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How do you calculate the density of water vapour?

Published in Water Vapor Density 3 mins read

The density of water vapor, specifically the actual vapor density in the air, can be calculated using the relative humidity (RH) and the saturation vapor density (SVD).

Understanding the Terms

  • Relative Humidity (RH): This is the percentage of water vapor present in the air compared to the maximum amount of water vapor the air could hold at a given temperature. It's expressed as a percentage.
  • Saturation Vapor Density (SVD): This is the maximum amount of water vapor the air can hold at a specific temperature. This value increases with temperature. It represents the density of water vapor if the air is 100% saturated.
  • Actual Vapor Density: This refers to the real amount of water vapor present in the air. This is what we're calculating.

The Calculation

According to the provided reference, the formula to calculate the actual vapor density is:

Actual Vapor Density = RH × SVD

Where:

  • RH is the relative humidity expressed as a decimal (e.g., 50% RH becomes 0.50)
  • SVD is the saturation vapor density at the given temperature.

Practical Insights and Examples

Let's illustrate with an example:

  1. Obtain the RH: Let's assume the measured relative humidity is 60%. This means the air currently contains 60% of the maximum moisture it can hold. In decimal form, RH would be 0.60
  2. Find the SVD: For this example, let's say the saturation vapor density (SVD) at your current temperature is 20 g/m³. This value usually has to be found in a table based on the current temperature.
  3. Apply the formula:
    Actual Vapor Density = 0.60 * 20 g/m³
    Actual Vapor Density = 12 g/m³

Therefore, in this scenario, the actual water vapor density in the air is 12 g/m³.

Summary Table

Variable Description Units
RH Relative humidity (percentage of saturation) % (or decimal)
SVD Saturation Vapor Density (maximum water vapor at a given temperature) g/m³ (or similar)
Actual Vapor Density The density of water vapor in the air g/m³ (or similar)

Key Considerations

  • Temperature: SVD is highly dependent on temperature. Warmer air can hold much more moisture than cold air, hence why the SVD is dependent on temperature.
  • Units: Make sure that the units of SVD and the final density are consistent. For example if SVD is in g/m^3, then actual vapor density will be in g/m^3.
  • Accuracy: Accurate calculation depends on a precise reading of the relative humidity and knowing the correct value for SVD based on temperature.
  • Tools: You'll need a hygrometer to measure the RH and often a reference table or a calculator to determine the SVD for the current temperature.