Humidity table for different temperatures. Absolute and relative humidity

In this lesson, the topic of which is: “Humidity. Measuring Humidity”, we will discuss the properties of saturated and unsaturated water vapor, which is always present in the atmosphere.

In the previous lesson, we got acquainted with the concept of "saturated steam". As in the study of any topics and subjects, the question may arise: “Where do we use this concept, how will we apply it?”. The most important application of the properties of saturated steam will be discussed in this lesson.

You probably know the name of the topic well, because you hear the concept of “air humidity” every day when you watch or listen to the weather forecast. However, if you are asked: “What is meant by air humidity?”, You are unlikely to immediately give an exact physical definition.

Let's try to formulate what in physics is meant by air humidity. First of all, what is this water in the air? After all, such, for example, is fog, rain, clouds and other atmospheric phenomena passing with the participation of water in a particular state of aggregation. If all these phenomena are taken into account when describing humidity, then how to carry out measurements? Already from such simple considerations it becomes clear that intuitive definitions are indispensable here. In fact, we are talking First of all, about the water vapor that is contained in our atmosphere.

Atmospheric air is a mixture of gases, one of which is water vapor (Fig. 1). He contributes to Atmosphere pressure, this contribution is called partial pressure(as well as elasticity) of water vapor.

Rice. 1. Components of atmospheric air

Dalton's Law

The main regularities that you and I obtained in the framework of the study of molecular kinetic theory relate to the so-called pure gases, i.e. gases consisting of atoms or molecules of the same kind. However, very often one has to deal with a mixture of gases. The simplest and most common example of such a mixture is atmospheric air that surrounds us. As we know, it is 78% nitrogen, more than 21% oxygen, and the remaining percentage is occupied by water vapor and other gases.

Rice. 2. Composition of atmospheric air

Each of the gases that is part of air or any other mixture of gases, of course, contributes to the total pressure of this mixture of gases. The contribution of each individual such component is called partial gas pressure,t. that is, the pressure that a given gas would exert in the absence of other components of the mixture.

The English chemist John Dalton experimentally established that for rarefied gas mixtures, the total pressure is a simple sum of the partial pressures of all components of the mixture:

This relationship is called Dalton's law.

The proof of Dalton's law within the framework of molecular kinetic theory, although not particularly complicated, is rather cumbersome, so we will not present it here. Qualitatively, it is quite simple to explain this law if we take into account the fact that we neglect the interaction between molecules, i.e., molecules are elastic balls that can only collide with each other and with the walls of the vessel. In practice, the ideal gas model works well only for sufficiently rarefied systems. In the case of dense gases, deviations from the fulfillment of Dalton's law will be observed.

Partial pressurep water vapor is one of the indicators of air humidity, which is measured in pascals or millimeters of mercury.

Water vapor pressure depends on the concentration of its molecules in the air, as well as on the absolute temperature of the latter. Density is often taken as a characteristic of humidity. ρ water vapor in the air is called absolute humidity .

Absolute humidity shows how many grams of water vapor is contained in the air. Accordingly, the unit of absolute humidity is .

Both mentioned indicators of humidity are related by the Mendeleev-Clapeyron equation:

- molar mass of water vapor;

is its absolute temperature.

That is, knowing one of the indicators, for example, density, we can easily determine the other, that is, pressure.

We all know that water vapor can be both unsaturated and saturated. A vapor in thermodynamic equilibrium with a liquid of the same composition is said to be saturated. An unsaturated vapor is a vapor that has not reached dynamic equilibrium with its liquid. In this case, there is no equilibrium between the processes of condensation and evaporation.

In general, water vapor in the atmosphere, despite the presence of a large number of water bodies: oceans, seas, rivers, lakes, and so on, is unsaturated, because our atmosphere is not a closed vessel. However, moving air masses: winds, hurricanes and so on - lead to the fact that in different points Earth at each moment of time there is a different ratio between the rates of condensation and evaporation of water, as a result of which in some places the steam can reach saturation. What does this lead to? Moreover, in such an area, steam begins to condense, because we remember that saturated steam is always in contact with its liquid. As a result, fog or clouds may form, dew may fall. The temperature at which steam becomes saturated is called dew point . Denote the pressure of water vapor (saturated) at the dew point.

