This pages aims to answer some of the common questions asked by users of the calculator.
Minimum System Requirements  Recommended System Requirements 


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Parameter  Source 
wvp (e)  BS1339:1, 3.2.2 Saturation vapour pressure [Sonntag] 
swvp (es)  
Dew Point*  
Frost Point*  
Relative Humidity  BS1339:1, 3.2.9 Relative Humidity 
Water Activity  
ppmV (z)  BS1339:1, 3.2.10 Relationships between mixing ratio, mole ratio and vapour pressure 
Mixing Ratio (Y)  
ppmW  
Enthalpy (h)  BS1339:1, 3.2.14 Specific enthalpy of humid gas (“moist air”) 
Wet Bulb  The ASCE Standardized Reference Evapotranspiration Equation, ISBN: 9780784408056 
Absolute Humidity (dv)  BS1339:1, 3.2.12 Volumetric humidity 
*Uses iterative solution when calculating from water vapour pressure.
For water vapour pressure calculations the Sonntag equation without water vapour factor enhancement is used.
The reason for not using water vapour enhancement factor is that this only applies to air, over a small range of pressures. The uncertainties using the factor outside of this range are unknown, and many of our customers measure moisture in gas mixtures other than air, so it is safer not to use this.
Parameter  Source 
Dew / Frost Point  ISO 18453 or IGT Bulletin #8, depending on settings 
Absolute Humidity  Calculated at 15°C, atmospheric pressure 
The humidity calculator is designed to be used over the range of 100°C to +100°C dewpoint (at atmospheric pressure), calculations outside of this range may produce unexpected results. Calculations below atmospheric pressure may also produce unexpected results, as water vapour behaves differently in vacuum conditions.
The uncertainties (coverage factor approximately k=2) of the polynomials used in the Sonntag equation are:
Corresponding to dewpoint uncertainties of:
Source: BS 13392:2009 7.2.2
Example: A dewpoint measurement is made at 5barg pressure, but you want to know the value if you reduce the pressure of the sample to 1barg.
Enter 5barg in the Sample Pressure input, and 1barg in the Pressure Compensation box to calculate the dewpoint at the new pressure of 1barg.
This feature works in both directions.
The main reason for difference in calculations arises from the formula used to calculate water vapour pressure at a given temperature. This is the basis of all humidity calculations, and there are several different formula in use.
The most widely used formula is the Sonntag formula, found in BS13391, but other formula such as those by Hardy and Wexler are also. The uncertainties between the different formula are similar, and no formula is "more valid" than another.