphere: the processes of formation of clouds and education atmospheric fronts A large part is determined by the processes of saturation and condensation, the heat released during the condensation of the atmospheric water vapor provides the energy mechanism for the occurrence and development of tropical cyclones (hurricanes).
Relative humidity is the only hygrometric air indicator that allows direct instrument dimensions.
Evaluation of relative humidity
The relative humidity of the water mixture can be estimated if its temperature is known ( T.) and dew point temperature ( T D.), according to the following formula:
Rh \u003d P S (T D) P S (T) × 100%, (\\ DisplayStyle Rh \u003d ((P_ (S) (T_ (D))) \\ OVER (P_ (S) (T))) \\ Times 100 \\%,)where P S. - The pressure of the saturated pair for the corresponding temperature, which can be calculated by the Arden Tank formula:
P s (t) \u003d 6.1121 exp \u2061 ((18.678 - T / 234.5) × T 257.14 + T), (\\ DisplayStyle P_ (S) (T) \u003d 6.1121 \\ EXP \\ LEFT (((\\ FRAC (18.678-T / 234.5) \\ Times T) (257.14 + t)) \\ Right),)Approximate calculation
Relative humidity approximately can be calculated according to the following formula:
R H ≈ 100 - 5 (T - 25 T D). (\\ DisplayStyle R \\! H \\ APPROX 100-5 (T-25T_ (D)).)That is, with each degree Celsius, the difference in air temperature and dew point temperature relative humidity decreases by 5%.
Additionally, relative humidity can be assessed by a psychrometric diagram.
Limited water par
In the absence of condensation centers, with a decrease in temperature, the formation of an impregnated state is possible, that is, relative humidity becomes more than 100%. As the condensation centers, ions or particles of aerosols may be on the condensation of the delayed steam on the ions formed during the passage of the charged particle in such a pair, the principle of operation of the Wilson chamber and diffusion cameras are founded: water droplets condensing on the formed ions form a visible trail (track ) The charged particle.
Another example of the condensation of the late water vapor is the inversion traces of aircraft arising from the condensation of the late water vapor on the particles of the engine exhaust of the engines.
Means and methods of control
To determine air humidity, devices are used, which are called psychrometers and hygrometers. The psychrometer of August consists of two thermometers - dry and wet. A wet thermometer shows the temperature below than dry, since its tank is wrapped with a cloth moistened in water, which evaporated, cools it. The intensity of evaporation depends on the relative humidity. According to the testimony of dry and wet thermometers, relative humidity of air in psychrometric tables are found. IN lately Integral humidity sensors began to be widely used (as a rule, with voltage output), based on the property of some polymers change their electrical characteristics (Such as the dielectric permeability of the medium) under the action of water vapor contained in the air.
It is determined by a comfortable air humidity by such documents as GOST and SNiP. They regulate that in the winter in the room the optimal humidity for a person is 30-45%, in the summer - 30-60%. SNiP data differ slightly: 40-60% for any time of the year, the maximum level is 65%, but for very wet regions - 75%.
To determine and confirm the metrological characteristics of the moisture measurement devices, special reference (exemplary) installations are used - climatic chambers (hygrostats) or dynamic gases moisture generators.
Value
Relative humidity of air - an important environmental environment. With too low or too high humidity observed fast fatiguability man, deterioration of perception and memory. The mucous membranes of a person dry out, moving surfaces are cracking, forming microcracks, where viruses, bacteria, microbes penetrate directly. Low relative humidity (up to 5-7%) in the premises of the apartment, the office is marked in the regions with the continuous standing of low negative outdoor temperatures. Usually, duration up to 1-2 weeks at temperatures below -20 ° C leads to drying rooms. A significant deteriorating factor in maintaining relative humidity is an air exchange at low negative temperatures. The larger the air exchange in the premises, the faster in these premises the low (5-7%) relative humidity is created.
Airing the premises in the cold for the purpose of increasing humidity is a rough mistake - this is the most effective method Get the opposite. The reason for a widely rooted misconception in the perception of the relative humidity figures, known to all of the weather forecasts. This is an interest from a certain number, but this is the number for the room and the streets are different! You can learn from the table linking the temperature and absolute humidity. For example, 100% of outdoor air humidity at -15 ° C means 1.6 g of water in the cubic meter, but the same air (and the same grams) at +20 ° C means only 8% humidity.