Consider why dew tends to fall in the early morning? What happens to the temperature at this moment of the day, and, consequently, to the limiting pressure, to the pressure of saturated vapor? Obviously, knowing the absolute humidity or partial pressure of water vapor does not give us any idea of ​​how close or far a given vapor is from saturation. But it is precisely from this remoteness or proximity to saturation that the rate of evaporation and condensation processes, i.e., those processes that determine the vital activity of living organisms, depends.

If evaporation prevails over condensation, then organisms and soil lose moisture (Fig. 3). If condensation prevails, then drying processes become impossible (Fig. 4). We are faced with the need to improve the concept of moisture; the concept of absolute humidity, as we have just seen, does not fully describe all the phenomena we need.

Rice. 3. Evaporation prevails over condensation

Rice. 4. Condensation prevails over evaporation

Let's discuss the issue again. Let's do it on simple example. Imagine that there are 20 people in a certain vehicle. Is it a lot or a little, that is, is this absolute value of 20 people? Naturally, we will not be able to say whether this is a lot or a little, until we know the maximum capacity of a given car or vehicle. 20 people in a passenger car is, of course, a lot, it is virtually impossible, and 20 people in a big bus is not so much. Similarly, in the case of absolute humidity, i.e., with the partial pressure of water vapor, we need to compare it with something. With what to compare this partial pressure? The last lesson tells us the answer. What is the importance of water vapor pressure? This is the pressure of saturated water vapor. If we compare the partial pressure of water vapor at a given temperature with the pressure of saturated water vapor at the same temperature, we can more accurately characterize the very humidity of the air. To characterize the remoteness of the state of steam from saturation, a special quantity was introduced, called relative humidity .

relative humidity air is called the ratio of the pressure of water vapor contained in air, expressed as a percentage, to the pressure of saturated vapor at the same temperature:

Now it is clear that the lower the relative humidity, the further one or another vapor from saturation. So, for example, if the value relative humidity is 0, then there is actually no water vapor in the air. That is, condensation is impossible for us, and at a relative humidity value of 100%, all the water vapor that is in the air is saturated, because its pressure is equal to the pressure of saturated water vapor at a given temperature. In this way, we have now accurately determined what the very humidity is, the value of which is reported to us every time in weather forecasts.

Using the Mendeleev-Clapeyron equation, we can obtain an alternative formula for relative humidity, which now includes the value of the density of water vapor contained in the air and the density of saturated vapor at the same temperature.

Vapor pressure and density;

Pressure and density of saturated vapor at a given temperature;

Universal gas constant.

Relative Humidity Formula:

Density of water vapor contained in the air;

Density of saturated vapor at the same temperature.

The influence of the intensity of evaporation and condensation of water on living organisms

People are very sensitive to the value of relative humidity, the rate of evaporation of moisture from the surface of the skin depends on it. At high humidity, especially on a hot day, this evaporation decreases, as a result of which the normal heat exchange of the body with the environment is disturbed. In dry air, on the contrary, there is a rapid evaporation of moisture from the surface of the skin, from which, for example, mucous membranes dry out. respiratory tract. The most favorable for humans is relative humidity in the range of 40-60%.

The role of water vapor in the formation of weather conditions. Condensation of water vapor leads to the formation of clouds and subsequent precipitation, which, of course, is important for all aspects of our lives and for National economy. In many production processes, artificial humidity regimes are maintained. An example of such processes are weaving, confectionery, pharmaceutical shops and many others. In libraries and museums, in order to preserve books and exhibits, it is also important to maintain a certain value of relative humidity, therefore, in such institutions in all rooms, a psychrometer, a device for measuring relative humidity, must be hung on the wall.

To calculate relative humidity, as we have just seen, we need to know the value of the pressure or density of saturated vapor at a given temperature.

In the last lesson, studying saturated steam, we talked about this dependence, but its analytical form is very complicated, our mathematical knowledge is still not enough. How to be in this case? The way out is very simple: instead of writing these formulas in an analytical form, we will use tables of the pressure and density of saturated vapor at a given temperature (Table 1). These tables are found both in textbooks and in any reference book of technical quantities.

Tab. 1. Dependence of pressure and density of saturated water vapor on temperature

Now consider the change in relative humidity with temperature. The higher the temperature, the lower the relative humidity. Why and how, let's look at an example of a problem.

