Dew point

The dew point, the dew point temperature is the temperature of moist air, which has to be reached at unchanged pressure so that water vapor can be deposited as a dew or mist. The dew point of the relative humidity is 100%. The air is then saturated with water vapor. The more water vapor the air, the higher is the dew point temperature. The dew point can therefore be used to determine the absolute humidity. The dew point is measured with a hygrometer directly or indirectly with other hygrometric method. The term of the dew point shall apply mutatis mutandis to other gas mixtures with condensable constituents. For the dew point often the Greek letter is used as symbols.

• 5.1 Abbreviations
• 5.2 Basic relationships
• 5.3 saturation vapor pressure
• 5.4 depending on the dew point temperature of water content and barometric pressure
• 5.5 depending on the relative humidity and dew point temperature of air temperature

Physical background

The dew point is the temperature of a moist gas mixture, in which condensation and evaporation of the wet component adhered to the balance, the gas is so just saturated with the vapor. " Wetland " means in this context is usually that it is in the gas mixture is a water vapor-air mixture, it can also refer to another condensable component. The steam partial pressure prevailing in this state of equilibrium, the saturation vapor pressure. What is the saturation vapor pressure at a given temperature, is derived from the phase diagram of the water and of the substances other gas mixtures. Because the non - condensable components of the gas mixture did not influence the behavior of the steam almost, the dew point of a moist air is almost entirely dependent on the partial pressure of the water contained. Each such equilibrium state is determined by a point in the pT diagram. If you connect all these points together, we obtain the dew point as a phase boundary. In some textbooks dew point is the designation of the state of supersaturation used, the corresponding dew-point temperature then.

Below the triple point in the phase diagram there is no liquid but frost builds. We therefore speak here of the frost point instead of the dew point. But even here there is a dynamic equilibrium between solid and vapor phase can adjust so that sublimation and keep resublimation exactly the scale.

When no liquid phase is present, i.e., when the system consists of only the air -steam mixture, the condition is often far away from the dew point. The steam may still be caused to condense the one hand, can be cooled, the system, so that its temperature reaches the dew point temperature. On the other hand, it may be compressed so that the steam partial pressure increases up to the saturation vapor pressure. The relative humidity indicates how much the current steam partial pressure compared with the saturated vapor pressure ( at the same temperature ) expressed as a percentage. The difference between the actual temperature of air present and the dew point ( at the same pressure ), the dew point, also called the spread. It is an important variable in the prediction of thermal and cloud base and is often expressed in aerological measurements instead of the relative humidity. It is fairly large in dry air and small at high humidity.

Measurement

A direct measurement of the dew point can be carried out with a hygrometer. This includes a temperature-controlled mirror. If the mirror in mirror sinking temperature reaches or falls below the dew point, it fogs up. Thus its optical properties, especially its reflectivity change. An optical measurement system is in this way determined the dew point.

An indirect method to measure the room temperature combined with the relative humidity. From these two data, the absolute humidity is calculated and from this can determine the temperature at which the relative humidity reaches a value of 100%. In modern instruments, this calculation is done internally by a microprocessor is generally displayed the measured room temperature, the relative humidity and the dew point temperature.

Examples of use

Meteorology

In meteorology, the dew point is used as a measure for the air humidity. When the respective air temperature corresponds to the dew point, the relative humidity is 100%. The term humidity can be defined above the dew point: Sultry is experienced when the dew point exceeds about 16 ° C.

If the temperature of the air below its dew point, which is often the case in the lower atmosphere in the early morning hours, so liquefies some of the water vapor: dew or mist form, at lower temperatures, frost. It is sufficient that individual objects, such as automobiles to cool by radiation of heat at the dew point. Gone pointing out moist air then cools at the surface of the objects also below the dew point and from the windows steam up with condensation or frost flowers. If the temperature rises above the dew point, then these precipitates dissolve again.

Similar processes are also responsible for the formation of clouds and resolution: Heated air masses rise up from the ground and cool, this is about 1 ° C per 100 m from. One speaks of the " trockenadiabatischen temperature gradient ". First, the amount of water vapor contained in the air parcel remains unchanged. At a certain height, the temperature of the air mass approaches the dew point. This is where the condensation, a cumulus cloud forms. The cloud base is located exactly at this level. The air rises even further, so the temperature decreases slowly because condensation heat is released (see feuchtadiabatischer temperature gradient). In order to assess the evolution of the clouds are therefore created by using radiosondes regular vertical profiles of temperature and dew point. This information about the stratification of the atmosphere can be used to predict, for example thunderstorms.

