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.
- absolute humidity
- dewpoint / frostpoint - outputs dewpoint (equilibrium over supercooled water / ice) for values below the freezing point of water (0 °C/32 °F).
- mixing ratio
- parts per million
- relative humidity
- specific volume
- vapor pressure
- water content
- wet bulb temperature
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
For conversions between two humidity parameters, you need to
1) Adjust the ambient conditions:
2) Adjust the known humidity parameter value and corresponding unit. The field with the known humidity value is marked with a dark blue color.
- the temperature of the measured gas
- the pressure of the measured gas
- gas type (e.g. air)
- for wet bulb calculation, the psychrometer type
After providing the known values and pressing Enter or Calculate button, the other humidity
parameter values are calculated.
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
NOTE: The calculator recalculates all parameters that depend on the ambient conditions,
also the given humidity parameter.
- 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
Gas Type Selection
By default, Vaisala Humidity Calculator calculates humidity in air. User can choose different carrier gases from the list:
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.
- natural gas CH4
- carbon dioxide CO2
- sulfur hexafluoride SF6
- argon Ar
- oxygen O2
- nitrogen N2
- hydrogen H2
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.
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.
- standard calculation coefficients (corresponding to Vaisala humidity instruments)
- WMO psychrometer coefficients
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:
C= constant 216.679 gK/J
Pw= vapor pressure in hPa
T= temperature in K
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.
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.
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.
The mixing ratio (mass of water vapor/mass of dry gas) is calculated using:
The value of B depends on the gas. 621.9907 g/kg is valid for air.
In general the constant can be calculated using:
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):
Pw=water vapor pressure
II: Mass/mass ppmw dry gas (less commonly used than ppmv for gas humidity):
Pw=water vapor pressure
Mw=molecular mass of water
Md=molecular mass of dry gas
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
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).
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 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.
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:
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:
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.
(C) Copyright Vaisala Oyj, 2006-2014