Determination of absolute air humidity. Absolute and relative humidity

You will need

• - mercury thermometer;
• - sealed vessel;
• - table of the dependence of saturated water vapor on temperature;
• - psychrometer.

Instructions

To measure humidity directly, take a sample air into an airtight container and begin to cool it. When dew appears on the walls of the vessel (steam condenses), write down the temperature at which this occurs. Using a special table, find the density saturated steam at the temperature where it condensed. This will be absolute humidity air, the sample of which was taken.

Determining relative humidity with two thermometers Take two identical thermometers. Liquid mercury thermometers are better suited. Wrap gauze around a bottle of working fluid from one of them, then moisten it generously with water. After waiting for some time, take readings from the thermometers in Celsius. Then find the temperature difference on the wet and dry thermometer; the thermometer readings will be either the same or lower than the dry thermometer. In the psychrometric table, find the column of dry bulb readings and find the closest match to what the measurement showed. Then, using the line, find the value that corresponds to the calculated difference in the readings of the dry and wet thermometers; the cell will contain the relative humidity air in percentages.

Determination of relative humidity with a hair hygrometerSince horsehair changes its length depending on humidity air, tighten it and attach it to the sensitive dynamometer. By strength you can determine the relative humidity air. This measurement will be the least accurate.

Helpful advice

When calculating, the saturated vapor pressure can be replaced by its density, this will not affect the result.

Humidity shows how much water vapor is contained in the air. An important environmental indicator of the environment is relative humidity. If it takes too low or too high values, a person quickly gets tired, his perception, memory and well-being worsen.

Instructions

Humidity can be absolute or relative. Absolute humidity f shows the actual amount of water vapor by mass that is in one air. To find the absolute humidity of the air, divide the mass of vapor by the total volume. Units of measurement – ​​per cubic meter, g/m³.

There is a concept of maximum absolute humidity at a fixed temperature. The fact is that density cannot increase indefinitely, at a certain point there is thermodynamic equilibrium. This is the state of the system in which macroscopic parameters, such as temperature, volume, pressure, entropy, are constant in time. These values ​​fluctuate around their average values ​​if the external environment.

So, when thermodynamic equilibrium occurs between steam and air, the air is said to be saturated with steam. The humidity of air saturated with steam is maximum. It is also called the saturation limit. It is also in g/m³. We can designate it as F.

Absolute humidity

Absolute air humidity is the density of water vapor in the air, in other words, the mass of water vapor that actually fits in one cubic meter of air. The indicator is measured in grams per cubic meter.

The air is quite capable of reaching a state of complete saturation, this is due to the fact that at a constant temperature it is able to absorb only a certain amount of steam. This absolute humidity (when the air is completely saturated) is called moisture capacity.

Relative humidity

Moisture capacity directly depends on temperature, and increases sharply as it increases. If you calculate the ratio of the absolute humidity of air at a specific temperature to its moisture capacity at the same temperature, you get an indicator called.

If we analyze the values ​​of relative humidity on an Earth scale, then it is highest in equatorial zone, in polar latitudes and inside mid-latitude continents in winter time, and is lowest in subtropical and . As altitude increases, air humidity quickly decreases.

How to find out relative humidity

To determine the relative humidity of the air, a special device is used - a psychrometer. Essentially this is a system of two thermometers. A gauze cover is put on one of them, the tip of which is lowered into water. The second thermometer operates as usual and shows the current air temperature. The first one, a thermometer with a cover, shows more low temperature(after all, when moisture evaporates from the cover, heat is consumed).

The temperature value shown by a wet bulb thermometer is called the cooling limit, and the difference between the dry and wet bulb readings is called the psychrometric difference. In this case, the relative humidity of the air is inversely proportional to the psychrometric difference: the lower the humidity, the more moisture the air can absorb.

To obtain a numerical indicator of relative humidity, you need to divide the absolute humidity value by the maximum possible humidity. Usually the result is expressed as a percentage.

