Dewpoint Calculator for Moisture

Equation Courtesy of Alpha Moisture Systems


1. Dew Point or Absolute Values: Begin by entering data into the dew point fields or any of the four absolute value sections. Ensure the correct unit of measurement is chosen.

2. Advanced Calculations: For results pertaining to ‘dew point at line pressure’ or ‘ppm(w)’, it’s mandatory to provide details on ‘Line Pressure’ and ‘Gas Type’. This allows for precise adjustments to the calculations, accounting for real-world conditions where line pressure can influence dew point.

3. Supported Gases: The calculator is optimized for seven commonly used gases in the industry. For gases not included in this list, the user can manually input the molecular weight to ensure accurate conversions.

4. Line Pressure Units: For diverse applications, line pressure units can be toggled among bara, barg, psia, psig, KPa, and MPa, providing flexibility and precision.


Dewpoint Values

Dewpoint at Atmospheric Pressure:

Optional Inputs

Line pressure:

Gas Type:

Input a gas of known molecular weight or select blank from the dropdown menu to input a custom value

Absolute Values

Parts per million by volume:


Parts per million by weight:


Absolute humidity:



Pressure Dewpoint

Dewpoint at Line Pressure:

The Magnus formula provides a method to determine the saturation vapor pressure of water over a range of temperatures, serving as a foundation for various dew point calculations. Here’s a detailed explanation:

Magnus Formula: The equation for saturation vapor pressure, P, over liquid water is: P(T) = A * exp((B * T) / (C + T)) Where:

  • P(T) is the saturation vapor pressure at temperature T.
  • A, B, and C are constants determined from empirical data for specific temperature ranges.
  • T represents the temperature.

Dew Point Calculation: Dew point is the temperature at which air is saturated with moisture, and water vapor begins to condense. This is the temperature corresponding to a relative humidity of 100%.

To compute the dew point using the Magnus formula:

  1. First, find the actual vapor pressure of the air from relative humidity and current temperature measurements.
  2. Then, adjust the Magnus formula to compute T (dew point temperature) for a given vapor pressure: T_dew = (C * ln(P / A)) / (B – ln(P / A)) Where:
  • T_dew is the dew point temperature.
  • P is the actual vapor pressure, typically derived from relative humidity and temperature.
  • A, B, and C are the same constants from the Magnus formula.

Application in SF6 Gas: In the context of SF6 gas, the dew point is of significant importance. A lower dew point indicates drier SF6 gas, which is preferable as moisture can affect the dielectric properties of SF6, leading to potential issues in high voltage equipment. Using the Magnus formula, professionals can accurately measure the moisture content in SF6 gas and take necessary actions if the dew point is not within the desired parameters.