Building Physics

For water- vapor-permeable vapor permeable building materials Water vapor diffuses due to the concentration gradient through the component. At the point in the wall, at which the material temperature is lower than the dew point, the water vapor condenses and the component becoming waterlogged. Taupunktebene as the surface is referred to the temperature corresponding to the dew point of water vapor. The moisture transport can be reduced by a vapor barrier.

On poorly insulating windows, the air cools below the dew point and humidity fogs up the glass. At temperatures below the frost point to make ice-flowers on the window pane.

Dehumidification

Using moisture condensation dryers can be deposited from room air. The air is cooled below the dew point, humidity condensable runs off, and then the air is heated again by a recovery register. The operating principle is also used in the tumble dryer.

Pneumatics

A moist gas mixture is compressed, the vapor partial pressure, and thus the dew point temperature increases. The steam then condenses at elevated temperatures. In pneumatic therefore the compressed air is dried prior to use, such as with refrigeration dryers. Thereby, the compressed air can be used at low temperatures without condensation in the pressure lines or transport containers of water.

Dew points in other gas mixtures

In multi- component systems ( for example, exhaust gases, distillation mixtures, natural gas) can, analogous to boiling or condensation range, a dew point be specified.

The hydrocarbon dew describes the dew point of a hydrocarbon mixture, the dew points of the individual hydrocarbon components taken into account in physical separation processes. Especially with a gas mixture of the dew point is then called Erdgastaupunkt. In practice, the hydrocarbon dew point is also calculated indirectly from a gas chromatographic analysis, but this is subject to large errors.

The dew point temperature condensable components in exhaust gas is referred to as flue gas dew point, in flue gases as Rauchgastaupunkt. The avoidance of a dew point in the exhaust gas to prevent sooting of a chimney, however, is in the utilization of condensing technology falling below the dew point desired to use the present in the exhaust latent heat. The dew point of acids in the flue gas acid dew point is called. The sulfuric acid dew specifically describes the dew point of sulfuric acid in the flue gases. In order to delineate these, the dew point of water vapor in smoke and exhaust gases is called water vapor dew point.

Calculation of the dew point of humid air

Abbreviations

The following designations are used:

Basic relationships

The water content is the ratio between the mass of water contained in the mixture mW and the mass of dry air mL.

In the unsaturated state all the water is present as vapor. You can write this:

The partial pressures of the steam and the dry air pD pL describe the state of the mixture components. The sum of the partial pressures of the barometric pressure of the moist air.

The partial pressure is the pressure that the particular mixture component would assume if she θ, the mixture volume V at the same temperature alone occupied. With the thermal equation of state for ideal gases can be formulated:

The ratio of the molar masses of water to dry air is 0.622.

Using equation ( 3) can be formulated for the partial pressure of the vapor

The relative humidity φ, the ratio of partial pressure of the vapor to the saturated vapor pressure Ps.

From equation ( 7) can be derived

Saturated vapor pressure

The dependence of the saturation vapor pressure of water is known from measurements and shown in Table plants and expressed in approximate equations. A way for the description of the saturated vapor pressure is the Magnus formula, which has the form

For the saturation vapor pressure over water apply the parameters

In the temperature range ( below 0 ° C for subcooled water).

For the saturation vapor pressure over ice apply the parameters

In the temperature range. The following formulas are only applicable in this field.

Depending on the dew point temperature of water content and barometric pressure

At the dew point of the vapor is present at saturation pressure. Equation (7) can be equated with the saturation pressure of the dew point temperature.

Depending on which formulation is used for the temperature dependence of the saturation vapor pressure, can be an iterative solution, or an explicit formula for the dew point temperature place. By inserting the Magnus formula (10), equation ( 11 ) can be changed by the dew point temperature.

Depending on the relative humidity and dew point temperature of air temperature

The relative humidity at the dew point is equal to 1 ( = 100%)

Dividing equation (9) by equation ( 13) we obtain

After inserting the Magnus formula allows the connection to reformulate