The air humidity indicator is very important, because if the humidity is too low or high, a person’s well-being worsens, performance decreases, perception and memory deteriorate. In addition, food, building materials and many electronic components must be stored within strictly defined air humidity limits.

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 certain values ​​affect the human body.

To determine the saturation of air with water vapor, special instruments 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 cools its body as it evaporates. Based on the readings of these thermometers, the relative humidity of the air is determined from the tables. There are many different hydrometers; their operation can be based on weight, film, electric or hair, as well as a number of other operating principles. IN last years Integrated measurement sensors have gained popularity. Hydrostats are used to check accuracy.

What is this article about?

Definition

In addition to relative air humidity, there is also such a value as absolute humidity. The amount of water vapor per unit volume of air is called absolute humidity. Since mass is taken as a unit of measurement of quantity, and its values ​​for steam in a cubic meter of air are small, it was customary to measure absolute humidity in g/m³. These indicators vary from parts of a unit of measurement to more than 30 g/m³, depending on the time of year and geographical location surface above which humidity is measured.

Absolute humidity is the main indicator characterizing the state of the air, and great importance To determine its properties, it is necessary to compare humidity with ambient temperature, since these parameters are interrelated. For example, when the temperature drops, water vapor reaches a state of saturation, after which the condensation process begins. The temperature at which this occurs is called the dew point.

Instruments for determining absolute humidity

Determination of the absolute humidity value is based on its calculations based on thermometer readings. In particular, according to the readings of the Augustus psychrometer, consisting of two mercury thermometers - one of which is dry and the other wet (picture A in the picture). The evaporation of water from the surface indirectly in contact with the tip of the thermometer leads to a decrease in its readings. The difference between the readings of both thermometers is the basis of August's formula, which determines absolute humidity. Air flows and thermal radiation can influence the error in such measurements.

The aspiration psychrometer proposed by Assmann is more accurate (picture B). It is designed with a protective tube that limits the effects of thermal radiation and an aspiration fan that creates a stable air flow. Absolute humidity is determined by a formula that reflects its dependence on thermometer readings and barometric pressure during this period of time.

Absolute Humidity Measurement Value

Monitoring absolute humidity values ​​is necessary in meteorology, since these readings play a large role in predicting possible precipitation. Psychrometers are also used in mines. The need for constant monitoring of absolute humidity in many automation systems is a prerequisite for the development of more modern meters. These are electronic sensors that take the necessary measurements, analyze the readings and display the already calculated value of absolute humidity.

The saturated vapor pressure of water increases greatly with increasing temperature. Therefore, with isobaric (that is, at constant pressure) cooling of air with a constant vapor concentration, a moment (dew point) comes when the vapor is saturated. In this case, the “extra” steam condenses in the form of fog, dew or ice crystals. The processes of saturation and condensation of water vapor play a huge role in atmospheric physics: processes of cloud formation and formation atmospheric fronts are largely determined by the processes of saturation and condensation; the heat released during the condensation of atmospheric water vapor provides the energy mechanism for the emergence and development of tropical cyclones (hurricanes).

Relative humidity is the only hygrometric indicator of air that allows direct instrumental measurement.

Relative Humidity Estimation

The relative humidity of a water-air mixture can be estimated if its temperature is known ( T) and dew point temperature ( Td), By the following formula:

Where P s- saturated vapor pressure for the corresponding temperature, which can be calculated using the Arden Buck formula:

Approximate calculation

Relative humidity can be approximately calculated using the following formula:

That is, with every degree Celsius difference between the air temperature and the dew point temperature, the relative humidity decreases by 5%.

Additionally, relative humidity can be estimated using a psychrometric chart.

Supersaturated water vapor

In the absence of condensation centers, when the temperature decreases, a supersaturated state may form, that is, the relative humidity becomes more than 100%. Ions or aerosol particles can act as condensation centers; it is on the condensation of supersaturated vapor on ions formed during the passage of a charged particle in such a vapor that the operating principle of the Wilson chamber and diffusion chambers is based: water droplets condensing on the formed ions form a visible trace (track ) charged particle.

