来自德国的在线露点计算器与JAVA语言源代码

hillside

        http://www.decatur.de/javascript/dew/index.html   
     直接点击以上网址，即可在线计算露点。介绍该计算器使用方法与各参数含义的页面粘贴至此帖，版面会严重变形。需要者可点击前往观看。http://www.decatur.de/javascript/dew/about.html
       附件为网页提供的在线计算器的JAVA语言源代码。该计算器我未用过，只是介绍。

Dewpoint Calculator
Computes the dewpoint or frostpoint temperatures at a given ambient temperature and relative humidity.

Temperature scale
Celsius Fahrenheit Kelvin

• Specify ambient temperature
• Specify relative humidity
• Compute
  

Links
About the CalculatorTemperature scaleAccuracySteam Table GeneratorCopyright
Last modified by Wolfgang Kühn on 2011-09-08 home | e-mail

Dewpoint Calculator Explained

Table of Content How does it work Glossary Resources

Links Dewpoint Calculator Steam Table Generator More JavaScript

How does it work

For given ambient temperature T and relative humidity RH, the Dewpoint Calculator solves the defining Equation (1) for the Dewpoint temperatur Tdp.

The solving is done numerically using Newton's Method and by initially guessing the ambient temperature as the dewpoint. The algorithm must be able to evaluate the saturation vapor pressure of water at arbitrary temperatures, and therefore relies on interpolation formulas for this quantity.

At freezing conditions (sub-zero Deg. C temperatures), the Dewpoint Calculator uses the saturation vapor pressure over ice, thus computing the frostpoint.

Glossary

Dewpoint Tdp
If a body is exposed to moist air and then cooled down, water will condense on its surface when the temperature drops below the dewpoint temperature. At freezing conditions, both dewpoint (now the dew is super-cooled water) and frostpoint can coexist.

Note: For a fixed relative humidity, neither dewpoint nor frostpoint depend on the atmospheric pressure. However, if a moist air sample is pressurized at a constant temperature and at a constant absolute humidity, both the relative humidity and the dewpoint will rise.

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Dewpoint Versus Temperature Curve for RH=5%

Frostpoint Tfp
Corresponding to the dewpoint for freezing conditions, the frostpoint is the temperature at which ice will form on the cool body. The frostpoint for liquid water is slightly higher than the dewpoint. Because the formation of ice depends on a certain amount of impurity on the surface of the body, however, you may still experience dew at subzero temperatures.

Boiling point
The temperature at which the vapor pressure equals atmospheric pressure (about 101kPa at sea-level). Below the boiling point, the atmospheric pressure (which equals the pressure inside the water) will prevent evaporation inside the water body. At the boiling point, the water body becomes physically unstable in that vapor bubbles will start to form.

Relative Humidity RH
Degree of saturation of water in air. At a RH of 100%, the air is saturated and attains its most moist state. At a RH of 0%, no water is present. Relative humidity is defined by Equation (1) as a function of ambient temperature T and dewpoint Tdp only, and therefore does not depend on air pressure or altitude:

Pvs(Tdp)=RH Pvs(T).

Equation (1)

In practice, RH is often measured directly and propagated through weather reports.

Saturation Vapor Pressure Pvs
The intensity at which water vapor will evaporate from water (Pressure over liquid water) or from ice (Pressure over ice). The saturation vapor pressure is a function of temperature only. It can be measured directly, is recorded in Steam Tables, and can be evaluated with the help ofinterpolation formulas.


Chilled Sensor Hygrometer
Technique to directly measure the dewpoint by chilling on object (e.g. a mirror) to the point of condensation. Sensors of this type are quite accurate, but are expensive to operate and have a low response rate.


Capacitive RH Hygrometer
Type of sensor most commonly used to measure atmospheric humidity. These type of sensors have a typical accuracy of about 2% to 5% at RH between 10% and 90%. For very low or very high humidity, accuracy can be lower.

Temperature Scale
The Dewpoint Calculator uses the traditional temperature scales Deg.C = Deg.K - 273.15 and Deg.F = 9/5 Deg.C + 32.

Interpolation Formulas
There are a great deal of interpolation formulas for the saturation vapor pressure Pvs(T). These formulas are of varying accuracy and have different ranges of validity. The Dewpoint Calculator uses the formula put forward in 1997 by IAPWS, the International Association for the Properties of Water and Steam, see below. It is very accurate for 273.15 K < T < 647.096 K, with the relative accuracy never exceeding 0.06%. For details, please consult the source code.

Newton's Method
To solve an equation of the form f(x)=y for x, Newton's Method computes the iterates xn

xn+1 = xn + (y-f(xn))/f'(xn),

Equation (2)

starting at a suitable initial guess x0. If the problem is not ill-conditioned, iterates will converge quickly to the solution x.

Resources

• Download JavaScript code for the calculator
• Equations of IAPWS-IF97, Section 5. "Equations for Region 4".
• Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam, Section 5. "Equations for Region 4".
• ITS-90 FORMULATIONS FOR VAPOR PRESSURE, FROSTPOINT TEMPERATURE, DEWPOINT TEMPERATURE
  

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