A task

In a certain vessel, the vapor becomes saturated at . What will be its relative humidity at , , ?

Since we are talking about steam in a vessel, the volume of steam remains unchanged when the temperature changes. In addition, we need a table of dependence of pressure and density of saturated steam on temperature (Table 2).

Tab. 2. Dependences of pressure and density of saturated vapor on temperature

Solution:

It is clear from the text of the question that at , , because it is at this value that the steam becomes saturated, i.e. from the definition of relative humidity we have:

The numerator is the density of the water vapor present in the vessel, and the denominator is the density of the saturated steam absent in the vessel at the same temperature. What happens to the moisture content as the temperature increases? The numerator, taking into account the closedness of the vessel, will not change. Indeed, since there is no condensation and no exchange of matter with outside world, then the mass of the vapor, and with it its density, will retain their values. And the denominator, as we know from the last lesson, increases with temperature, so relative humidity will decrease. The vapor density in the vessel at can be calculated from the above formula:

The same vapor density will have at all other temperatures. Therefore, to calculate humidity, it will be enough for us to know the value of the density of saturated vapor at all given temperatures, and we can immediately get answers. We take the value of the density of saturated steam from the table. Substituting the values ​​in turn into the formula for humidity, we get the following answers:

Answer:

An example of solving a typical problem for determining relative humidity

When solving such problems, it is important to know that the saturation vapor pressure depends on temperature, but does not depend on volume.

The task:

The vessel contains air, the relative humidity of which at a temperature is . What will be the relative humidity after reducing the volume of the vessel by n times (n = 3) and heating the gas to a temperature ? The density of saturated water vapor at temperature is equal to .

Solution progress:

From the definition of relative humidity, we can write that at a temperature absolute humidity, before compression, is equal to:

And after compression:

That is, with a decrease in volume by a factor of at a constant mass, the density increases by a factor of 1.

After compression, the mass of moisture per unit volume of the vessel, not only in the form of vapor, but also in the form of a condensed liquid, if conditions for condensation arise, will be equal to:

At a temperature, the pressure of saturated water vapor is equal to normal atmospheric pressure, we talked about this in the last lesson, and is:

And their density, if you use the Mendeleev-Clapeyron equation, can be calculated by the formula:

Where , since there will be unsaturated steam in the vessel with relative humidity:

Expressing this humidity as a percentage, we get a value of 2.9%.

Answer: .

And now let's talk not only about what humidity is, but also about how this very humidity can be measured. The most common instrument for such measurements is the so-called hygrometric psychrometer, which is shown in Fig. 5.

Rice. 5. Hygrometric psychrometer

Two thermometers with identical scales are fixed on the rack. The mercury tank of one of them is wrapped in a damp cloth (Fig. 8).

Rice. 6. Thermometers of the hygrometric psychrometer

The water from this cloth evaporates, due to which the thermometer itself cools, respectively, the thermometers are called dry and wet (Fig. 7).

Rice. 7. Dry and wet bulb hygrometric psychrometer

The greater the relative humidity of the ambient air, the less intense, the weaker the evaporation of water from a damp cloth, the smaller the difference in the readings of dry and wet thermometers. That is, at ϕ = 100%, water will not evaporate, since all water vapor is saturated, and the readings of both thermometers will coincide. When the difference in thermometer readings will be maximum. Thus, according to the difference in the readings of thermometers using special psychometric tables (most often such a table is immediately placed on the body of the device itself) and determine the value of relative humidity.

As we know, most of the surface of our planet is covered by the oceans, so water and all the processes that occur with it, in particular evaporation and condensation, play a crucial role in all the processes of our life. We ourselves gave a strict definition of the concepts "absolute humidity" and "relative humidity". In fact, this is a physical quantity, relative humidity shows how much atmospheric vapor differs from saturated.

Bibliography

1. Kasyanov V.A. Physics grade 10. - M.: Bustard, 2010.
2. Myakishev G.Ya., Sinyakov A.Z. Molecular physics. Thermodynamics. - M.: Bustard, 2010.

1. Internet portal WorldOfSchool.ru ()
2. Internet portal “Physics. Old textbooks "()

Homework

1. What is the difference between absolute humidity and relative humidity?
2. What can be measured with a psychrometric hygrometer and what is its principle of operation?
3. What partial pressures make up atmospheric pressure?

