Completed:
Tinkov A.Yu.
Group 803001207
Checked by: Alekhina T.N.
Grade: _______
Physics Lab Report
“Determination of air humidity by psychrometric method”
Security questions:
1) What is absolute humidity air? In what units is it measured? What is relative humidity?
Absolute air humidity– magnitude, numerically equal to mass water vapor contained in a unit volume of air at a given temperature. Usually measured in g/m3.
Relative humidity(definition 1) – a value characterizing the degree of saturation of air with water vapor. This is the ratio of the partial pressure of vapor present in the air at a given temperature to the pressure of water vapor that would saturate the air at the same temperature.
Relative humidity(definition 2) - the ratio of the amount of water vapor to the amount of water vapor that the air can contain at a given temperature.
2) Name the methods for determining air humidity. Which one is more accurate?
There are several methods for determining absolute and relative humidity air. Instruments used to measure humidity include hygrometers and psychrometers.
To determine air humidity using a psychrometric method, the August psychrometer and the Assmann psychrometer are most often used, the operating principle of which is the same.
3) The operating principle of the Assmann psychrometer.
The simplest psychrometer consists of two alcohol thermometers, one is a regular dry thermometer, and the second has a humidification device. Thermometers have graduations with division values of 0.1-0.5 degrees. The temperature sensor of a wet bulb thermometer is wrapped in cambric, which is placed in a container of water. Due to the evaporation of moisture, the moistened thermometer cools. To determine relative humidity, readings are taken from dry and wet thermometers, and then a Psychrometric table is used. Typically, the input values in the Psychrometric table are the dry bulb readings and the temperature difference between the dry and wet bulbs. The design of the device may include a Psychrometric table for ease of use. A fan can also be turned on to aspirate air around the wet bulb. At the same time, the aspiration speed is small, usually 0.5-1 m/s. The psychrometer readings are also affected by atmospheric pressure, therefore, for accurate measurements, if the atmospheric pressure deviates from the nominal pressure, a correction is added to the results of the Psychrometric Table.
4) State the essence of the psychrometric method for determining air humidity.
Based on measuring the temperature difference between dry and wet thermometers using August and Assmann psychrometers. August's psychrometer consists of two identical mercury thermometers with a division value of 0.2 C, mounted side by side. The mercury reservoir of one of them is tightly wrapped with a piece of cambric or gauze, lowered into a vessel with clean water. Water evaporates from the surface of the fabric more easily, the less moisture in the air. Based on the difference between the readings of two thermometers, the relative air humidity is found using psychrometric tables supplied with the device.
5) What is dew point?
The dew point temperature of a gas (dew point) is the value of the gas temperature below which the water vapor contained in an isobarically cooled gas becomes saturated above a flat surface of water.
6) State formula (7) and determine the dimension of the psychrometer constant.
P = P H – A(t - t 1)H
The dimension of the constant of the device used is found by the formula:
Work progress:
Following the rules specified in the work, I wetted the cambric with water, which was used to wrap the can of one of the psychrometer thermometers. I turned on the fan. Having followed the readings of the wet thermometer, I established the temperature reading t 1 (=16°C). Set the dry bulb reading to t (=24°C).
Having calculated the temperature difference between the dry and wet thermometer (t - t 1) using the psychrometric table, I found the relative air humidity:
Knowing the ambient air temperature t (=24°C), I found the pressure value P0 (=22.4) of saturated water vapor at this temperature using the corresponding table, and using formula (2) I found the absolute air humidity:
P rel = P 0 *r/100 = 9.4 mm.
Absolute air humidity was determined using formula (8):
P abs = P n – 0.000662 (t – t 1) * H = 9.6 mmHg.
Data table:
|
Prel (mm) |
N (mmHg) |
R abs (mm) |
||||
Water vapor in the atmosphere. Water vapor in the air, despite the huge surfaces of oceans, seas, lakes and rivers, is not always saturated. The movement of air masses leads to the fact that in some places on our planet the evaporation of water currently prevails over condensation, while in others, on the contrary, condensation predominates. But there is almost always some amount of water vapor in the air.
The content of water vapor in the air, i.e. its humidity, can be characterized by several quantities.
