The dew point at a given pressure is the temperature to which the air must cool so that the water vapor contained in it reaches saturation and begins to condense into dew.
The dew point is determined by the relative humidity and air temperature. The higher the relative humidity, the higher the dew point and closer to the actual air temperature. The lower the relative humidity, the lower the dew point of the actual temperature. If the relative humidity is 100%, the dew point is the same as the actual temperature.
Real life example- any object is brought into a warm room from frost. The air above the surface of such a thing cools below the dew point (for the current humidity and temperature) and "dew" forms on the surface. The higher the moisture content in the air, the less the temperature difference between the air temperature and the temperature of the same object is needed in order for the condensation process to begin. Subsequently, the object heats up to room temperature and the condensate evaporates. Actually, this is the reason for the recommendation not to immediately turn on household appliances brought in from the cold.
The dew point of the air is the most important parameter that indicates the humidity and the possibility of condensation in the room, but it cannot be controlled. This is a physical term. The dew point can be found on the graphs showing the relationship between humidity and room temperature.
If the temperature of the inner glass in the glass unit is equal to or lower than the dew point temperature at this moment relative humidity of the indoor air, condensation may appear on the glass.
There are several ways to lower the humidity in the room:
1. It is recommended to maintain the air temperature in the room not lower than 20 ° С, and the relative humidity not higher than 30-40%.
2. It is recommended to ventilate the room at least 3 times a day for 10-15 minutes. At the time of buying plastic windows ask the managers about the additional capabilities of the microclimate controllers: combs, micro-ventilation, winter ventilation, ventilation valves allow you to choose the most comfortable and effective method airing the room.
3. Ventilation hood must have traction. It is recommended to keep the interior doors open. (provide a gap of 15-20mm between the door and the floor)
4. Heating devices (batteries) should be removed from obstructing objects (sofas, furniture, blackout curtains, etc.)
Dew point table. Example: if the room temperature is + 20 ° C, and the relative humidity is 40%; dew point at which condensation can occur on glasses is + 6 ° С
| Owner / t | 0 | 2,5 | 5 | 7,5 | 10 | 12,5 | 15 | 17,5 | 20 | 22,5 | 25 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 20 | -20 | -18 | -16 | -14 | 12 | -9,8 | -7,7 | -5,6 | -3,6 | -1,5 | -0,5 |
| 30 | -15 | -13 | -11 | -8,9 | -6,7 | -4,5 | -2,4 | -0,2 | 1,9 | 4,1 | 6,2 |
| 40 | -12 | -9,7 | -7,4 | -5,2 | -2,9 | -0,7 | 1,5 | 3,8 | 6,0 | 8,2 | 10,5 |
| 50 | -9,1 | -6,8 | -4,5 | -2,2 | 0,1 | 2,4 | 4,7 | 7,0 | 9,3 | 11,6 | 13,9 |
| 60 | -6,8 | -4,4 | -2,1 | 0,3 | 2,6 | 5,0 | 7,3 | 9,7 | 12,0 | 14,4 | 16,7 |
| 70 | -4,8 | -2,4 | 0,0 | 2,4 | 4,8 | 7,2 | 9,6 | 12,0 | 14,4 | 16,8 | 19,1 |
| 80 | -3,0 | -0,6 | 1,9 | 4,3 | 6,7 | 9,2 | 11,6 | 14,0 | 16,4 | 18,9 | 21,3 |
| 90 | -1,4 | 1,0 | 3,5 | 6,0 | 8,4 | 10,9 | 13,4 | 15,8 | 18,3 | 20,8 | 23,2 |
| 100 | 0,0 | 2,5 | 5,0 | 7,5 | 10,0 | 12,5 | 15,0 | 17,5 | 20,0 | 22,5 | 25,0 |
The partial pressure of water vapor contained in the room air (absolute humidity of indoor air e at) depends on the temperature of the indoor air t at and its relative humidity in how
e in \u003d E (t)
The dependence is presented graphically in Figure 1:
At a low outside temperature, the temperature on the inner surface of the glazing (τ vp) will be significantly lower than the indoor air temperature (in the middle of the room at a height of 1.5 m from the floor) In this case, the limiting value of the partial pressure of water vapor E, corresponding to the temperature τ c.p. may be lower than the calculated f in \u003d f (t in, in), which will lead to the fallout of "excess" water vapor on the cold inner surface of the glazing in the form of condensation or frost. Temperature value at which E \u003d f ( τ c.p.) and е в \u003d f (t в, in)will be equal, corresponds dew point temperature.
