Air Density: How to calculate Relative Humidity

[Pooven]

Hi there,

I'm trying to calculate air density; I noticed on http://en.wikipedia.org/wiki/Density#Density_of_air" that there is a table that makes it seem that air density is a function of temperature... I suppose I could safely say that temperature has a significant effect on air density? What error could one expect if they had to use measurements from this table?

Anyway, I know that humidity plays an important role in air density and there's a nice formula on http://en.wikipedia.org/wiki/Density_of_air#Water_vapor": ρ_{humid air} = p_d/R_d.T + p_v/R_v.T.

This seems like it's the only way to calculate the density of humid air. I have temperature, absolute humidity and ambient pressure of the system (all in the appropriate SI unit). This is insufficient information to calculate the relative humidity as specified on http://en.wikipedia.org/wiki/Relative_humidity" .

I understand that vapour pressure readings can be taken from steam tables to calculate the saturated vapour pressure. But then everything gets really blurry. But before I get into that... I saw this example from http://www.cheresources.com/questions/thermodynamics-41.html" that suggests that I can calculate the relative humidity from the absolute humidity... I haven't come across anything else that explicitly linked the two so I'd like to know if the answer is sound. The equation given was:

Humidity_absolute=Humidity_saturation * RelativeHumidity/100

Can anyone please advise if this would be a valid?

Kind regards
Pooven

 

[AndyW]

Hi Pooven,

You are correct in saying that air density is affected by temperature, as well as other factors such as humidity. The table on Wikipedia provides a general guideline for air density at different temperatures, but it is not a precise measurement. The actual air density at a given temperature will vary depending on the specific conditions of the air, such as humidity, pressure, and altitude.

In order to calculate the air density more accurately, you will need to consider the effects of humidity. The formula you mentioned, ρhumid air = pd/Rd.T + pv/Rv.T, takes into account the partial pressures of both dry air and water vapor. However, as you mentioned, this formula requires knowledge of the relative humidity, which can be difficult to determine without proper equipment.

The equation given from the example on cheresources.com is valid, as it takes into account the relationship between absolute humidity and relative humidity. However, this equation may not be accurate in all cases as it assumes that the air is at its saturation point. It is always best to use more precise calculations and measurements when possible.

In summary, while the table on Wikipedia provides a general idea of air density at different temperatures, it is not a precise measurement. To calculate air density more accurately, you will need to consider the effects of humidity and use appropriate equations and measurements. I hope this helps and please let me know if you have any further questions. Good luck with your calculations!