Contents

 

About Vaisala Humidity Calculator

Vaisala Humidity calculator is a software tool that provides an easy way for solving humidity conversions from one humidity parameter to another. It can also be used to calculate the effect of changing ambient conditions.

The Vaisala Humidity Calculator includes calculation of For gas dependent humidity parameters (e.g., ppm by weight and mixing ratio) a wide selection of carrier gases is available in addition to air (natural gas, CO2, SF6, Ar, O2, N2, H2). The user can specify additional carrier gases but must provide the molecular weight of the gas. Wet bulb calculation can be performed based on standard or WMO coefficients, as well as custom values given by the user.

Vaisala Humidity Calculator has a customizable user interface - you can choose which parameters are shown on the screen. The calculator will remember your choices and you can modify the list of shown parameters at any time by clicking the Settings icon.

Calculating with Vaisala Humidity Calculator

Humidity conversions

Known values

For conversions between two humidity parameters, you need to
1) Adjust the ambient conditions:
  • the temperature of the measured gas
  • the pressure of the measured gas
  • gas type (e.g. air)
  • for wet bulb calculation, the psychrometer type
2) Adjust the known humidity parameter value and corresponding unit. The field with the known humidity value is marked with a dark blue color.

Calculated values

After providing the known values and pressing Enter or Calculate button, the other humidity parameter values are calculated.

Unit conversions

The units of all parameters, both given and calculated, can be changed instantly, and the software recalculates the values. The newly changed unit is shown on the screen with a green arrow that fades away after several seconds.

The dewpoint/frostpoint value follows the unit chosen for temperature (°C or °F). Other parameters have the possible units listed in the drop down boxes.

Calculating the effect of temperature or pressure change

Vaisala Humidity Calculator allows for calculating the effect of changes in ambient conditions. This function is useful for example when simulating the compression or expansion of a gas and observing the effect on different humidity parameters (example: calculating what a frostpoint of -20 °C at 7 bar pressure would be at 1 bar pressure).

The known conditions are first entered and the calculation is performed as described in Humidity Conversions. To know the effect of pressure or temperature change for the humidity parameters
  • provide the new pressure or temperature value
  • press Enter or Calculate button, or move cursor to another field, to get the new values of each parameter
NOTE: The calculator recalculates all parameters that depend on the ambient conditions, also the given humidity parameter.

Gas Type Selection

Gas type

By default, Vaisala Humidity Calculator calculates humidity in air. User can choose different carrier gases from the list:
  • air
  • natural gas CH4
  • carbon dioxide CO2
  • sulfur hexafluoride SF6
  • argon Ar
  • oxygen O2
  • nitrogen N2
  • hydrogen H2
Additionally, the user can specify a gas by choosing the option "Add new" and providing the molecular weight of the gas. Changing the gas type will directly influence the calculations of mixing ratio and ppmw (parts per million by weight). The custom made gas types are stored in the computer memory as cookies. They can be edited and removed by the user.

Enhancement factor

The water vapor saturation pressures are exactly valid only in vacuum where water vapor is the only gas present. If other gases are present the real saturation vapor pressure Pws will increase. For normal atmospheric pressure and moderately above it this effect is typically ignored. At pressures significantly above atmospheric pressure this effect has to be taken into account. In Vaisala Humidity Calculator, enhancement factors are in use for air, natural gas (methane), oxygen, nitrogen and hydrogen. The calculations with gas types carbon dioxide, sulfur hexafluoride argon and Custom gas are made by the ideal law assumption.

Psychrometer Selection

The Psychrometer selection affects the wet bulb calculation. The selection includes wetbulb calculations with Additionally, the user can specify a custom psychrometer by choosing the option "Add new" and providing the custom coefficients for a psychrometer. The value given is the actual psychrometer constant multiplied by 106. For example a psychrometer constant 0.000575 is given as 575. The given value must be between 400...1000.

Humidity Definitions

Absolute humidity

Absolute humidity is defined as the mass of water vapor in a certain volume. If ideal gas behavior is assumed the absolute humidity can be calculated using:
Where
C= constant 216.679 gK/J
Pw= vapor pressure in hPa
T= temperature in K

Density

The gas density [mass/ volume unit] is calculated taking into account the mixing ratio of water vapor to dry gas, dry gas molecular weight and the temperature and pressure of the gas. For dry air the molecular weight 28.96443 g/mol is used. For water the molecular weight 18.015 is used.

Dewpoint

The Dewpoint temperature (Td) of a moist air or other gas sample is the temperature to which the sample must be cooled to reach saturation with respect to liquid water.

At dewpoint temperature,
NOTE: Dewpoint value is always expressed as dewpoint across the entire range of temperatures and assumes supercooled water below 0 °C/32 °F.