Let's determine the probability of condensation on the inner surface of a 4-12-4 single-chamber double-glazed unit installed with an internal air temperature t \u003d 20 ° C and an internal air humidity h \u003d 60%, provided that the outside temperature drops to a value of t n \u003d -30 ° C.
- According to GOST 24866-99 "Glued double-glazed windows", the reduced heat transfer resistance of a 4-12-4 double-glazed window is R o \u003d 0.30 m 2 ° C / W
- Determine the dew point at the indoor air temperature t in \u003d 20 ° C and relative humidity h \u003d 60%. In accordance with Figure 1, the limiting value of the partial pressure of water vapor E at a temperature of t at \u003d 20 ° C is 17.53 mm Hg. According to the equation е в \u003d E (t) absolute air humidity e \u003d 17.53 * 0.6 \u003d 10.52 mm Hg, which corresponds to the dew point t \u003d 12.0 ° C
- We determine the temperature on the inner surface of the glass unit.
τ c.p. when the outdoor temperature drops to -30 ° С. The total temperature difference in this case is equal to δ T \u003d T in -T n \u003d 20 + 30 \u003d 50 ° C.
Based on the fact that the temperature drop in the thickness of the enclosing structure from the inside to the outside is proportional to the change in thermal resistance, namely
δ t in \u003d (δ .Т / R o) xR in where
R in \u003d 0.12 - resistance to heat transfer at the inner surface of the glazing.
Accordingly, we get t in \u003d (50 / 0.30) x0.12 \u003d 19.99 ° C
The temperature on the inner surface of the glass unit will be equal to τ c.p. \u003d 20-19.99 \u003d 0.01 ° C, which is significantly lower than the dew point temperature for a given room (t \u003d 12 ° C)
Thus, the temperature on the inner surface of a single-chamber double-glazed unit installed in a room with an internal air temperature t in \u003d 20 ° С and an internal air humidity c \u003d 60%, provided that the outside temperature drops to a value of t n \u003d -30 ° C, it will be significantly lower than the dew point temperature, which will lead to abundant condensation and the formation of ice on the glass from inside the room.
So, to sum up, we can say that such conditions of the example are acceptable for some industrial enterprises, parking lots, shopping centers, etc. that is, for premises not intended for permanent residence of people
Window firms all the time faced with a dew point - the eternal problem of condensation, especially in winter (windows "flow", "cry" in the cold, condensation falls abundantly on glass and frames) does not give anyone rest. This problem especially worries those who have not yet installed windows for themselves and are very afraid to face this problem in the future.
Dew point determines the ratio of air temperature, air humidity and surface temperature at which water begins to condense on the surface.
Production and sale of materials, execution of works: Polymer floors Self-leveling floors
Dew point definition
Dew point determination is an extremely important factor in the installation of any polymer floors, coatings and self-leveling floors on any bases: concrete, metal, wood, etc. The appearance of a dew point and, accordingly, water condensation on the surface of the base at the time of laying polymer floors of self-leveling floors and coatings can cause the appearance of a variety of defects: shagreen, swelling and shells; complete peeling of the coating from the base. Visual determination of the dew point - the appearance of moisture on the surface - is almost impossible, therefore, the technology below is used to calculate the dew point.
Dew point table
The dew point table is used very simply - hover the mouse over it ... Dew point table - download
For example: air temperature + 16 ° С, relative air humidity 65%.
Find the cell at the intersection of air temperature + 16 ° С and air humidity 65%. It turned out + 9 ° С - this is the Dew Point.
