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Author Topic: How to determine air density?  (Read 97 times)

poorman plinker

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How to determine air density?
« on: August 18, 2017, 10:07:43 AM »
First what is HTHs? LOL

Please take a look at my air density formulae.

Air Density is a way of expressing the mass of air per unit of volume. Air density is an important value to consider in aerodynamics. The following steps will explain how to calculate dry air density:
Ad = p/(K*T)
Ad = Air density (kg/m^3)
p = Air pressure in Pascals (Pa)
K = Gas constant for dry air (287.05)
T = Temperature (Kelvin)
Here is an example :Use the ISA standard values for conditions at sea level. P = 101325 Pa and T = 15 deg C. The air density is calculated to be:
Ad = (101325) / (287.05 * (15 + 273.15)) = 1.2250 kg/m3

Density of Humid Air can be calculated as the sum of the densities of the two gases, dry air and water vapor, in proportion with their partial pressures. The moist air density formula:

HA= pd/K*T + pv/Kv*T
HA = Humid Air Density (kg/m^3)
pd = Partial pressure of dry air (Pa)
K = Gas constant dry air (287.05)
T = Temperature (Kelvin)
pv = Partial pressure of water vapor (Pa)
Kv = Gas constant for water vapor (461.495)

Your thoughts will be appreciated.

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Re: How to determine air density?
« Reply #1 on: August 18, 2017, 01:17:35 PM »
I haven't a clue, I never calculate it using a formula, I just use an online calculator, or the one in ChairGun.... and input the altitude, temperature and barometric pressure (or station pressure).... The humidity is of minor importance, but if you know it, you can input that also.... The major variable is altitude (which affects station pressure)....

Your dry air formula agrees with that calculator = 1.225 kg/m^3 or 0.0765 lb/ft^3 at sea level.... I have no idea how you relate relative humidity to the partial pressure of water vapour for use in your second formula....

The temperature is also necessary to compute the speed of sound, so that the Mach number of your velocity can be calculated.... Again, the humidity has a near-negligible effect....

« Last Edit: August 18, 2017, 01:22:03 PM by rsterne »
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poorman plinker

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Re: How to determine air density?
« Reply #2 on: August 21, 2017, 10:21:15 AM »
As always your input does help...

Temp is part of the Air Density formula and the Mach formulae use Air Density so I have just assumed it included.

Thanks for the comment.
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