Stationary
August psychrometer. The instrument is
mandatory at meteorological
stations. It consists of two identical
mercury thermometers mounted side by side
on a tripod. The reservoir of one of the thermometers
wrapped in thin matter, the end of which
dropped into a glass of distilled
water.

From the surface
wet bulb will evaporate the water
stronger than drier air, so it
shows a lower temperature,
than the dry bulb, the difference will be
more than dry air and vice versa.

Psychrometer
installed at a distance of 1.5 m from
floor, pour into a vessel under a thermometer
water and therefore moisturize it and
readings are taken after 15 minutes
thermometers. Absolute humidity
calculated according to Regnault's formula.

BUT
= M1- a(t— t 1 )
* H,
where:

A-absolute
humidity,

M is the maximum voltage of water
steam at a humid temperature
thermometer/cm, table 1/,

a-/alpha/'-psychrometric
coefficient equal to room
air 0.0011 and for open atmosphere
-0,00074,

t
- temperature for dry bulb,

t 1 - temperature
wet thermometer,

H-barometric
pressure.

TABLE#1

ELASTICITY
SATURATED WATER VAPOR
/selectively/

2.2.
Determination of absolute humidity
Assmann psychrometer .

It's more modern
psychrometer compared to stationary.
Both mercury thermometers are enclosed in
metal tubes through which
the investigated is evenly sucked
air with the help of a fan located,
on the top of the device. Such a device
provides tank protection
thermometer from radiant energy,
guarantees constant speed
air around the thermometer and due to
suction of a large mass of air
gives more accurate readings than
stationary psychrometer. Storage tank
wet bulb thermometer in the suction chamber
psychrometer wrapped in thin cloth
moistened before each observation
distilled water with
pipettes. The fan is turned on. Indications
thermometers are counted after 4-5 minutes
work in summer and 15 minutes in winter.
The device should not be held in the hands, it is necessary
mount it on a stand.

Absolute humidity
when working with an aspiration psychrometer
calculated according to the Sprunge formula:

A= M ! -0,5(t c — t in ) H/755,
where

A-absolute
humidity,

0.5-constant
psychrometric coefficient,

M! -maximum
water vapor pressure at temperature
wet thermometer,

T c -temperature
dry thermometer,

TV temperature
wet thermometer,

H-barometric
pressure,

755 - average
barometric pressure.

Translation
found absolute humidity in
relative is produced by the formula:

a=A/M x 100%, where:

a - desired relative
humidity,

A is the absolute humidity

M - maximum
humidity at dry temperature
thermometer.

For determining
relative humidity according to aspiration
psychrometer, you can use the table.
2, in which in the first vertical column
dry bulb readings are
at the time of observation. And in the top
horizontal row-indications of wet
thermometer.

For these two numbers
together crossing lines drawn
from | the first digit to the right and from the second down,
find relative humidity. Table
suitable for indoor and outdoor use
open air, but received
results are less accurate than computed
according to the formula.

studfiles.net

Relative humidity at temperature determination with a psychrometer (Table)

Relative Humidity

Relative Humidity is determined by the ratio (expressed as a percentage) of the pressure of water vapor in the air to the vapor pressure that saturates the air at the same temperature. In practice, in most cases, relative humidity is determined by the ratio of the weight of water vapor per unit volume of air (absolute humidity) to the weight of saturated water vapor in the same volume of air and at the same temperature.

Weight hygrometer

The reference table gives the amount of water in grams contained in 1 m 3 of air at saturation, if the total pressure is 760 mm Hg.

Aspiration hygrometer (psychrometer)

In meteorology, a simple expression is used

P w -P=AH (t-t w).

Where t w 0 C denotes the temperature of the wet bulb, P (mm) is the pressure of water vapor in the air, P w is the vapor pressure that saturates the air at a temperature of t w , H (mm) is the barometric pressure and A is a constant. Thus, the relative humidity of the air is equal to 100 R/R s , where R s denotes the saturated vapor pressure at temperature t, measured by dry bulb. The value of A, which depends on the air velocity near the wet bulb, is 0.00066 for the Assmann aspiration psychrometer and A=0.00080 for the Stevenson instrument used in the meteorological service.