This means that if the surface temperature is equal to or below + 9 ° C, moisture will condense on the surface.
| Tempe- ratura air | Dew point temperature at relative humidity (%) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30% | 35% | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% | |
| -10 ° C | -23,2 | -21,8 | -20,4 | -19 | -17,8 | -16,7 | -15,8 | -14,9 | -14,1 | -13,3 | -12,6 | -11,9 | -10,6 | -10 |
| -5 ° C | -18,9 | -17,2 | -15,8 | -14,5 | -13,3 | -11,9 | -10,9 | -10,2 | -9,3 | -8,8 | -8,1 | -7,7 | -6,5 | -5,8 |
| 0 ° C | -14,5 | -12,8 | -11,3 | -9,9 | -8,7 | -7,5 | -6,2 | -5,3 | -4,4 | -3,5 | -2,8 | -2 | -1,3 | -0,7 |
| + 2 ° C | -12,8 | -11 | -9,5 | -8,1 | -6,8 | -5,8 | -4,7 | -3,6 | -2,6 | -1,7 | -1 | -0,2 | -0,6 | 1,3 |
| + 4 ° C | -11,3 | -9,5 | -7,9 | -6,5 | -4,9 | -4 | -3 | -1,9 | -1 | 0 | 0,8 | 1,6 | 2,4 | 3,2 |
| + 5 ° C | -10,5 | -8,7 | -7,3 | -5,7 | -4,3 | -3,3 | -2,2 | -1,1 | -0,1 | 0,7 | 1,6 | 2,5 | 3,3 | 4,1 |
| + 6 ° C | -9,5 | -7,7 | -6 | -4,5 | -3,3 | -2,3 | -1,1 | -0,1 | 0,8 | 1,8 | 2,7 | 3,6 | 4,5 | 5,3 |
| + 7 ° C | -9 | -7,2 | -5,5 | -4 | -2,8 | -1,5 | -0,5 | 0,7 | 1,6 | 2,5 | 3,4 | 4,3 | 5,2 | 6,1 |
| + 8 ° C | -8,2 | -6,3 | -4,7 | -3,3 | -2,1 | -0,9 | 0,3 | 1,3 | 2,3 | 3,4 | 4,5 | 5,4 | 6,2 | 7,1 |
| + 9 ° C | -7,5 | -5,5 | -3,9 | -2,5 | -1,2 | 0 | 1,2 | 2,4 | 3,4 | 4,5 | 5,5 | 6,4 | 7,3 | 8,2 |
| + 10 ° C | -6,7 | -5,2 | -3,2 | -1,7 | -0,3 | 0,8 | 2,2 | 3,2 | 4,4 | 5,5 | 6,4 | 7,3 | 8,2 | 9,1 |
| + 11 ° C | -6 | -4 | -2,4 | -0,9 | 0,5 | 1,8 | 3 | 4,2 | 5,3 | 6,3 | 7,4 | 8,3 | 9,2 | 10,1 |
| + 12 ° C | -4,9 | -3,3 | -1,6 | -0,1 | 1,6 | 2,8 | 4,1 | 5,2 | 6,3 | 7,5 | 8,6 | 9,5 | 10,4 | 11,7 |
| + 13 ° C | -4,3 | -2,5 | -0,7 | 0,7 | 2,2 | 3,6 | 5,2 | 6,4 | 7,5 | 8,4 | 9,5 | 10,5 | 11,5 | 12,3 |
| + 14 ° C | -3,7 | -1,7 | 0 | 1,5 | 3 | 4,5 | 5,8 | 7 | 8,2 | 9,3 | 10,3 | 11,2 | 12,1 | 13,1 |
| + 15 ° C | -2,9 | -1 | 0,8 | 2,4 | 4 | 5,5 | 6,7 | 8 | 9,2 | 10,2 | 11,2 | 12,2 | 13,1 | 14,1 |
| + 16 ° C | -2,1 | -0,1 | 1,5 | 3,2 | 5 | 6,3 | 7,6 | 9 | 10,2 | 11,3 | 12,2 | 13,2 | 14,2 | 15,1 |