The density of water vapor in the air is called absolute humidity. Absolute humidity is therefore measured in kilograms per cubic meter (kg/m3).
Partial pressure of water vapor. Atmospheric air is a mixture of various gases and water vapor. Each of the gases contributes to the total pressure produced by the air on the bodies in it. The pressure that water vapor would produce if all other gases were absent is called partial pressure of water vapor. The partial pressure of water vapor is taken as one of the indicators of air humidity. It is expressed in pressure units - pascals or millimeters of mercury.
Atmospheric pressure is determined by the sum of the partial pressures of the components of dry air (oxygen, nitrogen, etc.) and water vapor.
Relative humidity. Based on the partial pressure of water vapor and absolute humidity, it is still impossible to judge how close water vapor is to saturation under these conditions. Namely, the intensity of water evaporation and moisture loss by living organisms depends on this. That is why a value is introduced that shows how close water vapor is to saturation at a given temperature - relative humidity.
Relative air humidity called the partial pressure ratio r water vapor contained in the air at a given temperature to pressure r n.p. saturated steam at the same temperature, expressed as a percentage:
Relative humidity is usually less than 100%.
Psychrometer. Air humidity is measured using special instruments. We will tell you about one of them - psychrometer.
The psychrometer consists of two thermometers ( Fig.11.4). The reservoir of one of them remains dry and it shows the air temperature. The reservoir of the other is surrounded by a strip of cloth, the end of which is dipped into the water. The water evaporates, and this cools the thermometer. The higher the relative humidity, the less intense evaporation occurs and the temperature indicated by a thermometer surrounded by a damp cloth is closer to the temperature of a dry thermometer.
At a relative humidity of 100%, water will not evaporate at all and the readings of both thermometers will be the same. Based on the temperature difference between these thermometers, using special tables, you can determine the air humidity.
Humidity value. The intensity of moisture evaporation from the surface of human skin depends on humidity. And the evaporation of moisture is of great importance for maintaining body temperature constant. IN spaceships the most favorable relative air humidity for humans is maintained (40-60%).
It is very important to know humidity in meteorology - in connection with weather forecasting. Although the relative amount of water vapor in the atmosphere is relatively small (about 1%), its role in atmospheric phenomena is significant. Condensation of water vapor leads to the formation of clouds and subsequent precipitation. This releases a large amount of heat. Conversely, the evaporation of water is accompanied by the absorption of heat.
In weaving, confectionery and other industries, a certain humidity is required for the normal course of the process.
Storing works of art and books requires maintaining air humidity at the required level. That's why you can see psychrometers on the walls of museums.
It is important to know not the absolute amount of water vapor in the atmosphere, but the relative one. Relative humidity is measured with a psychrometer.
Absolute and relative air humidity
Absolute and relative air humidity. Atmospheric air always contains some moisture in the form of vapor. Air humidity in rooms with natural ventilation is determined by the release of moisture by people and plants during breathing, the evaporation of household moisture during cooking, washing and drying clothes, as well as process moisture (in industrial premises) and the humidity of enclosing structures (in the first year of operation of buildings).
The amount of moisture in grams contained in 1 m3 of air is called absolute humidity f, g/m3. However, for calculations of vapor diffusion through building envelopes, the amount of water vapor must be estimated in units of pressure, which allows the driving force for moisture transfer to be calculated. For this purpose, building thermophysics uses partial pressure water vapor e, called the water vapor pressure and expressed in Pascals.
Partial pressure increases as the absolute humidity of the air increases. However, like absolute humidity, it cannot increase indefinitely. At a certain temperature and barometric air pressure, there is a limiting value of absolute air humidity F, g/m3, corresponding to complete saturation of the air with water vapor, beyond which it cannot increase. This absolute air humidity corresponds to the maximum pressure of water vapor
E, Pa, also called saturated water vapor pressure. With increasing air temperature, E and F increase. Consequently, both e and f do not give an idea of the degree of saturation of the air with moisture if the temperature is not indicated.
To express the degree of air saturation with moisture, the concept of relative air humidity j, % is introduced, which is the ratio of the partial pressure of water vapor e in the air environment under consideration to the maximum elasticity of water vapor E, corresponding to the temperature of the environment j = (e/E)100%.