Table of relative humidity values ​​(%) when measured with a psychrometer

The reference tables given refer to instruments with full (free) ventilation. More complete tables for temperature ranges from -30 to 55°C and from 30 to 350°C F.

1) Subcooled water (but not ice) on a wet bulb.

Relative Humidity Table - Wet Bulb Ice Covered 1)

1) Relative humidity is defined here as the ratio of absolute humidity, calculated per unit volume, to the amount of water vapor in the air that is in equilibrium with water (but not ice) at dry bulb temperature.

infotables.ru

8.1. Determination of air humidity with the Augusta psychrometer

Psychrometer
(type PBU-1M)
consists of two adjacent
upright liquid

wrapped in cloth, the end of which is
in a bottle cup filled with clean
water. Due to the inertia of the device
readings should be taken no earlier than 5-7
min. after its installation in the place of observation
or start blowing with a fan. Necessary
make sure that the thermometer reservoir is not
touched the water level.

Relative
humidity of still air,
located in close proximity
from a psychrometer, determined by indications
dry and wet bulb thermometers,
using a psychrometric table,
marked on the instrument panel.

Absolute
and relative humidity of the mobile
air can be determined either by
special psychrometric tables,
either by
formulas.

Absolute
the humidity of the moving air is calculated
according to the formula

Pa \u003d Pnv - a (tc-tv) B,

where Ra
— absolute
air humidity. Pa;

Rnv -
maximum
air humidity (partial pressure
saturated water vapor) at
wet bulb temperature, Pa,
determined by table.4;

a
- psychrometric coefficient,
speed dependent
air, determined according to table 5;

tc,tv —
dry and wet bulb readings,
°C;

AT
— barometric
pressure, Pa, determined by wall
barometer mounted over a laboratory
table (I mbar = 100 Pa).

Table
4

Relative
humidity j determined from
ratios

j =(Ra/Rn) 100%,
(2)

where pH —
maximum air humidity
(partial pressure of saturated
water vapor) at a temperature of dry
thermometer, Pa, determined according to Table 4.

Table 5

Psychrometer
PBU-1M can be used for
determination of air humidity in
industrial premises without
sources of thermal radiation.

studfiles.net

8.2. Determination of air humidity with an Assmann aspiration psychrometer

aspiration
psychrometer (type MV-4M) is more
perfect and precise instrument
compared with August's psychrometer. He
consists of two identical mercury
thermometers fixed in a special
frame. The thermometer tanks are
in double metal sleeves with
polished and nickel-plated outer
surface, which eliminates the influence
thermal radiation on the results
measurements. In the head of the psychrometer
there is a fan with a clockwork spring
mechanism. fan air
is sucked into the sleeves, flows around
mercury tanks thermometers, passes
through the air tube to the head
and thrown out. In this way
permanent conditions are created for
evaporation of moisture from the surface of the mercury
wet bulb reservoir and
the influence of air mobility is excluded
at work.

Order
work with aspiration psychrometer
next. First wet the cambric on
reservoir of the right thermometer. For
take a rubber balloon with a pipette,
pre-filled with water and light
by pressing bring the water no closer than I
cm to the edge of the pipette while holding it on this
level with a clamp. Thereafter
the pipette is inserted to failure into the internal
protection sleeve, wetting cambric. after waiting
some time without removing the pipette from
tubes, open the clamp, absorbing excess
water into a balloon, after which a pipette
take out.

Table
6

Then
start the fan almost to failure, but
be careful not to break the spring,
the psychrometer is suspended on a special
pin in a vertical position. Countdown
thermometer readings are made
4 minutes after starting the fan.

calculation
air humidity (absolute and
relative) is produced according to
special psychrometric tables
or formulas. Absolute humidity
found from the ratio

Ra
= Rnv
— 0.5(tс-
tv)B/99000,
(3)

where
99000 - average barometric pressure,
Pa, ;

rest
values ​​in accordance with (1).

Knowing
value Ra
, moisture content can be calculated
air d - the ratio of the mass of water
vapor to the mass of dry air in the same
volume, g/kg:

d=622Ra/(B-Ra).
(4)

Relative
air humidity j

is calculated by formula (2), and also
can be determined by psychrometric
table (Table 6) or psychrometric
graphics (Fig. 2), located on
laboratory table.