| + 17 ° C | -1,3 | 0,6 | 2,5 | 4,3 | 5,9 | 7,2 | 8,8 | 10 | 11,2 | 12,2 | 13,5 | 14,3 | 15,2 | 16,6 |
| + 18 ° C | -0,5 | 1,5 | 3,2 | 5,3 | 6,8 | 8,2 | 9,6 | 11 | 12,2 | 13,2 | 14,2 | 15,3 | 16,2 | 17,1 |
| + 19 ° C | 0,3 | 2,2 | 4,2 | 6 | 7,7 | 9,2 | 10,5 | 11,7 | 13 | 14,2 | 15,2 | 16,3 | 17,2 | 18,1 |
| + 20 ° C | 1 | 3,1 | 5,2 | 7 | 8,7 | 10,2 | 11,5 | 12,8 | 14 | 15,2 | 16,2 | 17,2 | 18,1 | 19,1 |
| + 21 ° C | 1,8 | 4 | 6 | 7,9 | 9,5 | 11,1 | 12,4 | 13,5 | 15 | 16,2 | 17,2 | 18,1 | 19,1 | 20 |
| + 22 ° C | 2,5 | 5 | 6,9 | 8,8 | 10,5 | 11,9 | 13,5 | 14,8 | 16 | 17 | 18 | 19 | 20 | 21 |
| + 23 ° C | 3,5 | 5,7 | 7,8 | 9,8 | 11,5 | 12,9 | 14,3 | 15,7 | 16,9 | 18,1 | 19,1 | 20 | 21 | 22 |
| + 24 ° C | 4,3 | 6,7 | 8,8 | 10,8 | 12,3 | 13,8 | 15,3 | 16,5 | 17,8 | 19 | 20,1 | 21,1 | 22 | 23 |
| + 25 ° C | 5,2 | 7,5 | 9,7 | 11,5 | 13,1 | 14,7 | 16,2 | 17,5 | 18,8 | 20 | 21,1 | 22,1 | 23 | 24 |
| + 26 ° C | 6 | 8,5 | 10,6 | 12,4 | 14,2 | 15,8 | 17,2 | 18,5 | 19,8 | 21 | 22,2 | 23,1 | 24,1 | 25,1 |
| + 27 ° C | 6,9 | 9,5 | 11,4 | 13,3 | 15,2 | 16,5 | 18,1 | 19,5 | 20,7 | 21,9 | 23,1 | 24,1 | 25 | 26,1 |
| + 28 ° C | 7,7 | 10,2 | 12,2 | 14,2 | 16 | 17,5 | 19 | 20,5 | 21,7 | 22,8 | 24 | 25,1 | 26,1 | 27 |
| + 29 ° C | 8,7 | 11,1 | 13,1 | 15,1 | 16,8 | 18,5 | 19,9 | 21,3 | 22,5 | 22,8 | 25 | 26 | 27 | 28 |
| + 30 ° C | 9,5 | 11,8 | 13,9 | 16 | 17,7 | 19,7 | 21,3 | 22,5 | 23,8 | 25 | 26,1 | 27,1 | 28,1 | 29 |
| + 32 ° C | 11,2 | 13,8 | 16 | 17,9 | 19,7 | 21,4 | 22,8 | 24,3 | 25,6 | 26,7 | 28 | 29,2 | 30,2 | 31,1 |
| + 34 ° C | 12,5 | 15,2 | 17,2 | 19,2 | 21,4 | 22,8 | 24,2 | 25,7 | 27 | 28,3 | 29,4 | 31,1 | 31,9 | 33 |
| + 36 ° C | 14,6 | 17,1 | 19,4 | 21,5 | 23,2 | 25 | 26,3 | 28 | 29,3 | 30,7 | 31,8 | 32,8 | 34 | 35,1 |
| + 38 ° C | 16,3 | 18,8 | 21,3 | 23,4 | 25,1 | 26,7 | 28,3 | 29,9 | 31,2 | 32,3 | 33,5 | 34,6 | 35,7 | 36,9 |
| + 40 ° C | 17,9 | 20,6 | 22,6 | 25 | 26,9 | 28,7 | 30,3 | 31,7 | 33 | 34,3 | 35,6 | 36,8 | 38 | 39 |
Dew point calculation
To calculate the dew point, you need devices: thermometer, hygrometer.