Food products, construction Materials And even many electronic components are allowed to be stored in a strictly defined range of relative humidity. Many technological processes occur only with strict control of the content of water vapors in the air of the production premises.
Air humidity indoors can be changed.
Humidifiers are used to enhance humidity.
The functions of drainage (lower humidity) of the air are implemented in most air conditioners and in the form of separate devices - air dryers.
In flower growing
The relative humidity of the air in the oranges and used for plant cultivation of residential premises is subject to fluctuations, which is due to the time of the year, air temperature, degree and frequency of watering and spraying plants, the presence of humidifiers, aquariums, or other containers with an open surface of water, the system of ventilation and heating. Cacti and many succulent plants are easier to carry dry air than many tropical and subtropical plants.
As a rule, for plants of the birthplace of which are wet rainforests, Optimal is 80-95% relative humidity of air (in winter it can be reduced to 65-75%). For plants of warm subtropics - 75-80%, cold subtropics - 50-75% (levko, cyclamen, cycherress, etc.)
When the content of plants in residential premises, many species suffer from dry air. First of all it is reflected on
IN this lesson The concept of absolute and relative humidity will be introduced, the terms and values \u200b\u200bassociated with these concepts will be discussed: saturated steam, dew point, moisture measurement devices. During the lesson we will get acquainted with the density tables and the saturated steam pressure and the psychrometric table.
For a person, moisture is a very important environmental parameter, since our body reacts very actively to its changes. For example, such a mechanism for regulating the functioning of the body, as a sweating, is directly related to the temperature and humidity. With high humidity, the process of evaporation of moisture from the surface of the skin is virtually compensated for by the processes of its condensation and the heat dissipation is disturbed from the body, which leads to the thermoregulation disorders. With low humidity, moisture evaporation processes prevail over condensation processes and the body loses too much fluid, which can lead to dehydration.
The magnitude of the humidity is important not only for man and other living organisms, but also for the flow technological processes. For example, due to the known properties of water electricity Its content in the air can seriously influence the correct work of most electrical appliances.
In addition, the concept of humidity is the most important evaluation criterion weather conditionsthat everyone is known from weather forecasts. It is worth noting that if we compare humidity at different times of the year in our usual for us climatic conditions, it is higher in summer and below in winter, which is associated, in particular, with the intensity of evaporation processes at different temperatures.
The main characteristics of wet air are:
- water vapor density in the air;
- relative humidity.
Air is a composite gas, it contains many different gases, including water vapor. To estimate its quantity in the air, it is necessary to determine which weight there are water vapors in a certain dedicated volume - such a value characterizes the density. Water vapor density in the air is called absolute humidity.
Definition.Absolute humidity - the amount of moisture contained in one cubic air meter.
Designationabsolute humidity: (as well as an ordinary density designation).
Unitsabsolute humidity: (in SI) or (for the convenience of measuring a small content of water vapors in the air).
Formula calculations absolute humidity:
Designations:
Mass of steam (water) in the air, kg (in C) or g;
Air volume in which the specified pair mass is contained.
On the one hand, the absolute humidity of the air is a clear and convenient value, since it gives an idea of \u200b\u200bthe specific content of water in the air by mass, on the other hand, this value is inconvenient from the point of view of humidity susceptibility by living organisms. It turns out that, for example, a person feels not a massive water content in the air, namely its content relative to the maximum possible value.
To describe such perception, this value is introduced as relative humidity.
Definition.Relative humidity - The value showing how far the par from saturation.
That is the magnitude of relative humidity, simple wordsThe following shows: if the pairs are far from saturation, then the humidity is low, if close is high.
Designationrelative humidity: .
Unitsrelative humidity: %.
Formula calculations relative humidity:
Designations:
Water vapor density (absolute humidity), (in SI) or;
The density of saturated water vapor at a given temperature, (in C) or.