At
Working with a vertical line chart
note the readings of the dry thermometer,
on inclined - wetted, at the intersection
these lines receive relative values
humidity, expressed as a percentage.

studfiles.net

Psychrometric table for determining the relative humidity of the air

Lab #13

Determination of air humidity using a psychrometer (dry and wet bulbs)

Equipment: thermometer, wet gauze, table of dependence of saturated water vapor pressure on temperature, psychrometric table.

Task number 1. Determination of relative and absolute humidity using a psychrometer (dry and wet thermometers).

To determine the humidity of the air, it is necessary to record the readings of dry and wet thermometers, find the difference between these readings and, using the psychrometric table, determine the value of the relative humidity of the air corresponding to these data in any three school premises.

Enter the obtained data in the table.

Calculate the absolute humidity value for each of these rooms. Show how you did these calculations in your notebook.

Also enter the obtained values ​​​​of absolute humidity in the table.

Compare and analyze the obtained results.

Calculate the errors of the obtained results.

Room numberAir temperature, o СWet-bulb thermometer readings, o СTemperature difference, o СRelative air humidityAbsolute air humidity

• Table 1. PSYCHROMETRIC TABLE for determining the relative humidity of the air

Difference between dry and wet thermometer readings thermometer012345678910 0 1008163452811——1 100 83 65 48 32 16 — — — — —2 100 84 68 51 35 20 — — — — —3 100 84 69 54 39 24 10 — — — —4 100 85 70 56 42 28 14 — — — —5 100 86 72 58 45 32 19 6 — — —6 100 86 73 60 47 35 23 10 — — —7 100 87 74 61 49 37 26 14 — — —8 100 87 75 63 51 40 28 18 7 — —9 100 88 76 64 53 42 34 21 10 — —10 100 88 76 65 54 44 34 24 14 5 —11 100 88 77 66 56 46 36 26 17 8 —12 10089 78 68 57 48 38 29 20 11 —13 100 89 79 69 59 49 40 31 23 14 614 100 89 79 70 60 51 42 34 25 17 915 100 90 80 71 61 52 44 36 27 20 1216 100 90 81 71 62 54 46 37 30 22 1517 100 90 81 72 64 55 47 39 32 24 1718 100 91 82 73 65 56 49 41 34 27 2019 100 91 82 74 65 58 50 43 35 29 2220 100 91 83 74 66 59 51 44 37 30 2421 100 91 83 75 67 60 52 46 39 32 2622 100 92 83 75 68 61 54 47 40 34 2823 100 92 84 76 69 61 55 48 42 36 3024 100 92 84 77 69 62 56 49 43 37 3125 100 92 84777063 57 50 44 38 3326 100 92 85 78 71 64 58 51 46 40 3427 100 92 85 78 71 65 59 52 47 41 3628 100 93 85 78 72 65 59 53 48 42 3729 100 93 85 79 72 66 60 54 49 43 3830 100 93 86 79 73 67 61 55 50 44 39 1 hectopascal = 10 2 Pa = 100 Pa.

koledj.ru

4. Determination of air humidity using the August psychrometer

Psychrometer
Augusta consists of two identical
thermometers. one of the tank
covered with a piece of cambric, loose
the end of which is lowered into the tank with
distilled water.

Due to
water evaporation wet readings
thermometer will be lower than dry. Knowing
difference between dry and wet
thermometers and dry bulb readings
according to psychrometric table 2, determine
relative humidity of the environment
air, and by formulas (1) and (2) find
absolute humidity
and
moisture deficiency.

Completing of the work

An exercise
№1 . Definition
humidity with psychrometer
Assman

wet
lawn on the reservoir of the psychrometer
Assman with water using a pipette.

Start
clockwise fan almost
to failure, but be careful not to break
spring.

Through
4 minutes after starting the fan remove
dry and wet thermometer readings.

Calculate
according to the formula (4) absolute humidity f.

For
do this: a) find the atmospheric pressure H 0
in mm. rt. Art. by barometer, b) Find R 1 in mmHg according to table 3
according to the wet bulb thermometer c)
constant BUT 0 \u003d 0.0013 1 / deg.

Define
maximum humidity F
thermometer.

By
formula (1) calculate the relative
humidity E.

Define


Data
enter in table 2

Table
1

results
measurements and calculations

An exercise
№2 . Definition
humidity using the Augusta psychrometer

wet
water batiste psychrometer Augusta.

