Humidity Calculator

Relative humidity is the ratio of the actual vapor pressure (equilibrium vapor pressure at dew point or frost point) to the equilibrium (saturation) vapor pressure at the gas temperature, expressed as a percentage. At gas temperatures below freezing, where possibility of super-cooling exists, the humidity calculator calculates relative humidity in either standard (%RH) or WMO (World Meteorological Organization) way. Since the super-cooling phenomenon does occur in clouds and the upper atmosphere, the WMO has decided that the relative humidity be calculated by computing the maximum possible water content over water rather than ice regardless of the temperature.

Dew point is the temperature at which dew, or condensation, forms, on cooling a gas. It is also the temperature, at which water vapor is in equilibrium with liquid water. The humidity calculator calculates the dew point from the inverse water vapor pressure formulae, selected by user.

Frost point is the temperature at which frost forms, on cooling a gas. It is also the temperature, at which water vapor is in equilibrium with ice. The humidity calculator calculates the frost point from the inverse water vapor pressure formulae, selected by user.

Parts per million by volume (PPMv) is the ratio of the water vapor volume to the volume of the dry carrier gas, multiplied by 106.

Parts per million by weight (PPMw) is the ratio of the water vapor mass to the mass of the dry carrier gas, multiplied by 106.

Wet-bulb temperature is the temperature indicated by a thermometer sheathed in a wet wicking, and influenced by a rate of evaporation from the wicking. The humidity calculator calculator finds the water vapor pressure e in the gas from the wet-bulb temperature tw , the gas temperature t, the gas pressure P and equilibrium vapor pressure e' at tw using the following equation:

e = e' - A.P.(t - tw).(1+B.tw)    ,

where A and B are the psychrometric constants, which can be set by user in the input boxes below.

A: °C-1                       B: °C-1

Absolute humidity is the mass of the water vapor present in unit volume of moist gas of a given temperature and pressure.

Specific humidity is the mass of water vapor per unit mass of moist gas.

Mixing (humidity) ratio is the mass of water vapor per unit mass of dry carrier gas with which it is associated.

Moist gas density is the mass of the moist gas in unit volume of moist gas of a given temperature and pressure.

Enthalpy is a measure of the total energy in a humid gas. Enthalpy of a gas can be defined as the sum of sensible and latent heat for each component in the gas. Values of enthalpy are conventionally expressed relative to a datum point (for a dry gas, this is normally the heat content at 0 °C). The humidity calculator calculates the enthalpy for different carrier gasses and gas temperatures, based on the following constants (if left in auto mode, the calculator will use the mean value of cp  with respect to the gas temperature, except for SF6 , for which a manual input value shall be used):   auto     manual specific heat capacity of gas at constant pressure:     J/g/°C specific heat capacity of water at constant pressure:     J/g/°C evaporation heat of water at 0 °C:     kJ/g

References Hardy R., ITS-90 Formulations for Vapor Pressure, Frost point Temperature, Dew point Temperature, and Enhancement Factors in the range –100 to +100 C, Papers and Abstracts of the Third International Symposium on Humidity and Moisture, Vol. 1, pp. 214-222, April 1998 Sonntag D., Advancements in the field of hygrometry. (Review Article) Meteorologische Zeitschrift, N.F., 3, pp. 51-66, 1994 Wexler A., Vapor Pressure Formulation for Water in the Range 0 to 100°C. A Revision, Journal of Research of the National Bureau of Standards, Physics and Chemistry, Vol. 80A, Nos. 5 and 6, pp. 775-785, 1976 Wexler A., Vapor Pressure Formulation for Ice, Journal of Research of the National Bureau of Standards – A. Physics and Chemistry, Vol. 81A, No. 1, pp. 5-19, 1977 Buck Research Manual (1996); updated equation from Buck A. L., New equations for computing vapor pressure and enhancement factor, J. Appl. Meteorol., 20, pp. 1527-1532, 1981 Greenspan L., Functional Equations for the Enhancement Factors of CO2-Free Moist Air, Journal of Research of the National Bureau of Standards, Physics and Chemistry, Vol. 30A, No. 1, pp. 41-44, 1976 A Guide to the Measurement of Humidity, ISBN 0-904457-24-9, The Institute of Measurement and Control, London, 1996 tables for specific heat capacities at constant pressure - http://www.engineeringtoolbox.com/

The information contained in the humidity calculator / moisture calculator is subject to change without notice. The author of the humidity calculator / moisture calculator makes no warranties, either express or implied, regarding the calculations, or the fitness of these procedures or program for a particular purpose. The humidity calculator / moisture calculator is made available solely on an "as is" basis, and the entire risk as to its quality and performance rests with the user. The author shall not be liable for any incidental or consequential damages in connection with or arising out of the furnishing, use, or performance of the humidity calculator / moisture calculator.

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