Dewpoint / Frostpoint

At temperatures above freezing (0 °C/32 °F), saturation vapor pressure (Pws) is always calculated with respect to water vapor at equilibrium over a water surface. The corresponding parameter is dewpoint temperature. At temperatures below freezing, equilibrium can be over either an ice surface (frostpoint) or a water surface (dewpoint).

NOTE: Dewpoint / Frostpoint value is expressed as dewpoint at and above 0 °C/32 °F, and as frostpoint below 0 °C/32 °F.

Enthalpy

The Specific enthalpy of moist air is defined as the total enthalpy of the dry air (sensible heat) and the water vapor (latent heat) mixture per unit mass of moist air. The value is calculated as a difference to a selected reference state. For metric units the (kJ/kg) the reference state is dry air at 0°C. For nonmetric units (Btu/lb) the reference state is dry air at 0°F.

Mixing ratio

The mixing ratio (mass of water vapor/mass of dry gas) is calculated using:
Where
B=621.9907 g/kg

The value of B depends on the gas. 621.9907 g/kg is valid for air.

In general the constant can be calculated using:
Where
M(H2O)=molecular weight of water
M(gas)=molecular weight of gas

Parts per million (ppm)

Parts per million values can be calculated either in relation to volume (ppmv) or weight (ppmw). For gas measurements, the ppmv is more commonly used, and is in many cases referred to as ppm.

I: Volume/volume ppmv dry gas (standard parameter for gas humidity):
Where
Pw=water vapor pressure
Ptot=total pressure

II: Mass/mass ppmw dry gas (less commonly used than ppmv for gas humidity):
Where
Pw=water vapor pressure
Ptot=total pressure
Mw=molecular mass of water
Md=molecular mass of dry gas

Relative humidity

Relative humidity is defined as the ratio of water vapor pressure (Pw) to the saturation water vapor pressure (Pws) at the gas temperature:
NOTE: Above the boiling point of water (100 °C/212 °F), the saturation vapor pressure Pws is greater than 1013 hPa (normal atmospheric pressure). Therefore relative humidity cannot reach 100%RH above 100 °C/212 °F in an unpressurized system.

Below the freezing point (0 °C/32 °F) the definition is also valid. Here 100 %RH is also impossible because condensation will occur at a lower humidity than 100%RH (when the vapor is saturated against ice). You can find out the maximum relative humidity value by choosing Tfrost/Tdew for dewpoint parameter and setting the frostpoint value equal to the ambient temperature. For example at an ambient temperature of -20 °C and a frostpoint of -20 °C the maximum %RH value is aproximately 82.2 %RH.

Saturation vapor pressure Pws

The saturation vapor pressure (Pws) is the equilibrium water vapor pressure in a closed chamber containing liquid water. It is a function only of temperature, and it indicates the maximum amount of water that can exist in the vapor state. This amount increases with increasing temperature. Vaisala Humidity Calculator uses the "Wagner, Pruß" formula to calculate the water vapor saturation pressure. (W. Wagner and A. Pruß: "The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use", Journal of Physical and Chemical Reference Data ,June 2002 ,Volume 31, Issue 2, pp. 387-535).

Specific volume

The Specific volume of the moist gas is the inverse of the gas density [volume/mass unit].

Vapor pressure Pw

Vapor pressure refers to the vapor pressure of water in air or other gas. Water vapor has a partial pressure Pw which is part of the total pressure (Ptot) of the gas according to Dalton's law
Ptot= Poxygen + Pnitrogen ...+ Pw

Water content

Water content is defined as the absolute humidity of the gas if it is brought to a standard pressure and temperature state. For metric units this state is normal ambient pressure (101325 Pa) and 0 °C. For nonmetric units (lb/MMscf) the standard temperature is 60 °F.

Wet bulb

The wet bulb temperature Twet depends on the vapor pressure Pw, the total absolute pressure Ptot, and the dry bulb temperature Tdry according to the following formulas.

In standard wetbulb temperature calculation:
Where
Pws=water vapour saturation pressure (over water above freezing and over ice below freezing)
Ptot= total ambient pressure
K=psychrometer constant 0.000662 °C-1 for water and 0.000583 for ice

In custom calculation of wet bulb:
As standard, but with custom value for K.

In wetbulb calculation according to WMO:
For Twet > 0°C:
For Twet <= 0°C:

Notice

The information contained in this document is subject to change without notice.

Vaisala Oyj makes no warranties, either express or implied, regarding the program, or the fitness of these procedures or program for a particular purpose. The program is made available solely on an "as is" basis, and the entire risk as to its' quality and performance rests with the user. Vaisala Oyj shall not be liable for any incidental or consequential damages in connection with or arising out of the furnishing, use, or performance of the program.
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