Through
10 min record dry and
wet thermometers.

By
psychrometric table 4 find
relative humidity E.

From
formulas (1) find the absolute humidity
f.

Define
maximum humidity F according to table 3 according to dry
thermometer.

Define
according to formula (2) moisture deficit D.

Data
enter in table 2.

Table
2

results
measurements

Table
3

Pressure
and saturated vapor density in the interval
temperatures from -5 o to 30 0

Table
4

Psychrometric
table

Relative Humidity

Relative Humidity is determined by the ratio (expressed as a percentage) of the pressure of water vapor in the air to the vapor pressure that saturates the air at the same temperature. In practice, in most cases, relative humidity is determined by the ratio of the weight of water vapor per unit volume of air (absolute humidity) to the weight of saturated water vapor in the same volume of air and at the same temperature.

Weight hygrometer

The reference table gives the amount of water in grams contained in 1 m 3 of air at saturation, if the total pressure is 760 mm Hg.

Aspiration hygrometer (psychrometer)

In meteorology, a simple expression is used

P w -P=AH (t-t w).

Where t w 0 C denotes the temperature of the wet bulb, P (mm) is the pressure of water vapor in the air, P w is the vapor pressure that saturates the air at a temperature of t w , H (mm) is the barometric pressure and A is a constant. Thus, the relative humidity of the air is equal to 100 R/R s , where R s denotes the saturated vapor pressure at temperature t, measured by dry bulb. The value of A, which depends on the air velocity near the wet bulb, is 0.00066 for the Assmann aspiration psychrometer and A=0.00080 for the Stevenson instrument used in the meteorological service.

Table of relative humidity values ​​(%) when measured with a psychrometer

The reference tables given refer to instruments with full (free) ventilation. More complete tables for temperature ranges from -30 to 55°C and from 30 to 350°C F.

1) Subcooled water (but not ice) on a wet bulb.

Relative Humidity Table - Wet Bulb Ice Covered

1) Relative humidity is defined here as the ratio of absolute humidity, calculated per unit volume, to the amount of water vapor in the air that is in equilibrium with water (but not ice) at dry bulb temperature.

DEFINITION

Absolute air humidity is the amount of water vapor per unit volume of air:

In the SI system, the unit of measure for absolute humidity

Humidity is a very important parameter environment. It is known that most of the Earth's surface is occupied by water (the World Ocean), from the surface of which evaporation continuously occurs. In different climatic zones the intensity of this process is different. It depends on the average daily temperature, the presence of winds and other factors. Thus, in certain places the process of water vaporization is more intense than its condensation, and in some places it is vice versa.

The human body actively reacts to changes in air humidity. For example, the process of sweating is closely 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.

In addition, the concept of humidity is the most important criterion for evaluating weather conditions, which is known to all from weather forecasts.

The absolute humidity of the air gives an idea of ​​the specific water content in the air by mass, but this value is inconvenient in terms of the susceptibility of humidity by living organisms. A person feels not the mass amount of water in the air, but its content relative to the maximum possible value. To describe the reaction of living organisms to changes in the water vapor content in the air, the concept of relative humidity is introduced.

Relative humidity

DEFINITION

Relative humidity- this is a physical quantity showing how far the water vapor in the air is far from saturation:

where is the density of water vapor in the air (absolute humidity); density of saturated water vapor at a given temperature.

Dew point

DEFINITION

Dew point is the temperature at which water vapor becomes saturated.

Knowing the dew point temperature, you can get an idea of ​​the relative humidity of the air. If the dew point temperature is close to the ambient temperature, then the humidity is high ( when the temperatures match, fog is formed). Conversely, if the values ​​of the dew point and air temperature at the time of measurement differ greatly, then we can talk about a low content of water vapor in the atmosphere.

When something is brought into a warm room from frost, the air above it cools, becomes saturated with water vapor, and water droplets condense on things. In the future, the thing warms up to room temperature, and all the condensate evaporates.

Another, no less well-known example is the fogging of windows in a house. Many people have condensation on their windows in the winter. This phenomenon is influenced by two factors - humidity and temperature. If a normal double-glazed window is installed and the insulation is carried out correctly, and there is condensate, it means that the room high humidity; Possibly poor ventilation or ventilation.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2