As can be seen from the formula, the absolute humidity appear in it, with which we are already familiar, and the density of the saturated pair at the same temperature. There is a question how to determine the last magnitude? For this there are special devices. We'll consider condensationhygrometer (Fig. 4) - a device that serves to determine the dew point.
Definition.Dew point- Temperature at which couple becomes saturated.
Fig. 4. Condensation hygrometer ()
Inside the tank of the device poured an easy-to-sleep fluid, for example, the ether, the thermometer (6) is inserted and the air is poured through the container with a pear. As a result of enhanced air circulation, an intensive evaporation of the ether begins, the temperature of the capacity is reduced due to this and the mirror performs dew (the droplets of the condensed pair). At the time of the appearance of dew on the mirror using a thermometer, the temperature is measured, this temperature is also a dew point.
What to do with the resulting temperature value (dew point)? There is a special table in which the data is entered - which density of the saturated water vapor corresponds to each specific dew point. It should be noted useful factAs increasing the value of the dew point, the value of the corresponding saturated pair density is growing. In other words, the warmer air, the large quantity It may contain moisture, and vice versa, than the air is colder, the maximum content in it is less.
We now consider the principle of action of other types of hygrometers, instruments for measuring humidity characteristics (from Greek. Hygros - "wet" and Metreo - "Measure").
Hair hygrometer (Fig. 5) - a device for measuring relative humidity in which hair acts as an active element, for example, human.
The effect of the hairless hygrometer is based on the property of a skimped hair to change its length when the air humidity changes (with increasing moisture, the hair length increases, with a decrease - decreases), which allows measuring relative humidity. Hair is stretched on a metal frame. The change in the length of the hair is transmitted by the arrow moving along the scale. It should be remembered that the hair hygrometer does not give exact values \u200b\u200bof relative humidity, and is used mainly in domestic purposes.
More convenient to use and accurately, such a device for measuring relative humidity, as a psychrometer (from Dr.-Greek. Ψυχρός - "Cold") (Fig. 6).
The psychrometer consists of two thermometers that are fixed on the general scale. One of the thermometers is called wet, since it is wrapped with a batter cloth, which is immersed in a water tank located on the back of the instrument. With wet fabric, water evaporates, which leads to cooling the thermometer, the process of reducing its temperature lasts until the step is reached until the pairs near the wet tissue reach saturation and the thermometer will not start showing the temperature of the dew point. Thus, a wet thermometer shows a temperature less than either equal to the real ambient temperature. The second thermometer is called dry and shows the real temperature.
On the instrument housing, as a rule, an even so-called psychrometric table is depicted (Table 2). Using this table by the value of the temperature that the dry thermometer shows, and on the temperature difference between dry and wet thermometers, the relative humidity of the surrounding air can be determined.
However, not even having such a table at hand, you can roughly determine the value of humidity using the following principle. If the testimony of both thermometers is close to each other, the evaporation of water from the wet is almost completely compensated by condensation, i.e. the humidity is high. If, on the contrary, the difference in thermometers is large, then evaporation from wet fabric prevails over condensation and dry air, and the humidity is low.
Turn to tables that allow you to determine the characteristics of air humidity.
|
Temperature, |
Pressure, mm. RT. Art. |
Pair density |
Table. 1. Density and pressure of saturated water vapor
Once again, we note that, as mentioned earlier, the value of the density of the saturated steam grows with its temperature, the same applies to the pressure of a saturated steam.
Table. 2. Psychometric table
Recall that relative humidity is determined by the value of the dry thermometer's readings (first column) and the difference in the readings of dry and wet (first string).
In today's lesson, we met with an important characteristic The air is his humidity. As we said, humidity in the cold season (winter) decreases, and in warm (in summer) rises. It is important to be able to regulate these phenomena, for example, if necessary, raise the humidity positioning in the room in winter time Several water tanks to enhance evaporation processes, however, this method will be effective only at the corresponding temperature, which is higher than on the street.
In the next lesson, we will look at what gas is the work and the principle of the internal combustion engine.
Bibliography
- Gentendestein L.E, Kaidalov AB, Kozhevnikov V.B. / Ed. Orlova V.A., Roizen I.I. Physics 8. - M.: Mnemozin.
- Pryrickin A.V. Physics 8. - M.: Drop, 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" ()
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