- Measure the temperature at a height of 50-60cm from the floor (or from the surface) and the relative humidity.
- Determine the dew point temperature from the table.
- Measure the surface temperature. If you do not have a dedicated non-contact thermometer, place a regular thermometer on a surface and cover it to insulate it from the air. Take readings after 10-15 minutes.
- The surface temperature must be at least four (4) degrees above the dew point.
Otherwise, it is NOT POSSIBLE to carry out work on the application of polymer floors and polymer coatings!
There are devices that immediately calculate the dew point in degrees C.
In this case, the thermometer, hygrometer and dew point table are not required - they are all combined in this device.
Various polymer coatings differently "refer" to moisture on the surface during application. The most "sensitive" to the occurrence of dew point are polyurethane materials: paint coatings, polyurethane self-leveling floors, varnishes, etc. This is due to the fact that water for polyurethane is a hardener, and with an excess of moisture, the polymerization reaction proceeds very quickly. The result is a variety of coating defects. A particularly unpleasant defect is a decrease in adhesion, which cannot be immediately determined, and over time this leads to partial or complete peeling of the coating or polymer floor.
It is important to keep in mind that the dew point is dangerous not only at the time of coating, but also during curing. This is especially dangerous for self-leveling floors, since the time of their initial curing is quite long (up to a day).
Epoxy self-leveling floors and coatings are "less sensitive" to moisture, but, nevertheless, dew point determination is a guarantee of quality when installing any polymer floors and paintwork.
Here are some examples that will clearly show what a dew point is, and then we will give a definition of this effect from the point of view of physics. And so, when bathing in the bathroom, the mirror fogs up without fail. We enter the store from frost, glasses fog up. On the glass lid of the frying pan that fries potatoes for us, drops of water are also found. Here are literally just three examples that show what a point of rossa is.
What is dew point
Many may be surprised to answer that this is a simple process of condensing steam, and they will be completely right. After all, the dew point is the temperature at which water vapor or humidity contained in the air around us decreases so much that this vapor turns into water droplets. That is, the process of condensation of water vapor occurs.
But it should be noted that the condensation process itself is influenced by two factors simultaneously - this is humidity and temperature. However, usually when confronted with the term "dew point", the main meaning is given to relative humidity. And here it is all interconnected. For example, if the relative humidity is higher, then the dew point is also higher and becomes closer to the ambient temperature. At 100% relative humidity, the dew point is the same as the temperature. Here is a purely mathematical alignment.
Example
I would like to give an example that will show exactly how dew forms on surfaces. We've all noticed that any vessel with an open mouth and filled with water will lower the water level over time. This is due to the process of water evaporation. So the air that surrounds the vessel will gradually be saturated with these water vapor. This saturation will occur to a certain level, which depends on the air temperature. That is, it is the temperature that affects the content of water vapor in the air.
But the air can accept so much vapor until it reaches the saturation limit. This, by the way, is called relative humidity. Here's an example of how temperature affects humidity. The air temperature is + 20C, the vapor saturation is 17.3 g / m3, that is, for a given temperature this is one hundred percent humidity. At the same saturation temperature of 8.7 g / m3, the relative humidity is already 50%. But if the temperature drops to + 10C, then the air will be considered saturated with a water vapor content of 9.4 g / m3. Now you should understand what is the difference and what is the dependence of the dew point, temperature and humidity.
And yet, as mentioned above, the Ross point is the temperature at which the saturation of air with water vapor is impossible, that is, the evaporation process stops and the condensation process begins.
Murat and Vera Baymirzaevs specially for
Dew point
The dew point is the temperature at which moisture in the air begins to condense on the surface, i.e. fall out in the form of that very dew.