Relative air humidity is of great importance when assessing it both from a hygienic and technical point of view; it determines the intensity of moisture evaporation from moistened surfaces and, in particular, from the surface of the human body. Relative air humidity of 30–60% is considered normal for humans. j determines the sorption process, i.e., the process of moisture absorption by capillary-porous materials located in the air. Finally, the process of moisture condensation in the air (formation of fogs) and on the surface of enclosing structures depends on j.
If you increase the temperature of air with a given moisture content, then the relative humidity will decrease, since the partial pressure of water vapor E remains constant, and the maximum elasticity E increases with increasing temperature.
When the temperature of air with a given moisture content decreases, the relative humidity increases, since at a constant partial pressure of water vapor E, the maximum elasticity E decreases with decreasing temperature. In the process of decreasing the air temperature at a certain value, the maximum elasticity of water vapor E turns out to be equal to the partial pressure of water vapor e. Then the relative air humidity j will be equal to 100% and the state of complete saturation of the cooled air with water vapor will occur. This temperature is called the dew point temperature for a given air humidity.
CONTENT: In the atmosphere, water is found in gaseous (water vapor), droplet-liquid and solid (ice crystals) states. Air humidity is one of the most significant characteristics of weather and climate. Air humidity is of great importance in some technological processes, the treatment of a number of diseases, the storage of works of art, books, etc.
In the atmosphere, water is found in gaseous (water vapor), droplet-liquid and solid (ice crystals) states.
Air humidity is one of the most significant characteristics of weather and climate. Air humidity is of great importance in some technological processes, the treatment of a number of diseases, the storage of works of art, books, etc. Several concepts are used to characterize air humidity.
Partial pressure of water vapor (e, Pa or mmHg) is the part of the total pressure that is created by water vapor.
Absolute air humidity (a, g/m) - moisture content per unit volume of air.
If the air contains the maximum amount of moisture that can be in it at a given temperature, then the partial pressure of water vapor
The process of evaporation of moisture by plants is called transpiration.
it is equal to the saturated vapor pressure of water at a given temperature.
The pressure of saturated water vapor (E, mm Hg) at temperature (t, ° C) can be found using the formula:
log E = 0.622 + 7.5t/(238+t)
Relative humidity (f, %) - the ratio of the actual moisture content to the maximum possible at a given temperature.
Relative humidity can be calculated as the ratio of partial pressure to saturated vapor pressure at a given temperature:
Thus, relative humidity gives an indication of the degree
saturation of air with water vapor.
Dew point (t, C) is the temperature to which the air must be cooled so that the water vapor contained in it reaches a state of saturation. Knowing the dew point and air temperature, you can determine the absolute and relative humidity of the air.
Absolute air humidity and partial pressure of water vapor characterize the content of water vapor in the air (moisture content), and relative humidity and saturation deficit characterize the relationship between the actual moisture content of the air and the maximum possible (saturated state).
2. At a temperature of 25°C, absolute air humidity is 20 g/m3. Determine the relative humidity. The necessary reference materials can be found in the training manual
log E = 0.622 + 7.5t/(238+t)
log E = 0.622 + 7.5*25/(238+25)=1.33
f=(20/22.96)*100%=87.1%
Answer: relative humidity is 87.1%
3. Air temperature 40°C, relative humidity 70%. Determine the content of water vapor in the air (in g/m). The necessary reference materials can be found in the study guide.
log E = 0.622 + 7.5t/(238+t)
e = 50.25*0.7=35.2g/m3
2. At a temperature of 40°C, absolute air humidity is 40 g/m3. Determine the relative humidity. The necessary reference materials can be found in the study guide.
log E = 0.622 + 7.5t/(238+t)
log E = 0.622 + 7.5*40/(238+40)=1.7
f=(40/50.25)*100%=79.6%
Answer: relative humidity is 79.6%
3. Air temperature 10°C, relative humidity 70%. Determine the content of water vapor in the air (in g/m3). The necessary reference materials can be found in the study guide.
log E = 0.622 + 7.5t/(238+t)
log E = 0.622 + 7.5*10/(238+10)=1.7
