Michell Humidity Calculator FAQ

This page aims to answer some of the common questions asked by users of the calculator.

System Requirements

Online Calculator

Minimum System Requirements Recommended System Requirements
  • Internet Explorer 11
  • Edge (Latest) or
  • Chrome (Latest) or
  • Firefox (Latest)

All modern, standards-compliant browsers with HTML5 support should work without issue.

Sources of formulae used in the calculator

Humidity Calculator

wvp (e)BS1339:1, 3.2.2 Saturation vapor pressure [Sonntag]
swvp (es)
Dew Point*
Frost Point*
Relative HumidityBS1339:1, 3.2.9 Relative Humidity
Water Activity
ppmV (z)BS1339:1, 3.2.10 Relationships between mixing ratio, mole ratio and vapor pressure
Mixing Ratio (Y)
Enthalpy (h)BS1339:1, 3.2.14 Specific enthalpy of humid gas (“moist air”)
Wet BulbThe ASCE Standardized Reference Evapotranspiration Equation, ISBN: 9780784408056
Absolute Humidity (dv)BS1339:1, 3.2.12 Volumetric humidity

*Uses iterative solution when calculating from water vapor pressure.

For water vapor pressure calculations the Sonntag equation without water vapor factor enhancement is used.

The reason for not using water vapor 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.

Moisture in Natural Gas Calculator

Dew / Frost PointISO 18453 or IGT Bulletin #8, depending on settings
Absolute HumidityCalculated at 15°C, atmospheric pressure

Limits and Uncertainties

The humidity calculator is designed to be used over the range of -100°C to +100°C dew-point (at atmospheric pressure), calculations outside of this range may produce unexpected results. Calculations below atmospheric pressure may also produce unexpected results, as water vapor behaves differently in vacuum conditions.

The uncertainties (coverage factor approximately k=2) of the polynomials used in the Sonntag equation are:

Corresponding to dew-point uncertainties of:

Source: BS 1339-2:2009 7.2.2

How does the pressure compensation feature work?

Example: A dew-point 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 dew-point at the new pressure of 1barg.

This feature works in both directions.

Why are results different to calculator from company X, Y or Z?

The main reason for difference in calculations arises from the formula used to calculate water vapor 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 BS1339-1, 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.