Why is this happening? The reason is an interesting feature of air: the higher the temperature, the more it can hold water vapor. Conversely, at a lower temperature, the air can hold less moisture. Hence the concepts of relative and absolute air humidity appear.
If you heat a closed volume of air, then the absolute humidity in it will not change, because the amount of water in it in milligrams does not change. But the relative humidity of the heated air will decrease, because the air will be able to contain large quantity water vapor.
That is why condensation (dew) falls on a cold bottle taken from the refrigerator: the air in contact with the bottle cools down, the relative humidity reaches 100% and the excess steam condenses.
That is why in winter, with properly functioning ventilation, the air in the room is too dry, I wrote about this in an article about ventilation. Feel the difference: at -20 ° C at normal atmospheric pressure, air can hold less than 1 g of moisture per m 3, and at + 20 ° C as much as 17 g of moisture. The difference is 17 times!
Click on the photo to enlarge.
I have already shown in the article double-glazed windows my double-glazed window with frozen condensate. Quite dry air with 40% relative humidity condensed on the glass unit at an indoor temperature of + 20 ° C. The single-chamber double-glazed window at the bottom is frozen at an external temperature of -25 ° C. The air in contact with the glass unit cooled down, quickly reached a relative humidity of 100% (that is, a dew point appeared) and condensed on the glass unit.
The dew point can also occur inside the wall. Water vapor flowing through the wall can condense inside the wall material. Read more in the article Wall vapor permeability and dew point. If you don't know anything about vapor permeability building materialsthen be sure to read this article.
Dew point table
| Temperature ° C | 0 | 2,5 | 5 | 7,5 | 10 | 12,5 | 15 | 17,5 | 20 | 22,5 | 25 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Relative humidity,% |
|||||||||||
| 20 | -20 | -18 | -16 | -14 | -12 | -9,8 | -7,7 | -5,6 | -3,6 | -1,5 | -0,5 |
| 25 | -18 | -15 | -13 | -11 | -9,1 | -6,9 | -4,8 | -2,7 | -0,6 | 1,5 | 3,6 |
| 30 | -15 | -13 | -11 | -8,9 | -6,7 | -4,5 | -2,4 | -0,2 | 1,9 | 4,1 | 6,2 |
| 35 | -14 | -11 | -9,1 | -6,9 | -4,7 | -2,5 | -0,3 | 1,9 | 4,1 | 6,3 | 8,5 |
| 40 | -12 | -9,7 | -7,4 | -5,2 | -2,9 | -0,7 | 1,5 | 3,8 | 6,0 | 8,2 | 10,5 |
| 45 | -10 | -8,2 | -5,9 | -3,6 | -1,3 | 0,9 | 3,2 | 5,5 | 7,7 | 10,0 | 12,3 |
| 50 | -9,1 | -6,8 | -4,5 | -2,2 | 0,1 | 2,4 | 4,7 | 7,0 | 9,3 | 11,6 | 13,9 |
| 55 | -7,8 | -5,6 | -3,3 | -0,9 | 1,4 | 3,7 | 6,1 | 8,4 | 10,7 | 13,0 | 15,3 |
| 60 | -6,8 | -4,4 | -2,1 | 0,3 | 2,6 | 5,0 | 7,3 | 9,7 | 12,0 | 14,4 | 16,7 |
| 65 | -5,8 | -3,4 | -1,0 | 1,4 | 3,7 | 6,1 | 8,5 | 10,9 | 13,2 | 15,6 | 18,0 |
| 70 | -4,8 | -2,4 | 0,0 | 2,4 | 4,8 | 7,2 | 9,6 | 12,0 | 14,4 | 16,8 | 19,1 |
| 75 | -3,9 | -1,5 | 1,0 | 3,4 | 5,8 | 8,2 | 10,6 | 13,0 | 15,4 | 17,8 | 20,3 |
| 80 | -3,0 | -0,6 | 1,9 | 4,3 | 6,7 | 9,2 | 11,6 | 14,0 | 16,4 | 18,9 | 21,3 |
| 85 | -2,2 | 0,2 | 2,7 | 5,1 | 7,6 | 10,1 | 12,5 | 15,0 | 17,4 | 19,9 | 22,3 |
| 90 | -1,4 | 1,0 | 3,5 | 6,0 | 8,4 | 10,9 | 13,4 | 15,8 | 18,3 | 20,8 | 23,2 |
| 95 | -0,7 | 1,8 | 4,3 | 6,8 | 9,2 | 11,7 | 14,2 | 16,7 | 19,2 | 21,7 | 24,1 |
| 100 | 0,0 | 2,5 | 5,0 | 7,5 | 10,0 | 12,5 | 15,0 | 17,5 | 20,0 | 22,5 | 25,0 |
I took the table from Wikipedia, but I double-checked the numerical data in the cells just in case.