Another example of the condensation of supersaturated water vapor is the contrails of aircraft, which occur when supersaturated water vapor condenses on soot particles from engine exhaust.

Means and methods of control

To determine air humidity, instruments called psychrometers and hygrometers are used. August's psychrometer consists of two thermometers - dry and wet. A wet thermometer shows a lower temperature than a dry thermometer because its reservoir is wrapped in a cloth soaked in water, which cools it as it evaporates. The intensity of evaporation depends on the relative humidity of the air. Based on the readings of dry and wet thermometers, the relative humidity of the air is found using psychrometric tables. IN Lately Integrated humidity sensors (usually with voltage output) have become widely used, based on the property of some polymers to change their electrical characteristics (such as the dielectric constant of the medium) under the influence of water vapor contained in the air.

Comfortable air humidity for humans is determined by documents such as GOST and SNIP. They regulate that in winter the optimal humidity for a person in a room is 30-45%, in summer – 30-60%. The data on SNIP are slightly different: 40-60% for any time of year, the maximum level is 65%, but for very humid regions - 75%.

To determine and confirm the metrological characteristics of instruments for measuring humidity, special reference (model) installations are used - climatic chambers (hygrostats) or dynamic generators of gas humidity.

Meaning

Relative air humidity is an important environmental indicator of the environment. If the humidity is too low or too high, a person becomes fatigued quickly, and perception and memory deteriorate. Human mucous membranes dry out, moving surfaces crack, forming microcracks into which viruses, bacteria, and microbes directly penetrate. Low relative humidity (up to 5-7%) in apartments and offices has been observed in regions with prolonged low negative outdoor temperatures. Typically, a duration of up to 1-2 weeks at temperatures below −20 ° C leads to drying out of the premises. A significant deteriorating factor in maintaining relative humidity is air exchange at low negative temperatures. The greater the air exchange in the rooms, the faster low (5-7%) relative humidity is created in these rooms.

Ventilating rooms in cold weather to increase humidity is blunder- this is the most effective method achieve the opposite. The reason for the widespread misconception is the perception of relative humidity figures, known to everyone from weather forecasts. These are percentages of a certain number, but this number is different for a room and a street! You can find out this number from the table linking temperature and absolute humidity. For example, 100% humidity of street air at −15 °C means 1.6 g of water per 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 can be stored in a strictly defined range of relative humidity. Many technological processes occur only with strict control of the water vapor content in the air of the production premises.

The humidity in the room can be changed.

Humidifiers are used to increase humidity.

The functions of dehumidifying (reducing humidity) of air are implemented in most air conditioners and in the form of separate devices - air dehumidifiers.

In floriculture

The relative air humidity in greenhouses and residential premises used for cultivating plants is subject to fluctuations, which is determined by the time of year, air temperature, the degree and frequency of watering and spraying of plants, the presence of humidifiers, aquariums or other containers with an open water surface, ventilation and heating systems. Cacti and many succulent plants tolerate dry air more easily than many tropical and subtropical plants.
As a rule, for plants whose homeland is wet rainforests, the optimal relative air humidity is 80-95% (in winter it can be reduced to 65-75%). For plants of warm subtropics - 75-80%, cold subtropics - 50-75% (Levy, cyclamen, cineraria, etc.)
When keeping plants in residential areas, many species suffer from dry air. First of all, this affects

When we're talking about about our health, then knowledge of relative air humidity and the formula for determining it comes first. However, it is not necessary to know the exact formula, but it would be nice to at least general outline imagine what it is, why measure humidity in the house, and in what ways this can be done.

What should be the optimal humidity?

Humidity in a room where a person works, spends leisure time or sleeps is of particular importance. Our respiratory organs are designed in such a way that air that is too dry or saturated with water vapor is harmful to them. Therefore there are state standards, which regulate what the air humidity in the room should be.

Optimal humidity zone

In general, there are a dozen ways to control air humidity and bring it back to normal. This will create the most favorable conditions for study, sleep, sports, will increase performance and improve well-being.