How to use the table: select the air humidity in the left column, and the temperature in the top line. Example: room relative humidity 70%, room temperature + 25 ° C. The dew point is + 19.1 ° C. Those. in this room, condensation will fall on the glass of the bottle if it has a temperature of only + 19.1 ° C and below.
Another example. Winter, relative humidity in the room 55%, temperature + 22.5 ° C. Comfortable living conditions. According to the table, the dew point is + 13 ° C. This means that if there is somewhere a frozen corner of the wall or a slope near a window with a temperature of + 13 ° C and below in the room, then condensation will constantly fall out there. And this is a high probability of developing mold.
How to deal (if required) with dew point and condensation? Three ways:
- reduce relative humidity
- increase the temperature of the place where condensation occurs
- reduce the temperature while maintaining the relative humidity
Today's lesson we will devote to discussing the concept of air humidity and methods of measuring it. The main phenomenon that affects the humidity of the air will be the process of evaporation of water, which we talked about earlier, and the most important concept that we will use will be saturated and unsaturated steam.
If you single out the different states of the vapor, then they will be determined by the interaction between the vapor and its liquid. If we imagine that some liquid is in a closed vessel and the process of its evaporation takes place, then sooner or later this process will come to a state when evaporation at equal intervals of time will be compensated by condensation and the so-called dynamic equilibrium of the liquid with its vapor will come.
Definition.Saturated steam is a vapor that is in thermodynamic equilibrium with its liquid. If the steam is not saturated, then there is no such thermodynamic equilibrium.
With the help of these two concepts, we will describe such an important characteristic of air as humidity.
Definition.Air humidity - the content of water vapor in the air.
The question arises, why is the concept of humidity important for consideration, and how does water vapor get into the air? It is known that most of the Earth's surface is occupied by water (the World Ocean), from the surface of which evaporation occurs continuously (Fig. 1). Of course, in different climatic zones, the intensity of this process is different, which depends on the average daily temperature, the presence of winds, etc. These factors determine the fact that in certain places the process of water vaporization is more intense than its condensation, and in some it is vice versa. On average, it can be argued that the vapor that forms in the air is not saturated, and its properties must be described.
For humans, 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 interconnected with the temperature and humidity of the environment: at high humidity, the processes of moisture evaporation from the surface of the skin are practically compensated by the processes of its condensation and heat removal from the body is disturbed, which leads to violations of thermoregulation; at low humidity, moisture evaporation takes precedence over condensation and the body loses too much liquid, which can lead to dehydration.
The value of humidity is important not only for humans and other living organisms, but also for the course of technological processes. For example, due to the well-known property of water to conduct electric current, 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 assessing weather conditions, as everyone knows from weather forecasts. It is worth noting 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.
What is the characteristic of air humidity? This value is primarily characterized by the density of the water vapor that it contains. Air is a compound gas and contains many different gases, including water vapor. To estimate its amount in air, it is necessary to determine what mass water vapor has in a certain allocated volume - this value is characterized by density. The density of water vapor in the air is called absolute humidity.
Definition.Absolute air humidity - the amount of moisture contained in one cubic meter of air.
Designationabsolute humidity: (like the usual density notation).
Unitsabsolute humidity: (in SI) or (for the convenience of measuring a small content of water vapor in the air).
Formula calculations absolute humidity:
Legend:
Mass of steam (water) in air, kg (in SI) or g;
The volume of air in which the indicated mass of steam is contained,.
On the one hand, the absolute humidity of the air is an understandable 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 moisture susceptibility to living organisms. It turns out, for example, that a person does not feel the mass content of water in the air, but precisely its content relative to the maximum possible value.
To describe this perception, a quantity such as relative humidity.
Definition.Relative humidity - a value that shows how far the steam is from saturation.
That is, the value of the 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:
Designations:
Density of water vapor (absolute humidity), (in SI) or;
The density of saturated water vapor at a given temperature, (in SI) or.
As you can see from the formula, it contains the absolute humidity, which we are already familiar with, and the density of saturated steam at the same temperature. The question arises as to how to determine the last value. There are special devices for this. We'll consider condensinghygrometer (Fig. 2) - a device that serves to determine the dew point.
Definition.Dew point- the temperature at which the steam becomes saturated.
Figure: 2. Condensing hygrometer. ()
An easily evaporating liquid, for example, ether, is poured into the container of the device (enlarged in Figure 2 in section), a thermometer (6) is inserted inside, and air is pumped through the container using a pear (5). As a result of the enhanced air circulation, intensive evaporation of ether begins, the temperature of the container decreases due to this, and dew (droplets of condensed steam) appears on the mirror (4). At the moment dew appears on the mirror, the temperature is measured with 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 the 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 value of the dew point, the value of the corresponding density of saturated steam 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, instruments for measuring the characteristics of humidity (from the Greek hygros, wet and metreo I measure).
Hair hygrometer (Fig. 3) - a device for measuring relative humidity, in which hair, for example, human hair, acts as an active element.
Figure: 3. Hair hygrometer. ()
The action of a hair hygrometer is based on the property of defatted hair to change its length when the air humidity changes (with an increase in humidity, the length of the hair increases, with a decrease, it decreases), which allows you to measure the relative humidity. The hair is pulled over a metal frame. The change in hair length is transmitted to the arrow moving along the scale. It should be remembered that the hair hygrometer gives inaccurate values \u200b\u200bof relative humidity and is mainly used for domestic purposes.
A more convenient and accurate device for measuring relative humidity is a psychrometer (from ancient Greek ψυχρός - cold) (Fig. 4).
Figure: 4. Psychrometer. ()
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 a cambric cloth, which is immersed in a reservoir of water located on the back of the device. Water evaporates from the wet cloth, which leads to cooling of the thermometer, the process of lowering its temperature lasts until the stage is reached, until the steam near the wet cloth reaches saturation and the thermometer begins to show the dew point temperature. Thus, a wet bulb shows a temperature less than or equal to the actual ambient temperature. The second thermometer is called dry and shows the real temperature.
As a rule, the so-called psychrometric table is also depicted on the body of the device. Using this table, the relative humidity of the ambient air can be determined from the temperature value shown 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 value of humidity using the following principle: if the readings of both thermometers are close to each other, then the evaporation of water from the humid one is almost completely compensated for by condensation, i.e. the air humidity is high, if, on the contrary , the difference in thermometer readings is large, then evaporation from a damp cloth prevails over condensation, and the air is dry, and the humidity is low.
Let us refer to the tables that allow you to determine the characteristics of air humidity.
Pressure table (in mm Hg) and saturated vapor density (in) depending on temperature
|
Temperature, |
Pressure, mm. rt. Art. |
Density, |
Note again that, as indicated earlier, the value of the density of saturated vapor increases with its temperature, the same applies to the pressure of saturated vapor.
Psychrometric table
Recall that relative humidity is determined from the dry bulb reading (first column) and the difference between dry and wet bulb readings (first row).
In today's lesson, we got acquainted with an important characteristic of air - its humidity. As we have already said, humidity decreases during the cold season (winter), and increases during the warm season (summer). It is important to be able to regulate these phenomena, for example, if it is necessary to increase the humidity, place several tanks with water in the room in winter in order to enhance the evaporation processes, but this method will be effective only at an appropriate temperature, which is higher than outside.
